(1) The electronic configuration of C - atoms is: `1s^(2)2s^(2)2p^(2)`, yet its valency is four - why?
(2) The four C - H bonds of methane molecule are equivalent - explain with reasons.

Explanation

(1) During chemical reaction, the carbon atoms gain energy and promote one of the two electrons of 2s - orbital to the higher `2p_(2)` - orbital. Thus in the excited state, the electronic configuration of carbon becomes `1s^(2)2s^(1)2p^(1/2)2p^(1/y)2p^(1/z)`. At this condition, the valence shell of C - atom contains four unpaired electrons. Thus, the C - atom can from four covalent bonds using four unpaired electrons. This explains why carbon having electronis configuration, `1s^(2)2s^(2)sp^(2)` is tetravalent.

(2) Equivalency of four C - H bonds in methame `(CH_(4))` can be explained by the concept of hybridisation of orbitals. In the excited state, the four valence orbitals of carbon, i.e. one 2s and three 2p orbitals possessing slightly different energies mix up and results in the formation fo four equivalent `sp^(3)` - hybrid orbitals. These hybrid orbitals overlap with the four 1 s - orbitals of four H - atoms to form four C - H bonds which are also equivalent (same bond lenght and bond strenght).