Across Period 3, first ionisation energy generally increases from sodium to argon, with two small dips: from magnesium to aluminium, and from phosphorus to sulfur. Both dips come up often in exam questions, and both reward the same habit: a statement of fact first, then the reason.
What first ionisation energy means
First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, to form one mole of gaseous 1+ ions. For magnesium:
Mg(g) → Mg⁺(g) + e⁻
Learn the equation with its state symbols. The atoms are gaseous, and one electron is removed from each one.
The general trend: it increases
Across the period:
- the nuclear charge increases, because each element has one more proton than the one before
- the electrons go into the same main energy level (shell 3), so the shielding is similar
- the atomic radius decreases, so the outer electron is closer to the nucleus and more strongly attracted
So more energy is needed to remove the outer electron.
Dip 1: magnesium to aluminium
Aluminium has a lower first ionisation energy than magnesium.
- Magnesium's outer electron is in the 3s sub-level. Aluminium's is in the 3p sub-level.
- The 3p sub-level is higher in energy than 3s, and its electron is slightly further from the nucleus.
- The 3p electron is also partly shielded by the 3s electrons.
So less energy is needed to remove it, even though aluminium has one more proton.
Dip 2: phosphorus to sulfur
Sulfur has a lower first ionisation energy than phosphorus.
- In phosphorus, each of the three 3p electrons is in a separate orbital.
- In sulfur, the fourth 3p electron pairs up with another electron in the same orbital.
- The two paired electrons repel each other, so one of them is easier to remove.
Writing it in the exam
Rich's tip: Think in bullet points, write in sentences.
Plan the answer as the three points above, then write each one as a full sentence.
Rich's tip: Statement of fact first.
Start with what happens: "Aluminium has a lower first ionisation energy than magnesium." Then give the reason, naming the sub-levels: "Its outer electron is in the 3p sub-level, which is higher in energy than the 3s sub-level." Name 3s and 3p: "a different sub-level" on its own is too vague to show what you mean.