A student models lithium’s successive ionizations by assuming the outer 2s electron is
CHEMISTRY · Single correct · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
A student models lithium’s successive ionizations by assuming the outer 2s electron is, on average, four times farther from the nucleus than a 1s electron.
1. From this assumption, the student predicts that the ratio IE1IE2 should be 16.
2. Experimentally, the ratio is approximately 14.
Which option both reproduces the student’s predicted ratio and provides the best explanation for why the experimental ratio is slightly smaller?
(a) Predicted ratio: 4; the experimental ratio is smaller because ionization energy is inversely proportional to the average distance, not its square, so the simple model overestimates the energy needed for the second ionization.
(b) Predicted ratio: 16; the experimental ratio is smaller because the remaining 1s electron partially shields the nucleus, so the effective nuclear charge it feels is slightly less than +3.
(c) Predicted ratio: 16; the experimental ratio is smaller because electron–electron repulsion within the 2s subshell increases after the first ionization.
(d) Predicted ratio: 9; the experimental ratio is smaller because the 2s electron experiences an effective nuclear charge of about +1 while a 1s electron feels about +3, and the ratio of the squares of those charges gives 9 rather than 16.