The answer of your first and third Q is:
the electronic configuration of C is 1s2 ,2s2 ,2px1 ,2py1 and 2pz is empty.so C has two unpaired electron for bond formation.As you know that when bonds are formed, energy is released.So in start of reaction, C form 2 bonds and energy is released after bond formation.There is only a small energy gap b/w the 2s and 2p orbitals,and so it pays the C to provide a small amount of energy to promote an electron from 2s to the empty 2pz to give 4 unpaired electrons.it would be to the carbon' s benefit to try to maximize the no.of bonds it can form.For this reason, C will form an excited state by promoting one of its 2s e' into empty 2p orbital.By forming this excited state, C will be able to form 4 bonds.
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