CBSE Class 10 Science 2019 · Paper 31/1/1 · Question 17(b)

Methane, ${\text{CH}}_4$, is the simplest hydrocarbon (the first alkane), a compound of one carbon atom and four hydrogen atoms; it is the main constituent of natural gas, CNG and biogas. Electron dot structure (see the figure): carbon shares one pair of electrons with each of the four hydrogen atoms, so carbon has 8 outer electrons and each hydrogen 2. Type of bonds: covalent bonds (four single covalent bonds between carbon and hydrogen). (i) Such compounds are poor conductors of electricity because their electrons are shared and no ions (charged particles) are formed, so nothing can carry the current. (ii) They have low melting and boiling points because they are covalent compounds whose molecules are held together only by weak forces of attraction, so little energy is needed to separate the molecules. When methane burns in oxygen it forms carbon dioxide and water, with a lot of heat and light: ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}$ + heat and light.