Ions typically pack into extremely regular crystalline structures, in an arrangement that minimizes the lattice energy (maximizing attractions and minimizing repulsions). The lattice energy is the summation of the interaction of all sites with all other sites. For unpolarizable spherical ions only the charges and distances are required to determine the electrostatic interaction energy. For any particular ideal crystal structure, all distances are geometrically related to the smallest internuclear distance. So for each possible crystal structure, the total electrostatic energy can be related to the electrostatic energy of unit charges at the nearest neighbour distance by a multiplicative constant called the Madelung constant that can be efficiently computed using an Ewald sum.
Structure
Ionic compound
Ionic compound | Structure
Ions typically pack into extremely regular crystalline structures, in an arrangement that minimizes the lattice energy. Ions typically pack into extremely regular crystalline structures, in an arrangement that minimizes the lattice energy The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on inclusion of other materials, impurities, and structural or isotopic abnormalities in the crystals. Sodium and chloride
describe in general terms the structure of ionic solids such as nacl
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Ionic compounds in the solid state form lattice structures. The two principal factors in determining the form of the lattice are the relative charges of the ions and their relative sizes. Some structures are adopted by a number of compounds; for example, the structure of the rock salt sodium chloride is also adopted by many alkali halides, and binary oxides such as magnesium oxide. Pauling's rules provide guidelines for predicting and rationalizing the crystal structures of ionic crystals
Ionic bonding | Structures
When a reasonable form is assumed for the additional repulsive energy, the total lattice energy can be modelled using the Born–Landé equation, the Born–Mayer equation, or in the absence of structural information, the Kapustinskii equation. Using an even simpler approximation of the ions as impenetrable hard spheres, the arrangement of anions in these systems are often related to close-packed arrangements of spheres, with the cations occupying tetrahedral or octahedral interstices. Depending on the stoichiometry of the ionic compound, and the coordination (principally determined by the radius ratio) of cations and anions, a variety of structures are commonly observed, and theoretically rationalized by Pauling's rules.
Ionic compound | Structure
In some cases the anions take on a simple cubic packing, and the resulting common structures observed are: Some ionic liquids, particularly with mixtures of anions or cations, can be cooled rapidly enough that there is not enough time for crystal nucleation to occur, so an ionic glass is formed (with no long-range order).
Ionic compound | Structure
In such a lattice, it is usually not possible to distinguish discrete molecular units, so that the compounds formed are not molecular in nature. However, the ions themselves can be complex and form molecular ions like the acetate anion or the ammonium cation. For example, common table salt is sodium chloride. When sodium (Na) and chlorine (Cl) are combined, the sodium atoms each lose an electron, forming cations (Na), and the chlorine atoms each gain an electron to form anions (Cl). These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl). However, to maintain charge neutrality, strict ratios between anions and cations are observed so that ionic compounds, in general, obey the rules of stoichiometry despite not being molecular compounds.
Ionic bonding | Formation
The word "ion" is the Greek , "ion", "going", the present participle of , "ienai", "to go". This term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that "goes" from one electrode to the other through an aqueous medium. In 1913 the crystal structure of sodium chloride was determined by William Henry Bragg and William Lawrence Bragg. This revealed that there were six equidistant nearest-neighbours for each atom, demonstrating that the constituents were not arranged in molecules or finite aggregates, but instead as a network with long-range crystalline order. Many other inorganic compounds were also found to have similar structural features.
Ionic compound | History of discovery
According to the nomenclature recommended by IUPAC, ionic compounds are named according to their composition, not their structure. In the most simple case of a binary ionic compound with no possible ambiguity about the charges and thus the stoichiometry, the common name is written using two words. The name of the cation (the unmodified element name for monatomic cations) comes first, followed by the name of the anion. For example, MgCl is named magnesium chloride, and NaSO is named sodium sulfate (, sulfate, is an example of a polyatomic ion). To obtain the empirical formula from these names, the stoichiometry can be deduced from the charges on the ions, and the requirement of overall charge neutrality.
Ionic compound | Nomenclature
The compressibility of an ionic compound is strongly determined by its structure, and in particular the coordination number. For example, halides with the caesium chloride structure (coordination number 8) are less compressible than those with the sodium chloride structure (coordination number 6), and less again than those with a coordination number of 4.
Ionic compound | Compressibility
In solid sodium chloride, each ion is surrounded by six ions of the opposite charge as expected on electrostatic grounds. The surrounding ions are located at the vertices of a regular octahedron. In the language of close-packing, the larger chloride ions are arranged in a cubic array whereas the smaller sodium ions fill all the cubic gaps (octahedral voids) between them. This same basic structure is found in many other compounds and is commonly known as the halite or rock-salt crystal structure. It can be represented as a face-centered cubic (fcc) lattice with a two-atom basis or as two interpenetrating face centered cubic lattices.
Sodium chloride | Solid sodium chloride
The exact arrangement of ions in an ionic lattice varies according to the size of the ions in the solid.
Ionic crystal | Introduction
In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na) and chloride (Cl) in sodium chloride, or polyatomic species such as the ammonium () and carbonate () ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a crystalline structure.
Ionic compound | Introduction
A standard ionic solid consists of atoms held together by ionic bonds, that is by the electrostatic attraction of opposite charges (the result of transferring electrons from atoms with lower electronegativity to atoms with higher electronegativity). Among the ionic solids are compounds formed by alkali and alkaline earth metals in combination with halogens; a classic example is table salt, sodium chloride. Ionic solids are typically of intermediate strength and extremely brittle. Melting points are typically moderately high, but some combinations of molecular cations and anions yield an ionic liquid with a freezing point below room temperature. Vapour pressures in all instances are extraordinarily low; this is a consequence of the large energy required to move a bare charge (or charge pair) from an ionic medium into free space.
Bonding in solids | Ionic solids
In many simple compounds, valence bond theory, the Valence Shell Electron Pair Repulsion model (VSEPR), and the concept of oxidation number can be used to explain molecular structure and composition. An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and the ionic bond is the electrostatic force of attraction between them. For example, sodium (Na), a metal, loses one electron to become an Na cation while chlorine (Cl), a non-metal, gains this electron to become Cl.
Chemistry | Bonding
Ionic compounds containing basic ions hydroxide (OH) or oxide (O) are classified as bases. Ionic compounds without these ions are also known as salts and can be formed by acid–base reactions. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation, freezing, a solid-state reaction, or the electron transfer reaction of reactive metals with reactive non-metals, such as halogen gases. Ionic compounds typically have high melting and boiling points, and are hard and brittle. As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions are mobilized.
Ionic compound | Introduction
Ionic compounds can be produced from their constituent ions by evaporation, precipitation, or freezing. Reactive metals such as the alkali metals can react directly with the highly electronegative halogen gases to form an ionic product. They can also be synthesized as the product of a high temperature reaction between solids. If the ionic compound is soluble in a solvent, it can be obtained as a solid compound by evaporating the solvent from this electrolyte solution. As the solvent is evaporated, the ions do not go into the vapour, but stay in the remaining solution, and when they become sufficiently concentrated, nucleation occurs, and they crystallize into an ionic compound.
Ionic compound | Formation
Sodium nitride seems to be about 90% ionic at room temperature, but has the band gap typical for a semiconductor. It adopts the anti-ReO structure with a simple lattice made up of NNa octahedra. The compound has N-Na bond lengths of 236.6 pm. This structure has been confirmed through X-ray diffraction and more recently neutron diffraction on powder and single-crystals.
Sodium nitride | Structure
Sodium azide is an ionic solid. Two crystalline forms are known, rhombohedral and hexagonal. Both adopt layered structures. The azide anion is very similar in each form, being centrosymmetric with N–N distances of 1.18 Å. The ion has octahedral geometry. Each azide is linked to six Na+ centers, with three Na-N bonds to each terminal nitrogen center.
Sodium azide | Structure
Electrostatic forces between particles are strongest when the charges are high, and the distance between the nuclei of the ions is small. In such cases, the compounds generally have very high melting and boiling points and a low vapour pressure. Trends in melting points can be even better explained when the structure and ionic size ratio is taken into account. Above their melting point ionic solids melt and become molten salts (although some ionic compounds such as aluminium chloride and iron(III) chloride show molecule-like structures in the liquid phase). Inorganic compounds with simple ions typically have small ions, and thus have high melting points, so are solids at room temperature.
Ionic compound | Melting and boiling points
Boiling points exhibit similar trends to melting points in terms of the size of ions and strength of other interactions. When vapourized, the ions are still not freed of one another. For example, in the vapour phase sodium chloride exists as diatomic "molecules".
Ionic compound | Melting and boiling points
Solid YCl adopts a cubic structure with close-packed chloride ions and yttrium ions filling one third of the octahedral holes and the resulting YCl octahedra sharing three edges with adjacent octahedra, giving it a layered structure. This structure is shared by a range of compounds, notably AlCl.
Yttrium(III) chloride | Structure
The formula indicates the presence of aluminium metal centres in the +3 oxidation state and acetate groups in a ratio of 1:3. Images used to represent this substance, such as those shown at left, represent two highly oversimplified approximations of the solid-state structure: the first is as a purely ionic salt with a single aluminium(III) cation (Al) surrounded by and associated electrostatically with three acetate anions (), but this should not be taken to convey information about the crystal structure. For example, sodium chloride (NaCl) has a cation-to-anion stoichiometry of 1:1, but it has a cubic structure with each ion surrounded octahedrally by six ions of the opposite charge.
Aluminium triacetate | Structure
An ionic crystal is a crystal consisting of ions bound together by their electrostatic attraction. Examples of such crystals are the alkali halides, including potassium fluoride, potassium chloride, potassium bromide, potassium iodide, sodium fluoride, and other combinations of sodium, caesium, rubidium, or lithium ions with fluoride, bromide, chloride or iodide ions. NaCl has a 6:6 co-ordination. The properties of NaCl reflect the strong interactions that exist between the ions. It is a good conductor of electricity when molten, but very poor in the solid state. When fused the mobile ions carry charge through the liquid. They are characterized by strong absorption of infrared radiation and have planes along which they cleave easily.
Ionic crystal | Introduction
NaH is produced by the direct reaction of hydrogen and liquid sodium. Pure NaH is colorless, although samples generally appear grey. NaH is ca. 40% denser than Na (0.968 g/cm). NaH, like LiH, KH, RbH, and CsH, adopts the NaCl crystal structure. In this motif, each Na ion is surrounded by six H centers in an octahedral geometry. The ionic radii of H (146 pm in NaH) and F (133 pm) are comparable, as judged by the Na−H and Na−F distances.
Sodium hydride | Basic properties and structure
The concept of ionic radii is based on the assumption of a spherical ion shape. However, from a group-theoretical point of view the assumption is only justified for ions that reside on high-symmetry crystal lattice sites like Na and Cl in halite or Zn and S in sphalerite. A clear distinction can be made, when the point symmetry group of the respective lattice site is considered, which are the cubic groups "O" and "T" in NaCl and ZnS. For ions on lower-symmetry sites significant deviations of their electron density from a spherical shape may occur. This holds in particular for ions on lattice sites of polar symmetry, which are the crystallographic point groups "C", "C", "C" or "C", "n" = 2, 3, 4 or 6.
Ionic radius | Non-spherical ions
The decavanadate ion consists of 10 fused VO octahedra and has D symmetry. The structure of Na[VO]·18HO has been confirmed with X-ray crystallography. The decavanadate anions contains three sets of equivalent V atoms (see fig. 1). These include two central VO octahedra (V) and four each peripheral tetragonal-pyramidal VO groups (V and V). There are seven unique groups of oxygen atoms (labeled A through G). Two of these (A) bridge to six V centers, four (B) bridge three V centers, fourteen of these (C, D and E) span edges between pairs of V centers, and eight (F and G) are peripheral.
Sodium decavanadate | Structure
Any salt that melts without decomposing or vaporizing usually yields an ionic liquid. Sodium chloride (NaCl), for example, melts at into a liquid that consists largely of sodium cations () and chloride anions (). Conversely, when an ionic liquid is cooled, it often forms an ionic solid—which may be either crystalline or glassy. The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids. For that reason, common salts tend to melt at higher temperatures than other solid molecules. Some salts are liquid at or below room temperature. Examples include compounds based on the 1-ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIM dicyanamide, ()()·, that melts at ; and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below .
Ionic liquid | Introduction
The difficulty in determining the exact structure of Nafion stems from inconsistent solubility and crystalline structure among its various derivatives. Advanced morphological models have included a core-shell model where the ion-rich core is surrounded by an ion poor shell, a rod model where the sulfonic groups arrange into crystal-like rods, and a sandwich model where the polymer forms two layers whose sulfonic groups attract across an aqueous layer where transport occurs. Consistency between the models include a network of ionic clusters; the models differ in the cluster geometry and distribution. Although no model has yet been determined fully correct, some scientists have demonstrated that as the membrane hydrates, Nafion's morphology transforms from the Cluster-Channel model to a rod-like model.
Nafion | Structure/morphology
The anhydrous solid has the triclinic crystal structure, with space group (a = 5.8007(8) Å, b = 11.5811(15) Å, c = 23.157(3) Å, α = 89.709(10)°, β = 88.915(11)°, γ = 89.004(11)°, V = 1555.1(4) Å, Z = 8, Dx = 2.615 g·cm, μ(Mo‐Kα) = 7.94 cm). The anions are arranged in layers parallel to the (100) plane, with the sodium cations distributed in 24 distinct crystallographic positions, coordinated by 4 to 6 near oxygen atoms. Some of the 4-coordinated sodium atoms can be interpreted as parallel columns of edge-sharing tetrahedra. The columnar arrangement forms tunnels that house the remaining sodium cations.
Sodium pyrosilicate | Structure
The journal is abstracted and indexed in: According to the "Journal Citation Reports", the journal has a 2013 impact factor of 2.112.
Solid State Ionics | Abstracting and indexing
The charged components that make up ionic solids cannot exist in the high-density sea of delocalized electrons characteristic of strong metallic bonding. Some molecular salts, however, feature both ionic bonding among molecules and substantial one-dimensional conductivity, indicating a degree of metallic bonding among structural components along the axis of conductivity. Examples include tetrathiafulvalene salts.
Bonding in solids | Metallic to ionic
A thorough analysis of the bonding geometry was recently carried out for pyrite-type compounds, where monovalent chalcogen ions reside on "C" lattice sites. It was found that chalcogen ions have to be modeled by ellipsoidal charge distributions with different radii along the symmetry axis and perpendicular to it.
Ionic radius | Non-spherical ions
Many inorganic compounds are ionic compounds, consisting of cations and anions joined by ionic bonding. Examples of salts (which are ionic compounds) are magnesium chloride MgCl, which consists of magnesium cations Mg and chloride anions Cl; or sodium oxide NaO, which consists of sodium cations Na and oxide anions O. In any salt, the proportions of the ions are such that the electric charges cancel out, so that the bulk compound is electrically neutral. The ions are described by their oxidation state and their ease of formation can be inferred from the ionization potential (for cations) or from the electron affinity (anions) of the parent elements.
Inorganic chemistry | Key concepts
Sodium fluoride is an inorganic ionic compound, dissolving in water to give separated Na and F ions. Like sodium chloride, it crystallizes in a cubic motif where both Na and F occupy octahedral coordination sites; its lattice spacing, approximately 462 pm, is somewhat smaller than that of sodium chloride.
Sodium fluoride | Chemical structure
Most ionic compounds are very brittle. Once they reach the limit of their strength, they cannot deform malleably, because the strict alignment of positive and negative ions must be maintained. Instead the material undergoes fracture via cleavage. As the temperature is elevated (usually close to the melting point) a ductile–brittle transition occurs, and plastic flow becomes possible by the motion of dislocations.
Ionic compound | Brittleness
In 1929, Pauling published five rules which help to predict and explain crystal structures of ionic compounds. These rules concern (1) the ratio of cation radius to anion radius, (2) the electrostatic bond strength, (3) the sharing of polyhedron corners, edges and faces, (4) crystals containing different cations, and (5) the rule of parsimony.
Linus Pauling | Ionic crystal structures
NaCl, for example), temperature, and the common ion effect. One can calculate the amount of AgCl that will dissolve in 1 liter of water, some algebra is required.
Solubility | Solubility of ionic compounds in water
The circumstances under which a compound will have ionic or covalent character can typically be understood using Fajans' rules, which use only charges and the sizes of each ion. According to these rules, compounds with the most ionic character will have large positive ions with a low charge, bonded to a small negative ion with a high charge. More generally HSAB theory can be applied, whereby the compounds with the most ionic character are those consisting of hard acids and hard bases: small, highly charged ions with a high difference in electronegativities between the anion and cation. This difference in electronegativities means that the charge separation, and resulting dipole moment, is maintained even when the ions are in contact (the excess electrons on the anions are not transferred or polarized to neutralize the cations).
Ionic compound | Bonding
The net ionic equation of the proceeding reactions is:
Chemical equation | Ionic equations
Ions typically pack into extremely regular crystalline structures, in an arrangement that minimizes the lattice energy (maximizing attractions and minimizing repulsions). The lattice energy is the summation of the interaction of all sites with all other sites. For unpolarizable spherical ions only the charges and distances are required to determine the electrostatic interaction energy. For any particular ideal crystal structure, all distances are geometrically related to the smallest internuclear distance. So for each possible crystal structure, the total electrostatic energy can be related to the electrostatic energy of unit charges at the nearest neighbour distance by a multiplicative constant called the Madelung constant that can be efficiently computed using an Ewald sum.
Ionic compound | Structure
CuNe is predicted to have a square pyramid shape. CuNe has a seriously distorted octahedral shape. CuNe has an icosahedral shape. Anything beyond that is less stable, with extra neon atoms having to make an extra shell of atoms around an icosahedral core.
Neon compounds | Ionic clusters
If there are multiple different cations and/or anions, multiplicative prefixes ("di-", "tri-", "tetra-", ...) are often required to indicate the relative compositions, and cations then anions are listed in alphabetical order. For example, KMgCl is named magnesium potassium trichloride to distinguish it from KMgCl, magnesium dipotassium tetrachloride (note that in both the empirical formula and the written name, the cations appear in alphabetical order, but the order varies between them because the symbol for potassium is K). When one of the ions already has a multiplicative prefix within its name, the alternate multiplicative prefixes ("bis-", "tris-", "tetrakis-", ...) are used.
Ionic compound | Nomenclature
In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923, are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table: Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (AlI) (with a high positive charge (+3) and a large anion) is covalent. Polarization will be increased by:
Fajans' rules | Introduction
On the basis of the fluorides, one would say that Ag is larger than Na, but on the basis of the chlorides and bromides the opposite appears to be true. This is because the greater covalent character of the bonds in AgCl and AgBr reduces the bond length and hence the apparent ionic radius of Ag, an effect which is not present in the halides of the more electropositive sodium, nor in silver fluoride in which the fluoride ion is relatively unpolarizable.
Ionic radius | Trends
The space group of the rock-salt (NaCl) structure is called Fmm (in Hermann–Mauguin notation), or "225" (in the International Tables for Crystallography). The Strukturbericht designation is "B1". In the rock-salt or sodium chloride (halite) structure, each of the two atom types forms a separate face-centered cubic lattice, with the two lattices interpenetrating so as to form a 3D checkerboard pattern. Alternately, one could view this structure as a face-centered cubic structure with secondary atoms in its octahedral holes. Examples of compounds with this structure include sodium chloride itself, along with almost all other alkali halides, and "many divalent metal oxides, sulfides, selenides, and tellurides".
Cubic crystal system | Rock-salt structure
Twinning at a microscopic scale simulates a much larger monoclinic C centered lattice (V′ = 6220 Å, Z = 32).
Sodium pyrosilicate | Structure
Crystal growth, structure determination by X-ray and neutron diffraction, and thermal analysis are also important aspects of Dr. Haile's research. The group showed for example, that a broad range of proton containing solids undergo a monoclinic to cubic transition that is accompanied by an increase in conductivity of several orders of magnitude. In another example, her group demonstrated that Ba0.5Sr0.5Co0.8Fe0.2O3-d has exceptional activity as a cathode for ceria-based solid oxide fuel cells.
Sossina M. Haile | Solid-state ionics
The compound crystallizes in a hexagonal motif, often called the sodium arsenide structure. Like KP, solid NaP features pentacoordinate P centers.
Sodium phosphide | Structure and Properties
The caesium chloride structure adopts a primitive cubic lattice with a two-atom basis, where both atoms have eightfold coordination. The chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the center of the cubes. This structure is shared with CsBr and CsI and many binary metallic alloys. In contrast, the other alkaline halides have the sodium chloride (rocksalt) structure. When both ions are similar in size (Cs ionic radius 174 pm for this coordination number, Cl 181 pm) the CsCl structure is adopted, when they are different (Na ionic radius 102 pm, Cl 181 pm) the sodium chloride structure is adopted.
Caesium chloride | Crystal structure
Examination of the crystallographic data shows the presence of polyatomic units consisting of boron and nitrogen. These units have structures similar to those of isoelectronic anions, which have π-bonded structures. The bonding in some of these compounds is ionic in character, such as Ca[BN], other compounds have metallic characteristics, where the bonding has been described in terms of π-bonded anions with extra electrons in anti-bonding orbitals that not only cause a lengthening of the B–N bonds but also form part of the conduction band of the solid. The simplest ion BN is comparable to the ion, but attempts to prepare the compound CaBN analogous to CaC calcium carbide failed.
Nitridoborate | Structures and bonding
This process occurs widely in nature, and is the means of formation of the evaporite minerals. Another method of recovering the compound from solution involves saturating a solution at high temperature and then reducing the solubility by reducing the temperature until the solution is supersaturated and the solid compound nucleates. Insoluble ionic compounds can be precipitated by mixing two solutions, one with the cation and one with the anion in it. Because all solutions are electrically neutral, the two solutions mixed must also contain counterions of the opposite charges. To ensure that these do not contaminate the precipitated ionic compound, it is important to ensure they do not also precipitate.
Ionic compound | Formation
For example, Ba(BrF) is named barium bis(tetrafluoridobromate). Compounds containing one or more elements which can exist in a variety of charge/oxidation states will have a stoichiometry that depends on which oxidation states are present, to ensure overall neutrality. This can be indicated in the name by specifying either the oxidation state of the elements present, or the charge on the ions. Because of the risk of ambiguity in allocating oxidation states, IUPAC prefers direct indication of the ionic charge numbers. These are written as an arabic integer followed by the sign (... , 2−, 1−, 1+, 2+, ...) in parentheses directly after the name of the cation (without a space separating them).
Ionic compound | Nomenclature
Different compounds that obey the octet rule (sp-elements) and hydrogen can be placed on the triangle. Unfortunately, d-elements cannot be analysed using van Arkel-Ketelaar triangle, as their electronegativity is so high that it is taken as a constant. Using electronegativity - two compound average electronegativity on x-axis and electronegativity difference on y-axis, we can rate the dominant bond between the compounds. Example is here On the right side (from ionic to covalent) should be compounds with varying difference in electronegativity. The compounds with equal electronegativity, such as Cl (chlorine) are placed in the covalent corner , while the ionic corner has compounds with large electronegativity difference, such as NaCl (table salt).
Van Arkel–Ketelaar triangle | Usage
As a more complex example, the ionic strength of a mixed solution 0.050 M in NaSO and 0.020 M in KCl is:
Ionic strength | Calculation example
In the solid state it has the YCl layer structure.
Holmium(III) chloride | Structure
BASE is used as a membrane in several types of molten salt electrochemical cell.
Ionic conductivity (solid state) | Introduction
The lattice energy of an ionic compound depends upon charges of the ions that comprise the solid. More subtly, the relative and absolute sizes of the ions influence "ΔH".
Lattice energy | Theoretical treatments
The formula of the nitrite ion is .
Lewis structure | Example
This structure is shared with CsBr and CsI, and many other compounds that do not contain Cs. In contrast, most other alkaline halides have the sodium chloride (NaCl) structure. The CsCl structure is preferred because Cs has an ionic radius of 174 pm and 181 pm.
Caesium | Halides
The crystal structure of ScS is closely related to that of sodium chloride, in that it is based on a cubic close packed array of anions. Whereas NaCl has all the octahedral interstices in the anion lattice occupied by cations, ScS has one third of them vacant. The vacancies are ordered, but in a very complicated pattern, leading to a large, orthorhombic unit cell belonging to the space group Fddd.
Scandium(III) sulfide | Structure
Although ionic compounds contain charged atoms or clusters, these materials do not typically conduct electricity to any significant extent when the substance is solid. In order to conduct, the charged particles must be mobile rather than stationary in a crystal lattice. This is achieved to some degree at high temperatures when the defect concentration increases the ionic mobility and solid state ionic conductivity is observed. When the ionic compounds are dissolved in a liquid or are melted into a liquid, they can conduct electricity because the ions become completely mobile. This conductivity gain upon dissolving or melting is sometimes used as a defining characteristic of ionic compounds.
Ionic compound | Electrical conductivity
In the soft-sphere model, formula_6 has a value between 1 and 2. For example, for crystals of group 1 halides with the sodium chloride structure, a value of 1.6667 gives good agreement with experiment. Some soft-sphere ionic radii are in the table. These radii are larger than the crystal radii given above (Li, 90 pm; Cl, 167 pm). Inter-ionic separations calculated with these radii give remarkably good agreement with experimental values. Some data are given in the table. Curiously, no theoretical justification for the equation containing formula_6 has been given.
Ionic radius | Soft-sphere model
If the solvation energy exceeds the lattice energy, the negative net enthalpy change of solution provides a thermodynamic drive to remove ions from their positions in the crystal and dissolve in the liquid. In addition, the entropy change of solution is usually positive for most solid solutes like ionic compounds, which means that their solubility increases when the temperature increases. There are some unusual ionic compounds such as cerium(III) sulfate, where this entropy change is negative, due to extra order induced in the water upon solution, and the solubility decreases with temperature.
Ionic compound | Solubility
A large molecule with an ionized group is technically an ion, but its behavior may be largely the result of non-ionic interactions. For example, sodium stearate (the main constituent of traditional soaps) consists entirely of ions, yet it is a soft material quite unlike a typical ionic solid. There is a continuum between ionic solids and molecular solids with little ionic character in their bonding.
Bonding in solids | Molecular to ionic
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Cyclotetradecane | References
Ionic compounds containing hydrogen ions (H) are classified as acids, and those containing electropositive cations and basic anions ions hydroxide (OH) or oxide (O) are classified as bases. Other ionic compounds are known as salts and can be formed by acid–base reactions. If the compound is the result of a reaction between a strong acid and a weak base, the result is an acidic salt. If it is the result of a reaction between a strong base and a weak acid, the result is a basic salt. If it is the result of a reaction between a strong acid and a strong base, the result is a neutral salt.
Ionic compound | Acidity/basicity
The distance between two ions in an ionic crystal can be determined by X-ray crystallography, which gives the lengths of the sides of the unit cell of a crystal. For example, the length of each edge of the unit cell of sodium chloride is found to be 564.02 pm. Each edge of the unit cell of sodium chloride may be considered to have the atoms arranged as Na∙∙∙Cl∙∙∙Na, so the edge is twice the Na-Cl separation. Therefore, the distance between the Na and Cl ions is half of 564.02 pm, which is 282.01 pm. However, although X-ray crystallography gives the distance between ions, it doesn't indicate where the boundary is between those ions, so it doesn't directly give ionic radii.
Ionic radius | Determination
This often leads to much higher coordination numbers. In NaCl, each ion has 6 bonds and all bond angles are 90°. In CsCl the coordination number is 8. By comparison carbon typically has a maximum of four bonds. Purely ionic bonding cannot exist, as the proximity of the entities involved in the bonding allows some degree of sharing electron density between them. Therefore, all ionic bonding has some covalent character. Thus, bonding is considered ionic where the ionic character is greater than the covalent character. The larger the difference in electronegativity between the two types of atoms involved in the bonding, the more ionic (polar) it is.
Ionic bonding | Comparison with covalent bonding
The polysulfide anions form chains with S---S bond distances around 2 Å in length. The chains adopt skewed conformations. In the solid state, these salts are dense solids with strong association of the sodium cations with the anionic termini of the chains.
Sodium polysulfide | Structure
Ions in ionic compounds are primarily held together by the electrostatic forces between the charge distribution of these bodies, and in particular the ionic bond resulting from the long-ranged Coulomb attraction between the net negative charge of the anions and net positive charge of the cations. There is also a small additional attractive force from van der Waals interactions which contributes only around 1–2% of the cohesive energy for small ions. When a pair of ions comes close enough for their outer electron shells (most simple ions have closed shells) to overlap, a short-ranged repulsive force occurs, due to the Pauli exclusion principle.
Ionic compound | Bonding
This solid solution can be formed over the whole composition range at room temperature. The highest ionic conductivity are achieved at compositions of LiSiPO and LiSiP, with conductivity on the order of 10 S/cm. This results from the substitution of some Si for P in the lattice, resulting in the addition of interstitial Li-ions which diffuse much more easily. The ionic conductivity is further improved with the doping of Cl to replace O in the lattice. The compositions LiSiPCl and LiGePCl achieved ionic conductivities of 1.03 * 10 S/cm and 3.7*10 S/cm respectively. This Is theorized to be the due to the widening of the “bottlenecks” between interstitial points due to the Cl ions smaller size, and the weakening of the ionic bonding Li ions experienced due to chlorine's lower electronegativity.
LISICON | LISICON-Like Materials
Namely, the fully reduced compound can be interpreted as ··. As the compound is oxidized, sodium cations exit the structure and the cobalt formally approaches the state. For "x" above 0.5, the sodium ions adopt many different arrangements in which Na ions occupy two inequivalent Wyckoff sites, 2b and 2d, of the space group P6/mmc. In galvanostatic experiments, the arrangements transition at specific values of "x" as the sodium content is electrolytically varied. The diffusion rate of the ions, plotted as a function of "x", shows sharp dips (from about 10 to 10 cm/s at ambient temperature) at values of "x" that correspond to particular regular arrangements, namely 1/3, 1/2, and 5/7.
Sodium cobalt oxide | Structure
Similar charge-separation effects were observed for grain boundaries in crystalline ionic conductors.
Solid state ionics | Nanostructures
Its crystal structure has a tetragonal tI space group, which is a distorted version of the NHCl crystal structure; the unit cell has approximate dimensions 634×634×462 pm. The hydrogen bonding in the system causes the cations to orient such that the hydrogen atoms point toward the anions.
Phosphonium iodide | Structure
In some unusual ionic compounds: fast ion conductors, and ionic glasses, one or more of the ionic components has a significant mobility, allowing conductivity even while the material as a whole remains solid. This is often highly temperature dependent, and may be the result of either a phase change or a high defect concentration. These materials are used in all solid-state supercapacitors, batteries, and fuel cells, and in various kinds of chemical sensors.
Ionic compound | Electrical conductivity
NASICON is an acronym for sodium (Na) Super Ionic CONductor, which usually refers to a family of solids with the chemical formula NaZrSiPO, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. NASICON compounds have high ionic conductivities, on the order of 10 S/cm, which rival those of liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.
NASICON | Introduction
In the solid state the coordination sphere for niobium is a distorted octahedron. The Nb–O bonds and Nb–Cl bonds are unequal. This structure can be described as planar NbCl core connected by O–Nb–O bridges. In this way, the compound is best described as a polymer, consisting of a double stranded chain. In the gas phase above 320 °C the Raman spectrum is consistent with a pyramidal monomer containing a niobium–oxygen double bond.
Niobium oxychloride | Structure
For a solid crystalline ionic compound the enthalpy change in forming the solid from gaseous ions is termed the lattice energy. The experimental value for the lattice energy can be determined using the Born–Haber cycle. It can also be calculated (predicted) using the Born–Landé equation as the sum of the electrostatic potential energy, calculated by summing interactions between cations and anions, and a short-range repulsive potential energy term. The electrostatic potential can be expressed in terms of the interionic separation and a constant (Madelung constant) that takes account of the geometry of the crystal. The further away from the nucleus the weaker the shield.
Ionic bonding | Strength of the bonding
The hydroxide ion appears to rotate freely in crystals of the heavier alkali metal hydroxides at higher temperatures so as to present itself as a spherical ion, with an effective ionic radius of about 153 pm. Thus, the high-temperature forms of KOH and NaOH have the sodium chloride structure, which gradually freezes in a monoclinically distorted sodium chloride structure at temperatures below about 300 °C. The OH groups still rotate even at room temperature around their symmetry axes and, therefore, cannot be detected by X-ray diffraction. The room-temperature form of NaOH has the thallium iodide structure. LiOH, however, has a layered structure, made up of tetrahedral Li(OH) and (OH)Li units.
Hydroxide | Structural chemistry
At room temperature and atmospheric pressure, CsH has the same structure as NaCl.
Caesium hydride | Crystal structure
The nature of chemical interactions and their description is one of the most fundamental problems in chemistry. The concepts of covalent and ionic bonds which emerged in the beginning of the 20th century specify the profound differences between their electronic structures. These differences, in turn, lead to dramatically different behavior of covalent and ionic compounds both in the solution and solid phase. In the solid phase, ionic compounds, e.g. salts, are prone to formation of crystal lattices; in polar solvents, they dissociate into ions surrounded by solvate shells, thus rendering the solution highly ionic conductive. In contrast to covalent bonds, ionic interactions demonstrate flexible, dynamic behavior, which allows tuning ionic compounds to obtain desired properties.
Ioliomics | Fundamental nature
The crystal structure of NASICON compounds was characterized in 1968. It is a covalent network consisting of ZrO octahedra and PO/SiO tetrahedra that share common corners. Sodium ions are located at two types of interstitial positions. They move among those sites through bottlenecks, whose size, and thus the NASICON electrical conductivity, depends on the NASICON composition, on the site occupancy, and on the oxygen content in the surrounding atmosphere. The conductivity decreases for x < 2 or when all Si is substituted for P in the crystal lattice (and vice versa); it can be increased by adding a rare-earth compound to NASICON, such as yttria.
NASICON | Properties
• NaTl consists of a polyanion (—Tl—) with a covalent diamond structure. Na ions are located between the anions. Concept: Tl ~ C. • NaSi: the polyanion is tetrahedral (Si), similar to P. Concept: Si ~ P. • NaTl: the polyanion is tetrahedral (Tl), similar to P. Concept: Tl ~ P.
Zintl phase | Examples
NaCl is one of the simplest compounds and was thought to be well-understood, and yet, unexpected complexities related to it were uncovered at high pressure and in low-dimensional states. Here, exotic hexagonal NaCl thin films on the (110) diamond surface were crystallized in the experiment following a theoretical prediction based on ab initio evolutionary algorithm USPEX.
Two-dimensional materials | 2D Sodium Chloride (NaCl)
The use of the term hydrate is still acceptable e.g. NaSO·10HO, sodium sulfate decahydrate. The recommended method would be to name it sodium sulfate—water(1/10). Similarly other examples of lattice compounds are:
IUPAC nomenclature of inorganic chemistry 2005 | Naming of hydrates and similar lattice compounds
The numbers beside the compounds show the electronegativity difference between the elements based on the Pauling scale. The higher the number is, the more ionic the solid is.
Alkali metal halide | Properties
This reaction produces metal cations and nonmetal anions, which are attracted to each other to form a "salt".
Ion | Ionic bonding
In the solid state, it crystallises in the UCl motif. The Eu centres are nine-coordinate.
Europium(III) chloride | Structure
For typical ionic solids, the cations are smaller than the anions, and each cation is surrounded by coordinated anions which form a polyhedron. The sum of the ionic radii determines the cation-anion distance, while the cation-anion radius ratio formula_1 (or formula_2) determines the coordination number (C.N.) of the cation, as well as the shape of the coordinated polyhedron of anions. For the coordination numbers and corresponding polyhedra in the table below, Pauling mathematically derived the "minimum" radius ratio for which the cation is in contact with the given number of anions (considering the ions as rigid spheres). If the cation is smaller, it will not be in contact with the anions which results in instability leading to a lower coordination number.
Pauling's rules | First rule: the radius ratio rule
The model of ionic atmosphere is less adequate for concentrated ionic solutions near saturation. These solutions as well as molten salts or ionic liquids have a structure similar to the crystalline lattice where water molecules are located between ions.
Ionic atmosphere | Limits to the model
Compounds consisting of oppositely charged ions are known as ionic compounds, or salts.
Chemical substance | Chemical compounds
These other types of substances, such as ionic compounds and network solids, are organized in such a way as to lack the existence of identifiable molecules "per se". Instead, these substances are discussed in terms of formula units or unit cells as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as table salt), solids like carbon and diamond, metals, and familiar silica and silicate minerals such as quartz and granite.
Chemistry | Molecule
X-ray diffraction shows that in NaCN, for example, the bonds between sodium cations (Na) and the cyanide anions (CN) are "ionic", with no sodium ion associated with any particular cyanide. However, the bonds between C and N atoms in cyanide are of the "covalent" type, so that each carbon is strongly bound to "just one" nitrogen, to which it is physically much closer than it is to other carbons or nitrogens in a sodium cyanide crystal. When such crystals are melted into liquids, the ionic bonds are broken first because they are non-directional and allow the charged species to move freely.
Chemical bond | Ionic bond
NiCl adopts the CdCl structure. In this motif, each Ni center is coordinated to six Cl centers, and each chloride is bonded to three Ni(II) centers. In NiCl the Ni-Cl bonds have "ionic character". Yellow NiBr and black NiI adopt similar structures, but with a different packing of the halides, adopting the CdI motif. In contrast, NiCl·6HO consists of separated "trans"-[NiCl(HO)] molecules linked more weakly to adjacent water molecules. Only four of the six water molecules in the formula is bound to the nickel, and the remaining two are water of crystallization. Cobalt(II) chloride hexahydrate has a similar structure. The hexahydrate occurs in nature as the very rare mineral nickelbischofite.
Nickel(II) chloride | Structure of NiCl and its hydrates
Such behavior, called Faraday transition, is observed in the cation conductors NaS and LiSiO and anion conductors PbF, CaF, SrF, SrCl and LaF. Later in 1891, Johann Wilhelm Hittorf reported on the ion transport numbers in electrochemical cells, and in the early 20th century those numbers were determined for solid electrolytes.
Solid state ionics | Foundations
An iron ball would thus weigh about as much as three aluminum balls. Metals are typically malleable and ductile, deforming under stress without cleaving. The nondirectional nature of metallic bonding is thought to contribute significantly to the ductility of most metallic solids. In contrast, in an ionic compound like table salt, when the planes of an ionic bond slide past one another, the resultant change in location shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal. Such a shift is not observed in a covalently bonded crystal, such as a diamond, where fracture and crystal fragmentation occurs.
Metal | Form and structure
For ions, the charge on a particular atom may be denoted with a right-hand superscript. For example, Na, or Cu. The total charge on a charged molecule or a polyatomic ion may also be shown in this way. For example: HO or SO. Note that + and - are used in place of +1 and -1, respectively. For more complex ions, brackets [ ] are often used to enclose the ionic formula, as in [BH], which is found in compounds such as Cs[BH]. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in [Co(NH)]Cl. Here, (NH) indicates that the ion contains six NH groups bonded to cobalt, and [ ] encloses the entire formula of the ion with charge +3.
Chemical formula | Ions in condensed formulae
Repeating units will help to show how easy it is and clear it is to represent molecules through balls that represent atoms. The binary compounds sodium chloride (NaCl) and caesium chloride (CsCl) have cubic structures but have different space groups. This can be rationalised in terms of close packing of spheres of different sizes. For example, NaCl can be described as close-packed chloride ions (in a face-centered cubic lattice) with sodium ions in the octahedral holes. After the development of X-ray crystallography as a tool for determining crystal structures, many laboratories built models based on spheres. With the development of plastic or polystyrene balls it is now easy to create such models.
Molecular model | Models based on spheres
Solid is dimeric, consisting of , similar to that of AlBr. The structure of monomeric and dimeric forms have been characterized in the gas phase. The monomer, is trigonal planar with a bond length of 2.448(6) Å, and the bridged dimer, at 430 K is a similar to AlCl and AlBr with bond lengths of 2.456(6) Å (terminal) and 2.670(8) Å (bridging). The dimer is described as floppy with an equilibrium geometry of D.
Aluminium iodide | Structure
The attraction between cationic and anionic sites is a noncovalent, or intermolecular interaction which is usually referred to as ion pairing or salt bridge. It is essentially due to electrostatic forces, although in aqueous medium the association is driven by entropy and often even endothermic. Most salts form crystals with characteristic distances between the ions; in contrast to many other noncovalent interactions salt bridges are not directional and show in the solid state usually contact determined only by the van der Waals radii of the ions. Inorganic as well as organic ions display in water at moderate ionic strength I similar salt bridge as association ΔG values around 5 to 6 kJ/mol for a 1:1 combination of anion and cation, almost independent of the nature (size, polarizability etc) of the ions.
Intermolecular force | Ionic bonding
As with other types of atomic radius, ionic radii increase on descending a group. Ionic size (for the same ion) also increases with increasing coordination number, and an ion in a high-spin state will be larger than the same ion in a low-spin state. In general, ionic radius decreases with increasing positive charge and increases with increasing negative charge. An "anomalous" ionic radius in a crystal is often a sign of significant covalent character in the bonding. No bond is "completely" ionic, and some supposedly "ionic" compounds, especially of the transition metals, are particularly covalent in character. This is illustrated by the unit cell parameters for sodium and silver halides in the table.
Ionic radius | Trends
Alkali metals form many well-known stable salts. Sodium chloride (common table salt), NaCl, illustrates the usual role of an alkali metal such as sodium. In the empirical formula for this ionic compound, the positively charged sodium ion is balanced by a negatively charged chloride ion. The traditional explanation for stable Na is that the loss of one electron from elemental sodium to produce a cation with charge of +1 produces a stable closed-shell electron configuration.
Alkalide | "Normal" alkali metal compounds