Why is N2 a gas whereas P4 a solid?
C, N, and O are able to form 2p-π 2p-π bonds because being 1st row (2nd period) elements two atoms can closely approach each other for effective overlap of the 2p AOs on each atom. Only the 1s2 core electrons can prevent this approach and these e-s are close to the nucleus. For this reason bonds such as C=C, CΞC, CΞN, C=O, N=O, O=O are stable. For the 3rd period elements Si, P, and S the 2s22p6 core electrons are thought to prevent close approach for good 3p-π 3p-π bonding. Although cmpds with Si=Si, P=P, bonds are known they are not very stable. N2 with a NN σ bond and two NN 2p-π 2p-π bonds gives NΞN with a V. strong triple bond that is more stable than N4 tetrahedron (all attempts to make N4 even at low T have failed; think of the ring strain). P unable to form stable π bonds adopts a structure with only P-P single bonds. It is a strained structure and in white P is spontaneously flammable in air to give P4O10. It is lucky for us that O=C=O is a gas and not glass (SiO2: -O-Si-O-Si-)
