Question

In this course you have seen that hydrogen, oxygen, nitrogen, and carbon form one, two, three, and four bonds, respectively. Explain why this pattern of bonding is seen with these elements.

Answer 1

That pattern comes from the atoms' valence electron configurations and their tendency to attain a noble‑gas electron arrangement (the octet rule for second‑row elements, the duet for H).

- Hydrogen: 1s1. H needs one more electron to fill its 1s (duet), so it forms one covalent bond.

- Carbon: 1s2 2s2 2p2 (four valence electrons). Carbon needs four more electrons to complete an octet, so it can form four bonds. In practice carbon promotes/hybridizes its 2s and 2p orbitals (e.g. to sp3) to make four singly occupied orbitals and form four equivalent bonds.

- Nitrogen: 1s2 2s2 2p3 (five valence electrons). N needs three more electrons for an octet and has three unpaired electrons, so it normally forms three bonds (plus one lone pair).

- Oxygen: 1s2 2s2 2p4 (six valence electrons). O needs two more electrons for an octet and has two unpaired electrons, so it normally forms two bonds (plus two lone pairs).

Thus the number of bonds corresponds to how many electrons each atom needs to gain/share to reach a stable electron configuration (and to the number of available/unpaired valence orbitals).