Solutions/Answers to Exercises
Exercise 1. Arrange the following elements in order of increasing ionization energy: Ar, Al, Na, Cl.
Na < Al < Cl < Ar
Exercise 2. Phosphorus has a first ionization energy of 1011.8 kJ/mol and sulfur’s first ionization energy is 999.6 kJ/mol. Both P and S are in the same period. Explain why first ionization energy of P is higher than that of S.
The electron configuration for P is 1s22s22p63s23p3 and the electron configuration for S is 1s22s22p63s23p4
The electron in a half-filled p subshell will take more energy to remove. The S atom has a paired electron in the 3p subshell and it will take less energy to remove it.
Exercise 3. Magnesium has a first ionization energy of 737.7 kJ/mol and Aluminum is 577.5 kJ/mol. Explain why the first ionization energy is higher for Mg and lower for Al.
The electron configuration for Al is 1s22s22p63s23p1 and the electron configuration for Mg is 1s22s22p63s2. It will take more energy to remove the electron from the full 3s orbital than it would to remove the unpaired electron from the p orbital
Exercise 4. Explain why the fourth ionization energy of boron is so much higher (approximately 8 times higher) than Ei3
The electron configuration for B is 1s22s22p1. If a 4th electron is removed from B, it would be removed from a filled outer shell, the 1s. This would take a significant amount of energy.
Exercise 5. A second row element has the following ionization energies:
Ei2 = 1757 kJ/mol
Ei3 = 14,849 kJ/mol
Ei4 = 21,007 kJ/mol
What is the likely identity of the element?
The element is likely Be. The electron configuration for Be is 1s22s2. Once two electrons are removed from Be, the third electron would be taken from a full outer shell, 1s. The final electron would require much more energy because it is held so tightly to the nucleus.
Exercise 6. A third row element has the following ionization energies:
Ei2 = 1577 kJ/mol
Ei3 = 3232 kJ/mol
Ei4 = 4356 kJ/mol
Ei5 = 16,091 kJ/mol
What is the likely identity of the element?
The element is Si. The first ionization energy is larger than that for Al. The first three ionization energies increase, but the fourth ionization energy is significantly higher. This s because once Si loses 4 electrons, the 5th electron is removed from a full outer shell.