Summary — the whole chapter in one scan
- First ionisation energy generally increases across a period (left → right) because nuclear charge goes up while shielding stays about the same, so the atom shrinks and holds its electrons tighter.
- First ionisation energy decreases down a group because extra electron shells add distance and shielding, which outweighs the bigger nuclear charge.
- There are two predictable "dips" in the ionisation energy graph across Period 2 and Period 3 — one from a change in subshell (Be→B, Mg→Al) and one from electron-electron spin-pair repulsion (N→O, P→S).
- Melting points across Period 3 rise steeply from Na to Si, then crash from Si to Ar — because the bonding and structure change completely partway through the period.
- Na, Mg, Al are giant metallic structures; Si is a giant covalent (molecular) structure; P, S, Cl, Ar are simple molecular structures held together by weak intermolecular forces.
1. Ionisation Energy Across a Period
First, let's be precise about what "first ionisation energy" (IE₁) actually means, because half of getting this topic right is knowing the definition cold:
Definition
First ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms, to form one mole of gaseous 1+ ions.
X(g) → X⁺(g) + e⁻
The periodic table isn't just a random grid — elements are lined up in order of increasing atomic number, arranged into groups (columns) and periods (rows). Because electron configuration repeats in a pattern as atomic number increases, properties like ionisation energy repeat too. That repeating pattern is called periodicity — and it's the reason a graph of IE₁ against atomic number looks like a jagged sawtooth that resets at the start of every new period.
FIRST IONISATION ENERGY (kJ/mol) — Period 2 example
2500 | He
| *
2000 |
|
1500 | N F Ne
| * \ * *
1200 | C * O
| * \ /
900 | Be *
| * \ /
600 | / B
| / *
300 | H Li
| * *
0 +--------------------------------------------- Na
1 2 3 4 5 6 7 8 9 10 11 (atomic number)
Notice: overall trend rises across the period, but dips at
Be→B and N→O. Then it CRASHES back down at Na — a new
period, a new outer shell, much easier to remove an electron.
Think of it like this: imagine the nucleus is a magnet and the outer electrons are paperclips stuck to it. As you move across a period, you're adding protons to the nucleus (stronger magnet) but the paperclips are still in the same shell — same rough distance away. A stronger magnet pulling on paperclips at the same distance means it's harder to pull one off. That's the whole trend in one sentence.
Why ionisation energy increases across a period
Four linked facts, each one causing the next:
- Nuclear charge increases
- Shielding stays roughly constant