A researcher is comparing the likelihood of ionic compound formation for the four…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A researcher is comparing the likelihood of ionic compound formation for the four element pairs listed in the table below. All data are measured at 298 K and 1 atm.
Element Pair
First Ionization Energy of Metal (kJ mol⁻¹)
Electron Affinity of Non-metal (kJ mol⁻¹)
Mg + O
738
−141
Al + Cl
578
−349
K + Br
419
−325
Ca + S
590
−200
Assume that each metal loses the minimum number of electrons required to obtain a noble-gas electron configuration and that each non-metal gains electrons to complete its valence shell. Neglect second and higher ionization energies for this qualitative analysis.
Using only the numerical data provided and general periodic trends for ionic radii, analyze which of the four element pairs is expected to form an ionic compound most readily under standard conditions.
(a) For each pair, calculate the algebraic sum of the first ionization energy of the metal and the electron affinity of the non-metal. Present each result with its proper sign.
(b) Rank the four pairs from most favorable to least favorable for initial electron transfer based solely on your results from part (a). Briefly explain how the sign and magnitude of each calculated value supports your ranking.
(c) Incorporating qualitative lattice-enthalpy considerations related to ionic charge and ionic radii, revise your ranking to predict which compound will actually be most stable once the ionic lattice forms. Provide a concise justification that directly references the periodic position of each element.