WAEC Chemistry 2010 Paper

Concept:A molecule is linear when its central atom has two bonding regions and no lone pairs.Explanation:Use VSEPR theory to predict molecular shape from bonds and lone pairs.In $\mathrm{CO}_2$, the central carbon atom forms two double bonds with oxygen atoms.Carbon has no lone pairs of electrons.Therefore, the electron pairs spread as far apart as possible, making the molecule linear.The bond angle in $\mathrm{CO}_2$ is $180^\circ$.Look at the other options to confirm why they are not linear.$\mathrm{CH}_4$ is tetrahedral because carbon has four bonding pairs and no lone pairs.$\mathrm{H}_2\mathrm{S}$ is bent because sulfur has two bond pairs and two lone pairs.$\mathrm{NH}_3$ is trigonal pyramidal because nitrogen has three bond pairs and one lone pair.Among the given molecules, only carbon dioxide has a linear shape.Answer:B. $\mathrm{CO}_2$