CIE Chemistry Advanced Subsidiary (AS) Level Practice Test

A linear shape with a 180° bond angle occurs when the central atom has only two regions of electron density and no lone pairs, so the bonding pairs repel to form a straight line. Carbon dioxide fits this description: the carbon forms two double bonds to oxygen and has no lone pairs, giving two regions of electron density that lie opposite each other and produce a straight O=C=O arrangement with a 180° angle. Water has a bent shape because the oxygen carries two lone pairs in addition to the two bonding pairs, which pushes the hydrogens closer together and reduces the angle to about 104.5°. Ammonia has three bonding pairs and one lone pair, giving a trigonal pyramidal shape with angles around 107°. Methane has four bonding pairs and no lone pairs, yielding a tetrahedral geometry with angles about 109.5°.