Give reason for the following :
a. The first ioisation eathalpy of carbon carbon is greater than that foboron, whereas the reverse is true fore the second ionisation enthalpy .
b. Solid carbon dioxie is known as dry ice .
c. Why does not silicon form an analogue fo graphite ?

Explanation

a. Electronic configuration of carbon is ` 1s^2 , 2s^2 , 2s^2` , whereas that of boron is ` 1s^2 2s^2 2p^1` . The first ionisation enthalpy of carnon is grater than that of boron due to its smaller size and higher effective nuclear charge . The valence shell electons are nore tightly bound to the nucleus , hence , for its removal grater amount of enrgy is requreed .
The second ionisation enthalpy of carbon is less than that of boron , as the removal fo second electron in carbon has to be doen from `2p` orbital whereas in boron it has to gbe done from `2s` orbial . First , `2s` orbital , therfore , the electrons are more tightly bound . Also `2s^2` configuration in `C` being fully filled is more stable than `2p^(1)` in `B` Because of these two reasone , `IE_2` or `C lt IE_2` of B.
Altenrate explanation :

Therefore , `IE _21` of `C gt IE_1` of `B` and `IE_2` of `Clt IE_2` of `B`
b. Solid carbon dioxdie on veape on evaporation change directly to the gaseous state without changing to the liwuid state . It produces cooling diring evaporation and doen not wet the surface , therefore , solid `CO_2` is known as dry ice .
c. Due to small size and high electronegativity of `C` , it is capable of forming `p pi - p pi` bond , which occures in graphite , whereas in `Si` the size of the orbital involmed i.e.,e `3p` in `p pi -p pi` bond formation is large and ghecne multiple bond formation is not freasible `Si` . Hence silicon does not form an anlogous of grapbite .