CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 16(a)

(a) Carbon (electrons 2, 4) has four valence electrons. It cannot form ${\text{C}}^{4+}$ because removing four electrons would need a very large amount of energy, leaving six protons holding only two electrons. It cannot form ${\text{C}}^{4-}$ because a nucleus with six protons cannot hold on to ten electrons, that is, four extra electrons. So carbon shares its four valence electrons with other atoms and forms covalent bonds. (i) Covalent compounds are bad conductors of electricity because their electrons are shared between atoms and no ions are formed, so there are no free charged particles to carry the current. (ii) They have low melting and boiling points because the forces of attraction between their molecules are weak, so little energy is needed to separate the molecules. (b) Benzene, ${\text{C}}_6{\text{H}}_6$, is a hexagonal ring of six carbon atoms joined by alternate single and double bonds, with one hydrogen atom bonded to each carbon atom, as drawn in the figure.