Describe briefly allotropism in p- block elements with specific reference to carbon.

1. General characteristics of p-Block Elements:

(i) p-Block of the periodic table is the largest and unique block as it has all types of elements i.e., metals, non-metals and metalloids.

(ii) There are six groups of p-block elements in the periodic table numbering from $13 \mathrm{to} 18$. The general valence shell electronic configuration of the elements is $\mathrm{n}{\mathrm{s}}^2\mathrm{n}{\mathrm{p}}^{1-6}$ (except for He).

(iii) The elements belonging to a particular group show the group oxidation state; as well as other oxidation states differing from the total number of valence electrons by unit of two. The group oxidation state is the most stable for the lighter elements of the group; lower oxidation states become progressively more stable for the heavier elements. This is because of the inert pair effect.

(iv) The elements of the p-block have a strong ability to form π-bonds. This is attributable to the combined effect of (a) size and (b) availability of d orbitals. While the lighter elements form $\mathrm{p\pi } –\mathrm{p\pi }$ bonds, the heavier ones form $\mathrm{d\pi } – \mathrm{p\pi } \mathrm{or} \mathrm{d\pi } – \mathrm{d\pi }$ bonds. The absence of d-orbital in second period elements limits their maximum covalence to $4$ while heavier ones can exceed this limit.

2. Boron family:

(i) Boron is a typical non-metal and the other members are metals. Boron 2s22p1 has 3 valence electrons for covalent bond formation. The presence of empty vacant orbital in boron makes it a good electron acceptor and thus boron compounds behave as Lewis acids.

(ii) Boron forms covalent molecular compounds with dihydrogen known as boranes, the simplest of which is diborane, B2H6.

(iii) Diborane contains two bridging hydrogen atoms between two boron atoms; these bridge bonds are considered to be three-centre two-electron bonds.

(iv) The important compounds of boron with dioxygen are boric acid and borax.

(v) Boric acid, BOH3 is a weak monobasic acid; it acts as a Lewis acid by accepting electrons from hydroxyl ion.

(vi) Borax is a white crystalline solid of the formula Na2B4O5OH4.8H2O.

(vii) B3N3H6 (Borazine) is known as “inorganic benzene”.

(viii) The borax bead test gives characteristic colours of transition metals.

(ix) Aluminium is a metal. It exhibits $+3$ oxidation state in its compounds. Aluminium chloride exists as dimer up to $473 \mathrm{K}$ but above this temperature it becomes monomeric.

3. The Carbon family:

(i) The members of the carbon family mainly exhibit $+4$ and $+2$ oxidation states, compounds in $+4$ oxidation states are generally covalent in nature. The lower oxidation state of $+2$ becomes more favorable as we move down the group.

(ii) Carbon is a typical non-metal forming covalent bonds employing all its four valence electrons $\left(2{\mathrm{s}}^{2}2{\mathrm{p}}^2\right)$. It shows the property of catenation.

(iii) Three important allotropes of carbon are diamond, graphite and fullerene.

(iv) Carbon forms two important oxides: $\mathrm{CO}$ and ${\mathrm{CO}}_2$. Carbon monoxide is neutral whereas ${\mathrm{CO}}_2$ is acidic in nature. Carbon monoxide having lone pair of electrons on $\mathrm{C}$ forms metal carbonyls.

(v) Silica, silicates and silicones are important classes of compounds and find applications in industry and technology.