A researcher records the first-ionization energies (IE ) of the elements with atomic…
CHEMISTRY · Multi-part · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
A researcher records the first-ionization energies (IE1) of the elements with atomic numbers 1≤Z≤20 and produces the following verbal description of the graph of IE1 (in kJ mol−1) versus atomic number, Z:
• The graph begins with a small value at Z=1, rises sharply to a pronounced peak at Z=2, then drops steeply at Z=3.
• From Z=3 to Z=10 the values generally rise, but there is a noticeable "dip" at Z=5 and a larger peak at Z=10.
• A second steep drop occurs at Z=11, followed by a gradual rise punctuated by a dip at Z=13 and a second major peak at Z=18.
• A third sharp decrease is observed at Z=19, after which the value increases slightly at Z=20.
Use the information above to analyze periodic trends in ionization energy.
(a) On a qualitative sketch of the described graph, label each of the three highest peaks with the most likely family of elements to which those points correspond. Justify your labeling in terms of electron configurations.
(b) Identify the atomic numbers that correspond to the two local "dips" described at Z=5 and Z=13. For each dip, write the complete ground-state electron configuration of the element and explain why its IE1 is lower than that of the element immediately preceding it.
(c) Using effective nuclear charge, Zeff, and the shell model, explain why large decreases in IE1 are observed at Z=3, Z=11, and Z=19.
(d) Predict how the magnitude of the first-ionization energy at Z=21 would compare with the values at Z=19 and Z=20. Provide reasoning that references both periodic trends and the electron configuration of the element with Z=21.