Problems are listed in approximate order of difficulty. A single dot (•) indicates straightforward problems involving just one main concept and sometimes requiring no more than substitution of numbers in the appropriate formula. Two dots (••) identify problems that are slightly more challenging and usually involve more than one concept. Three dots (•••) indicate problems that are distinctly more challenging, either because they are intrinsically difficult or involve lengthy calculations. Needless to say, these distinctions are hard to draw and are only approximate.
•• Because of the way that atomic properties vary smoothly along the rows and columns of the periodic table, one can often predict the properties of an element from the known properties of its neighbors. (This is how Mendeleev predicted the existence and several properties of the elements now called scandium, gallium, and germanium.) (a) The ionization energies of 20Ca and 38Sr are 6.11 eV and 5.70 eV. If one guessed that ionization energies should change linearly as one moves through a group, what would one predict for the ionization energy of 56Ba? (The observed value is 5.21 eV.) (b) Use the same argument to predict the electron affinity of 35Br, given that 17Cl has electron affinity 3.61 eV and 53I has 3.06 eV. (The observed value is 3.36 eV.) (c) Predict the radius of the 9F atom, given that 7N and 8O have radii of 0.075 and 0.065 nm. (The actual value for 9F is 0.057 nm.)
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