2 8.1 Molecular CompoundsCovalent Bond are atoms held together by sharing electrons.Molecule is a neutral group of atoms joined together by covalent bonds.Diatomic molecule is a molecule consisting of two atomsMolecular compound is a compound composed of molecules.Example: water has two covalent bonds, the smallest particle of water is called a water molecule, and is a molecular compound
3 8.1 Molecular CompoundsMolecular compounds tent to have relatively lower melting and boiling points that ionic compoundsMost are gases or liquids at room temperature, and most molecular compounds are composed of two or more nonmetals.Molecular formula is the chemical formula of a molecular compoundExample: Water’s formula is H2OA molecular formula showshow many atoms of each elementa molecule contains
4 8.2 The nature of Covalent Bonding In covalent bonds, electrons sharing usually occur so that atoms attain the electron configuration of noble gases.In covalent bonds elements usually acquire a total of eight electrons (an octet) by sharing electrons.Single covalent bond iswhen atoms are held togetherby sharing a pair of electrons
5 8.2 The nature of Covalent Bonding An electron dot structure can be used to represent the shared pair of electrons of the covalent bond by two dots.Structural formula represents covalent bonds by dashes and shows the arrangement of covalently bonded atoms.Unshared pair (or lone pair) is a pair of valence electrons that is not shared between the atoms
6 8.2 The nature of Covalent Bonding Single covalent bond exampleWater:The oxygen atom has two unshared pair of electrons and two single covalent bonds.
7 8.2 The nature of Covalent Bonding Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.Double covalent bond is a bond that involves two shared pairs of electronsTriple covalent bond is a bond that involves three shared pairs of electrons
8 Steps for Building Lewis Structures Decide on a central atom. Generally the LEAST electronegative atom is central atom. Hydrogen can NEVER be central atom because it can only form one bond.Add up the number of valance electrons for ALL elements in compoundForm a single bond between the central atom and each of the other atomsAdd lone pair to elements to complete the octet (remember Hydrogen only want 2 electrons)Check each element to make sure they have access to 8 electrons (H = 2). Ask are they happy?Add up the number of electrons in drawing [count dots + 2(# of lines)] and see if this number match the number of valance electrons in step 2If you have too many form double or triple bonds as neededIf you have too few than you made a mistake in your drawing.
9 Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons = 3. Form bonds.C 4 e- O 6 e- X 2 O’s = 12 e- Total: 16 valence electronsThis leaves 12 electrons (6 pair).4. Place lone pairs on outer atoms.5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.
10 Carbon Dioxide, CO2C 4 e- O 6 e- X 2 O’s = 12 e- Total: 16 valence electronsHow many are in the drawing?6. There are too many electrons in our drawing. We must form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and replaced with another bond.How many double bonds is Carbon making?How many double bonds is one Oxygen making?How many lone pairs are on Oxygen? Carbon?
11 Now You Try One! Draw Carbon tetrachloride Central atom is CarbonOne C = 4 e-, Four Cl = 4 (7e-)Total: 32 e-Form single bondsAdd Lone PairsCarbon has 4 single bond and is happy, each Chlorine has 1 single bond and 3 lone pairs and is happyCheck electrons used24 dots + 2 (4 lines) = 32 e-Structure is correct
12 Now You Try One! Draw Sulfur Dioxide Central atom is SulfurOne S = 6 e-, Two O = 2 (6 e-)Total: 18 e-Form single bondsAdd Lone PairsSulfur has 2single bond and 2 lone pairs and is happy, each Oxygen has 1 single bond and 3 lone pairs and is happyCheck electrons used16 dots + 2 (4 lines) = 20 e-Structure is NOT CorrectMake double bonds12 dots + 2 (3 lines) = 18structure is correct
13 Diatomic MoleculesThere are 7 elements that do not exist in nature as a single atom; they always appear as pairsWhen atoms turn into ions, this NO LONGER HAPPENS! They can form bonds as single atoms.Hydrogen H2Nitrogen N2Oxygen O2Fluorine F2Chlorine Cl2Bromine Br2Iodine I2Remember: BrINClHOFOr remember the 7 elements in the shape of a 7 on the periodic table
14 8.4 Polar Bonds and Molecules Nonpolar covalent bond (nonpolar bond) is when the bonding electrons are shared equally.ALL diatomic halogen molecules are nonpolarPolar covalent bond (polar bond) is when the bonding electrons are NOT shared equallyThe more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge.When polar molecules are placed between oppositely charges plates they tend to become oriented with respected to the positive and negative plates.
15 8.4 Polar Bonds and Molecules To determine if a bond is polar or nonpolar look at the difference in electronegativity valuesNonpolar covalent is electronegativity difference range of 0.0 to 0.5polar covalent is electronegativity difference range of 0.51 to 1.6Ionic bonds is electronegativity difference range above 1.7 to 4.0
17 Bond PolarityHCl is POLAR because it has a positive end and a negative end. (difference in electronegativity)Cl has a greater share in bonding electrons than does H.Cl has slight negative charge (-d) and H has slight positive charge (+ d)
18 Bond PolarityThis is why oil and water will not mix! Oil is nonpolar, and water is polar.The two will repel each other, and so you can not dissolve one in the other
20 Compare and contrast Ionic and Covalent bonds CharacteristicIonic BondsCovalent BondsReason for formingHow they formStrength of bondMelting/Boiling PointsPhase at room temperatureBecause atoms want to have full outer energy levelsTransferring ElectronsSharing ElectronsVery Strong BondWeak BondVery HighLowMost are liquids or gasesMost are solids
21 Lewis Structure Summary The elements MOST of the time follow the guide lines below. There are a few exceptionsCarbon will form 4 bondNitrogen and Phosphorous will form 3 bonds and will have 1 lone pairOxygen, sulfur and Selenium will form 2 bonds and have 2 lone pairsF, Cl, Br, and I will form 1 bond and have 3 lone pairs.Hydrogen will from 1 bondThe bonds can be any combination of single, double or triple bonds.