Finding the group
The properties of an element depend on the number of electrons in its last energy level. These are called the valence electrons.
Look at lithium, sodium and potassium in table 3.2. Each has exactly one electron in its last level — and their chemical properties are largely alike. All three sit in the same vertical column.
That is the pattern: any element in an upper row of a column behaves much like the element below it. So the group an element belongs to is decided by its valence electrons.
| Electrons in the last level | Group |
|---|---|
| 1 | Group I |
| 2 | Group II |
| 3 | Group III |
| 4 | Group IV |
| 5 | Group V |
| 6 | Group VI |
| 7 | Group VII |
| 8, or a stable configuration | Group VIII / 0 |
Magnesium, worked through
Magnesium has atomic number 12, so its configuration is 2, 8, 2.
- Electrons are present in 3 energy levels → period 3.
- The last level holds 2 electrons → group II.
Magnesium is in period 3, group II.
Figure to be added
Potassium, worked through
Potassium has atomic number 19, so its configuration is 2, 8, 8, 1.
- Electrons are present in 4 energy levels → period 4.
- The last level holds 1 electron → group I.
Potassium is in period 4, group I.
Figure to be added