First ionisation energy
The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous positive ions.
For sodium:
First ionisation energies across Period 3
There is an overall increase in first ionisation energy from sodium to argon, although the pattern contains decreases from Mg to Al and from P to S.
| Element | First IE / kJ mol−1 |
|---|---|
| Na | 494 |
| Mg | 736 |
| Al | 577 |
| Si | 786 |
| P | 1060 |
| S | 1000 |
| Cl | 1260 |
| Ar | 1520 |
Explaining the overall increase
- The number of protons increases across the period, so nuclear charge increases.
- Electrons are being added within the same principal energy level, so shielding remains approximately constant.
- Atomic radius decreases, bringing the outer electron closer to the positively charged nucleus.
- The attraction between the nucleus and the outer electron therefore becomes stronger.
More energy is consequently required to remove the outer electron, giving the overall rise in first ionisation energy.