Two diagrams of each molecule are displayed, both ball-and-stick (top) and space-filling representations (bottom). Labels explain that they are arranged so the leftmost molecules are 'least crowded/most reactive' and the rightmost molecules are 'most crowded/least reactive'. From left to right, the molecules are (a) bromomethane, CH3Br; (b) bromoethane, CH3CH2Br; (c) 2-bromopropane, (CH3)2CHBr; (d) 2-bromo-2-methylpropane, (CH3)3CBr,