Why does carbon differ from the rest of the members of its group? Explain.

(A) Like the first member of other groups,carbon also differs from the rest of the members of its group. This is due to its smaller size,higher electronegativity,higher ionization enthalpy,and the unavailability of $d$-orbitals.
In carbon,only $s$ and $p$-orbitals are available for bonding,and therefore,it can accommodate only four pairs of electrons around it.
This limits the maximum covalence to four,whereas other members can expand their covalence due to the presence of $d$-orbitals.
Carbon also has a unique ability to form $p\pi-p\pi$ multiple bonds with itself and with other atoms of small size and high electronegativity. Examples of multiple bonding include $C=C$,$C \equiv C$,$C=O$,$C=S$,and $C \equiv N$.
Heavier elements do not form $p\pi-p\pi$ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.
Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called catenation.
This is because $C-C$ bonds are very strong. Down the group,the size increases and electronegativity decreases,and thereby,the tendency to show catenation decreases. This can be clearly seen from bond enthalpy values.
The order of catenation is $C \gg Si > Ge \approx Sn$. Lead does not show catenation. Due to the property of catenation and $p\pi-p\pi$ bond formation,carbon is able to show allotropic forms.

Bond	$C-C$	$Si-Si$	$Ge-Ge$	$Sn-Sn$
Bond enthalpy $(kJ \ mol^{-1})$	$348$	$297$	$260$	$240$