For a main-group element, the position of the first very large jump in successive ionization energies reveals the number of valence electrons and therefore indicates the element's group. For example, a large jump after the second ionization energy suggests two valence electrons and a Group 2 element.
How the Pattern Works
Successive ionization energies are the energies required to remove electrons one at a time from gaseous species. Each electron is removed from an increasingly positive ion, so successive values generally increase because the remaining electrons experience stronger attraction to the nucleus.
A particularly large increase occurs when all valence electrons have been removed and the next electron must be removed from an inner energy level. This electron is closer to the nucleus, experiences less shielding, and is much more strongly attracted to the nucleus.
For example, magnesium has approximate successive ionization energies of 738, 1451, and 7733 kJ mol⁻¹. The large jump between and shows that magnesium has two valence electrons:
Therefore, magnesium is in Group 2.
| Position of first large jump | Valence electrons | Likely group |
|---|---|---|
| After | 1 | Group 1 |
| After | 2 | Group 2 |
| After |
This method is most reliable for main-group elements. For transition elements, electrons from the and sublevels may have similar energies, so the group cannot always be identified from one obvious jump.
Exam Technique
This is Additional Higher Level (AHL) content from Structure 1.3. For a “deduce” question, identify where the major jump occurs, state the number of valence electrons, and then give the group. A common misconception is treating every increase as a shell change; look for a dramatic increase, not merely the largest ionization energy listed.