Edexcel A-Level Chemistry Paper 1, November 2020: Question 1

✅ Correct Answer

B: Al²⁺(g) → Al³⁺(g) + e⁻

💡 Key Knowledge

• The $n$-th ionisation energy is the energy required to remove one electron from each atom or ion in one mole of gaseous ions to form one mole of gaseous ions with a higher positive charge.
• State symbols must always be (g) for gaseous species.

❌ Common Errors

• Choosing option A, which represents the overall sum of the first three ionisation energies combined into a single equation.
• Selecting options C or D, which incorrectly show electrons being gained (reduction/electron affinity) rather than removed.

✅ Correct Answer

C (Element Y)

💡 Key Knowledge

• Look for a massive "jump" or large increase in successive ionisation energies. This indicates an electron is being removed from a new, inner quantum shell closer to the nucleus.
• For Element Y: First IE = 590, Second = 1145, Third = 4912. The huge leap between the 2nd and 3rd ionisation energy shows it has 2 electrons in its outer shell (Group 2).

🧠 Exam Technique

Calculate the ratio between successive values if unsure. A jump of 3x to 5x or more flags a change of shell. Element Z shows a huge jump between 1st and 2nd IE (496 to 4563), meaning Z is in Group 1.

✅ Correct Answer

C (letter c)

💡 Key Knowledge

• Oxygen (atomic number 8) follows nitrogen (atomic number 7, represented by b ).
• There is a characteristic drop in first ionisation energy from Group 5 (N) to Group 6 (O) due to electron-electron repulsion in the doubly-filled p -orbital.

❌ Common Errors

• Confusing the dip at Group 6 with the general upward trend. Note that c is lower than b , correctly depicting the drop for oxygen. Point a is carbon, b is nitrogen, and d is fluorine.

✅ Correct Answer

Any valid isoelectronic ion: N³⁻ , O²⁻ , F⁻ , Na⁺ , or Al³⁺

💡 Key Knowledge

• Isoelectronic means having the same number of electrons (and identical electronic configuration).
• Magnesium atom has 12 electrons; the Mg²⁺ ion has lost 2 electrons, leaving 10 electrons.
• Any stable ion with 10 electrons is accepted (e.g., O²⁻ or Na⁺ ).

❌ Common Errors

• Giving a neutral atom like Neon ( Ne ). The question explicitly asks for the formula of a stable ion.
• Writing incorrect charges or non-existent ions like C⁴⁻ or Si⁴⁺ .

💡 Core Definitions

• d-block element: An element whose last added electron enters a d-subshell.
• Transition metal: A d-block element that forms at least one stable ion with an incomplete d-subshell/d-orbital.

✅ Required Marking Points (4 marks available)

• Point 1: Both Sc and Zn have their last added electron in a d-subshell (making them d-block elements).
• Point 2: Neither forms a stable ion with an incomplete d-subshell (disqualifying them as transition metals).
• Point 3: Scandium forms Sc³⁺ which has an empty d-subshell ( 1s² 2s² 2p⁶ 3s² 3p⁶ or [Ar] ).
• Point 4: Zinc forms Zn²⁺ which has a full d-subshell ( 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ or [Ar] 3d¹⁰ ).

❌ Common Pitfalls

• Simply stating that the elements "contain d-electrons" without specifying that the last added electron is in the d-subshell.
• Failing to explicitly link the electronic configuration of the stable ions ( Sc³⁺ and Zn²⁺ ) to the definition of a transition metal.