Part 4: Ionisation energies

1. First ionisation energy

Ionisation energies are always positive (endothermic), because energy is needed to pull an electron away from the attraction of the nucleus. The units are kJ mol⁻¹. Always include the (g) state symbols, even for elements that are normally solids.

2. Successive ionisation energies

Each successive ionisation removes one more electron from each ion:

The second ionisation energy is not Mg(g) → Mg²⁺(g) + 2e⁻. That equation describes two ionisations added together. Check that charges balance: the left side's charge equals the right side's total.

3. What affects ionisation energy

Ionisation energy depends on how strongly the nucleus attracts the electron being removed. Three factors matter:

• Nuclear charge: more protons give a stronger attraction.
• Distance: an electron further from the nucleus (in a higher shell) is attracted less strongly.
• Shielding: inner-shell electrons repel the outer electrons, reducing the attraction they feel.

Explain trends using these factors and the attraction between the nucleus and the electron. "Full shells are stable" on its own does not explain anything.

4. Trend down Group 2

First ionisation energy decreases from Be to Ba.

Refer specifically to the outer electron.

5. Trend across Period 3

There is a general increase from Na to Ar.

The distance isn't exactly the same across a period: the radius gets smaller, which strengthens the attraction even more.

6. The Mg → Al and P → S drops

The same patterns explain the drops in Period 2 from Be to B and from N to O. Helium has the highest first IE of all: its electron is in 1s, very close to the nucleus with no inner shielding, and it has more protons than hydrogen.

7. Why successive ionisation energies rise

Each electron is removed from an increasingly positive ion. The nucleus stays the same, but there are fewer electrons to repel each other and the ion gets smaller. This means the remaining electrons are attracted more strongly. The number of protons does not change.

8. Big jumps and electron shells

For a main-group atom, if the first large jump comes after k electrons have been removed, the element has k outer-shell electrons: that jump marks the change from removing outer-shell electrons to starting on an inner shell. Later jumps reflect deeper shells, so use only the first. The next electron comes from an inner shell, which is closer to the nucleus, lower in energy and less shielded, so it is attracted much more strongly.

Worked example (illustrative data)

The large jump between the 3rd and 4th IEs means there are 3 outer electrons, so the element is in Group 13 (aluminium). The 4th electron is removed from the 2p sub-shell, which is closer to the nucleus and less shielded than 3s/3p. Note that this is Group 13, the old "Group 3", not the modern transition-metal Group 3.

9. Second ionisation energy across elements

This is different from successive IEs of a single element. When comparing the second IE of different elements, look at the configuration of the 1+ ion you are removing from:

• Na⁺ is [Ne]. Its second electron comes from 2p, an inner shell close to the nucleus, so sodium's second IE is unusually large.
• Mg⁺ is [Ne] 3s¹. Its second electron comes from 3s, so magnesium's second IE is much lower than sodium's.

Work out each ion's configuration rather than just shifting the first-IE graph along by one element.

10. Finesse practice

Original Finesse practice questions, not AQA past-paper questions.

11. Atomic structure topic checklist

• ☐Describe how the model of the atom developed, and interpret alpha-scattering evidence
• ☐Give the relative mass, charge and location of protons, neutrons and electrons
• ☐Calculate p, n and e for atoms and ions using A, Z and charge
• ☐Define isotopes, and compare the particles in neutral isotopes
• ☐Explain why isotopes have the same chemical properties
• ☐Describe the four stages of TOF mass spectrometry, including both ionisation methods
• ☐Use KE = ½mv² and v = d/t, with mass in kg per ion
• ☐Define Ar and calculate it from spectra or abundances; find missing abundances
• ☐Write configurations up to Kr, including Cr, Cu and ions
• ☐Define first and successive IEs with correct equations and state symbols
• ☐Explain the trends down Group 2 and across Period 3, including the drops
• ☐Use jumps in successive IEs to work out an element's group

12. Sources