CBSE Class 10 Science 2017 · Paper 531/1 · Question 19

Covalent bonding: carbon (atomic number 6, configuration 2, 4) has four valence electrons and needs four more to reach a noble-gas configuration. It could gain four electrons to form ${\text{C}}^{4-}$, but it would be difficult for a nucleus with six protons to hold ten electrons; or it could lose four to form ${\text{C}}^{4+}$, but that would need a large amount of energy, leaving six protons holding only two electrons. So carbon shares its valence electrons with other atoms (of carbon or of other elements) and forms covalent bonds, as in methane, where it shares one pair with each of four hydrogen atoms. Two main reasons for the large number of compounds: (i) catenation, the ability of carbon atoms to bond with one another to form long straight chains, branched chains and rings, with single, double or triple bonds; (ii) tetravalency, a valency of four, so that each carbon atom can bond with four other atoms of carbon or of elements such as hydrogen, oxygen, nitrogen, sulphur and chlorine. Stability: the bonds carbon forms with most other elements are very strong because the carbon atom is small, so its nucleus holds the shared pairs of electrons strongly; elements with bigger atoms form much weaker bonds.