AQA A-Level Chemistry AS Paper 1, June 2022: Question 1

✅ Model Answer

Mark 1: The outer electron is in a higher energy level / there are more shells / atomic radius increases / there is more shielding.

Mark 2: There is a weaker attraction between the nucleus and the outer electron.

💡 Key Knowledge

• Down any group, principal quantum number n increases, adding complete inner shells of electrons.
• Even though nuclear charge increases (more protons), the increased distance and shielding effect outweigh the extra nuclear charge.
• Net result: The outermost electron is less tightly held by the nucleus and requires less energy to remove.

🧠 Exam Technique: The Three-Step Template

Whenever explaining ionisation energy trends, always cover these three factors in your sentence:

1. Position/Shielding: More shells / higher energy level / increased shielding.
2. Attraction: Weaker electrostatic force between nucleus and outer electron.
3. Conclusion: Less energy required to remove the electron.

❌ Common Errors

• Vague terminology: Writing "the electrons are further away" without mentioning the outer electron or atomic radius.
• Nuclear charge trap: Claiming ionisation energy decreases because "nuclear charge decreases" — nuclear charge actually increases down the group!
• Missing the link: Stating there are more shells, but failing to state that this results in weaker attraction.

✅ Model Answer

Mg²⁺(g) → Mg³⁺(g) + e⁻

Also acceptable: Mg²⁺(g) + e⁻ → Mg³⁺(g) + 2e⁻

💡 Defining the "nth" Ionisation Energy

The nth ionisation energy is always the energy required to remove one mole of electrons from one mole of gaseous ions with a charge of +(n − 1):

X⁽ⁿ⁻¹⁾⁺(g) → Xⁿ⁺(g) + e⁻

For the 3rd IE: start with a +2 ion and form a +3 ion.

❌ Common Errors

• Starting from neutral Mg: Writing Mg(g) → Mg³⁺(g) + 3e⁻ is the sum of the 1st, 2nd, and 3rd IEs, not the 3rd IE itself!
• Missing or incorrect state symbols: Leaving out (g) or writing (s) . Ionisation energies are exclusively defined for gaseous species.
• Starting with the wrong charge: Writing Mg⁺(g) → Mg²⁺(g) + e⁻ (this is the 2nd IE).

🧠 Exam Checklist

• Did you start with a +2 ion on the left?
• Did you form a +3 ion and one electron on the right?
• Are all magnesium species labeled with (g)?

✅ Model Answer

Mark 1: The third electron is removed from the 2p sub-shell / a 2nd energy level / an inner shell / a shell that is closer to the nucleus.

Mark 2: This electron experiences less shielding (than the electron in the 3s sub-shell).

📐 Step-by-Step Electronic Configuration Comparison

Writing out the electron configurations makes the jump immediately obvious:

1. Mg atom: 1s² 2s² 2p⁶ 3s²
2. Mg⁺ ion (after 1st IE): 1s² 2s² 2p⁶ 3s¹
3. Mg²⁺ ion (after 2nd IE): 1s² 2s² 2p⁶
4. 3rd IE: Removes an electron from the completely full, stable inner shell: 2p⁶ → 2p⁵ .

❌ Common Errors & Misconceptions

• "Full shell stability" argument: Simply saying "it is harder because it is breaking a stable full octet" earns 0 marks in AQA mark schemes. You must explain it physically in terms of closer to the nucleus / lower energy level and less shielding.
• Nuclear charge confusion: Saying "the nucleus has more protons" is incorrect; magnesium still has 12 protons. The proton-to-electron ratio is higher, but the main factor causing the huge leap is the removal from an inner principal energy level.

🧠 Top-Scorer Strategy

Whenever an exam question notes that an IE is "much higher" (a big jump) rather than just "slightly higher":

• Explicitly name the sub-shell: identify that the 2nd electron came from 3s and the 3rd comes from 2p.
• State the direct consequences: closer to the nucleus, lower energy level, and substantially less shielding.