Anomalous Behaviour Of Carbon

Carbon, the first member of group 14 ,shows an anomalous behaviour i.e. differ from rest of the members of its family.

The main reason for this difference are :

1) exceptionally small atomic size

b) higher electronegativity

c)higher ionisation enthalpy

d) absence of d orbital in the valence shell

The main point of differences are:

1) Carbon in form of diamond is very hard as compared to other members of group 14.

2) It has higher melting point and boiling point than other members of the group14.

3) Carbon has only 4 valence orbitals ,therefore ,at the maximum, it can accommodate 4 pairs of electrons around it. The maximum covalency of carbon is 4. The other elements of this group due to the presence of d- orbital can expand their covalency to 6.

4) Due to small size and high electronegativity , carbon has a strong tendency to form pπ- pπ multiple bonds either with itself or with the other items such as oxygen, nitrogen and sulphur.

The remaining elements do not form pπ- pπ bonds because their atomic orbitals are too large and diffused to have effective overlapping. Instead due to the presence of d-orbitals, they have a tendency to form dπ-pπ bonds. Even this tendency to form dπ-pπ bonds decreases down the group from Si to Pb as the size of the d- orbitals increases.

5) Catenation : Carbon has the property of catenation which may be defined as the ability of like atoms to link with one another through covalent bonds. This is due to smaller size and higher electronegativity of carbon atoms and unique strength of carbon carbon bonds. The property of catenation mainly depends upon the strength of element – element bond. Since the bond energy of C-C bond is very large,  carbon forms long straight or branched chains or rings of different sizes and shapes. However ,as we move down the group, the element- element bond energies decrease rapidly ,and therefore, the tendency for catenation decreases in the order

C >> Si > Ge≅ Sn >> Pb

6) Due to the property of catenation and pπ- pπ bond formation , carbon show allotropic forms.