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AS · Practice questions · Ionisation energy and its trends

Name the factor.

Six original Cambridge-style questions on first ionisation energy: definition and equations, the Period 3 trend and its two dips, the group trend, and a prediction.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Charge, distance, shielding, pairing.

01
Recall
[2 marks]

Define first ionisation energy and write an equation, with state symbols, for the first ionisation energy of chlorine.

  • The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. ✓
  • Cl(g) → Cl+(g) + e−. ✓
02
Explain
[3 marks]

Explain why first ionisation energy generally increases across Period 3 from sodium to argon.

  • Nuclear charge increases by one proton for each element across the period. ✓
  • The outer electron being removed is in the same shell (n = 3) each time, and shielding by the inner n = 1 and n = 2 shells is approximately constant. ✓
  • The outer electron is therefore attracted more strongly and more energy is needed to remove it. ✓
03
Explain
[2 marks]

The first ionisation energies of magnesium and aluminium are 738 and 578 kJ mol−1. Explain why aluminium's is lower.

  • The electron removed from aluminium is in the 3p sub-shell, whereas magnesium's is in 3s; 3p is higher in energy than 3s. ✓
  • The 3p electron is also partly shielded by the 3s electrons, so less energy is needed to remove it despite aluminium's greater nuclear charge. ✓
04
Explain
[2 marks]

The first ionisation energies of phosphorus and sulfur are 1012 and 1000 kJ mol−1. Explain why sulfur's is lower.

  • Phosphorus (3p3) has one electron in each 3p orbital; in sulfur (3p4) two electrons share one 3p orbital. ✓
  • The paired electrons repel each other (spin-pair repulsion), so one of them is removed more easily than an unpaired 3p electron in phosphorus. ✓
05
Explain
[3 marks]

Explain why first ionisation energy decreases down Group 2 from beryllium (900 kJ mol−1) to barium (503 kJ mol−1).

  • Down the group the outer electron is in a shell further from the nucleus, so the attraction to the nucleus is weaker. ✓
  • There are more inner shells, so shielding of the outer electron increases. ✓
  • These two effects outweigh the increase in nuclear charge, so less energy is needed to remove the outer electron. ✓
06
Analysis
[2 marks]

Predict which has the higher first ionisation energy, potassium or calcium, and explain your prediction.

  • Calcium has the higher first ionisation energy (590 compared with 419 kJ mol−1). ✓
  • Both remove a 4s electron with the same inner shells and similar shielding, but calcium has 20 protons against potassium's 19, so its outer electron is held more strongly. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.