Group 16 Elements: The Oxygen Family -- General Trends and Classification of Oxides

Group 16, the oxygen family, comprises oxygen (O), sulphur (S), selenium (Se), tellurium (Te)

and polonium (Po), sharing the general outer configuration $ns^2np^4$ -- two electrons short of a

complete octet, which makes $-2$ the group's characteristic (and, for oxygen, essentially only

common) oxidation state, alongside the elemental state $0$. Sulphur, selenium and tellurium, with

their accessible $d$ orbitals, additionally show a much wider range including $+2$, $+4$ and

$+6$ (as in $\text{SO}_2$/$\text{SO}_3$ and their corresponding oxoacids), while oxygen -- again

showing the same first-member anomaly seen throughout the p-block -- is essentially restricted to

$-2$, since it lacks $d$ orbitals and is, apart from fluorine, the most electronegative element

of all; oxygen shows a positive oxidation state ($+1$ in $\text{O}_2\text{F}_2$, $+2$ in

$\text{OF}_2$) only in its compounds with fluorine, the one element more electronegative than

itself.

Across the group, atomic radius increases and ionisation enthalpy decreases on descending, in the

usual pattern (with the customary small anomaly between the second- and third-period members, O

and S). Electronegativity is highest for oxygen (second only to fluorine in the whole periodic

table) and falls steadily down the group; metallic character rises correspondingly, so that

polonium at the foot of the group is a genuine, if weakly, metallic and radioactive element.

A theme that runs throughout this entire chapter -- and indeed through inorganic chemistry

generally -- is the classification of oxides by their acid-base character, and Group 16 is

the natural place to state it explicitly. Acidic oxides are formed predominantly by

non-metals (and by metals in high oxidation states): they react with water to give an acid, or

with a base to give a salt and water, e.g. $\text{SO}_2$ (giving sulphurous acid,

$\text{H}_2\text{SO}_3$, with water) and $\text{CO}_2$ (giving carbonic acid). Basic oxides

are formed by metals, especially the more electropositive ones: they react with water to give a

base, or with an acid to give a salt and water, e.g. $\text{Na}_2\text{O}$ (giving NaOH) and

$\text{CaO}$. Amphoteric oxides, typically formed by metalloids or metals near the

metal/nonmetal borderline, react with both acids and bases, e.g. $\text{Al}_2\text{O}_3$ and

$\text{ZnO}$, each dissolving in strong acid to form the corresponding salt and in strong base to

form an aluminate/zincate complex. Neutral oxides, finally, show neither acidic nor basic …