Question 7.EP.1: Using data from Table 6.4 (page 236), predict the ions that ...
Using data from Table 6.4 (page 236), predict the ions that magnesium and aluminum are most likely to form.
Table 6.4
| Z | Element | IE_{1} | IE_{2} | IE_{3} | IE_{4} |
| 1 | H | 1.312 | – | – | – |
| 2 | He | 2.372 | 5,250 | – | – |
| 3 | Li | 520.2 | 7,298 | 11,815 | – |
| 4 | Be | 899.4 | 1,757 | 14,848 | 21,007 |
| 5 | Be | 800.6 | 2,427 | 3,660 | 25,026 |
| 6 | C | 1,086 | 2,353 | 4,620 | 6,223 |
| 7 | N | 1,402 | 2,856 | 4,578 | 7,475 |
| 8 | O | 1,314 | 3,388 | 5,300 | 7,469 |
| 9 | F | 1,681 | 3,374 | 6,050 | 8,408 |
| 10 | Ne | 2,081 | 3,952 | 6,122 | 9,370 |
| 11 | Na | 495.6 | 4,562 | 6,912 | 9,544 |
| 12 | Mg | 737.7 | 1,451 | 7,733 | 10,540 |
| 13 | Al | 577.6 | 1,817 | 2,745 | 11,578 |
| 14 | Si | 786.4 | 1,577 | 3,232 | 4,356 |
| 15 | P | 1,012 | 1,908 | 2,912 | 4,957 |
| 16 | S | 999.6 | 2,251 | 3,357 | 4,564 |
| 17 | CL | 1,251 | 2,297 | 3,822 | 5,158 |
| 18 | Ar | 1,520 | 2,666 | 3,931 | 5,771 |
Strategy
Scan the successive ionization energies for each element to find a point where removing one additional electron causes a dramatic increase in the value. It is not likely that sufficient energy would be available to compensate for such a large increase in ionization energy, so the ion formed will be dictated by the number of electrons lost before that jump in ionization energy.
For magnesium, the first significant jump occurs between the second ionization energy (1451 kJ/mol) and the third (7733 kJ/mol). We would expect two electrons to be removed, and the most commonly found ion should be Mg^{2+}. For aluminum, the third ionization energy is 2745 kJ/mol, whereas the fourth is 11,578 kJ/mol. We would expect three ionizations to occur, so aluminum should form Al^{3+}.
Discussion
This type of question is designed to help you see the reasoning behind some of the facts you may already know. In many cases, the way we learn chemistry is by learning information as facts first and then finding out the reason those facts are evident. In the examples from this problem, the answers are consistent with the idea that the ion formed is dictated by the number of electrons lost before the jump in ionization energy. They are also consistent with facts you may already have known, for example, that magnesium will form a 2+ ion because it is an alkaline earth element in Group 2 of the periodic table.