First 414 words of the document:
The properties of the elements organised in the periodic
table show trends which repeat themselves in each period.
These trends are known as periodic trends and the study of these trends is known as periodicity.
Trends in period III
Atomic radius decreases across the period because nuclear charge increases without any
significant increase in shielding, so the outer electrons are drawn closer to the nucleus since
they are attracted more strongly.
Melting and boiling points
increase across the period from Na to Al because there is
increasing nuclear charge as the atomic radius decreases, plus there is an increased number
of electrons in the delocalised cloud, and therefore increased electrostatic
attraction so more heat energy is required to overcome this.
Si is a macromolecule so has a high melting and boiling point due to the
many strong covalent bonds which need to be broken.
P, S and Cl are all molecular substances and the melting point of each
depends on the strength of van der Waals' forces, which is in turn
determined by the size of the molecule (biggest and most polarisable will have a higher
melting point). Each of these elements has a low melting point because van der Waals' is
weak and easily overcome (forces are never broken).
Ionisation energy generally increases across the period due to an
increase in nuclear charge but with no significant increase in
shielding, meaning the outer electrons are attracted more strongly.
There is a slight decrease in ionisation energy from Mg to Al because
the outer electron in Mg is in a 3s orbital however in Al it has moved
to a 3p orbital, meaning it is better shielded and therefore easier to
remove. There is another decrease from P to S because the outermost electron in S is paired,
leading to some repulsion.
Reactions of period III elements
Of the period 3 elements, only sodium, magnesium and chlorine react with water.
The products obtained when chlorine reacts with water depend on the conditions used. Under
normal laboratory conditions, a very pale green solution is formed, showing the presence of
2O HCl + HClO
This is a disproportionation reaction in which one species, in this case chlorine, is simultaneously
both oxidised and reduced.
Other pages in this set
Here's a taster:
If universal indicator is added to a solution of chlorine water, it first turns red since both the
reaction products are acids. The red colour then disappears and a colourless solutions is left
because chloric(I) acid is a very effective bleach.
If chlorine is bubbled through water in the presence of bright sunlight, or a green solution of
chlorine water is left in bright sunlight, a colourless gas is produced (oxygen) and the green colour
(due to chlorine) fades.…read more
Here's a taster:
Acid-base properties of the oxides of period III elements
Physical properties, structure and bonding
Ionic lattices are held together by strong electrostatic forces between ions, so that these lattices
have high melting points. Macromolecular solids also have high melting points, because the atoms
are held together by strong covalent bonds. Molecular solids involve weak intermolecular
dipole-dipole or van der Waals' forces and have low melting points.…read more