AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 3

✅ Correct Selections

Tick the two following boxes:

• Atoms have a charged nucleus. [1 mark]
• The mass of an atom is concentrated in the nucleus. [1 mark]

💡 Key Knowledge

• Most went straight through: atom is mostly empty space.
• Some deflected at large angles: the nucleus is positively charged (repelled the positive alpha particles).
• A very small fraction bounced straight back: the nucleus contains almost all the mass.

❌ Common Errors

• Ticking "Electrons orbit the nucleus at specific distances" — this was deduced later by Niels Bohr, not Rutherford.
• Ticking "Isotopes of an element have a different number of neutrons" — neutrons were discovered much later by James Chadwick (1932).

🧠 Exam Technique

Pay close attention to historical discoveries. Even though statements like "atoms are very small" or "electrons orbit in shells" are true scientific facts, they were not the direct conclusions from Rutherford's gold foil experiment.

✅ Correct Answer

Particle X has more electrons than protons (or "a different number of protons and electrons" / "atoms have equal numbers of protons and electrons"). [1 mark]

💡 Key Knowledge

• In any neutral atom: Number of protons = Number of electrons .
• When protons ≠ electrons, the particle has an overall charge and is an ion.
• Here, 36 electrons > 35 protons, so it has an overall 1− charge.

❌ Common Errors

Stating that it is not an atom because it has "a different number of neutrons". Neutrons have no charge; an imbalance between protons and neutrons just means it is an isotope, not an ion!

✅ Correct Answer

Tick the second box: ⁷⁹₃₅Br⁻ [1 mark]

📐 Step-by-Step Breakdown

• Atomic Number (bottom number): 35 protons → element is Bromine (Br), so bottom number = 35 .
• Mass Number (top number): protons + neutrons = 35 + 44 = 79 .
• Charge: 35 protons (+35) and 36 electrons (−36) = −1 charge ( Br⁻ ).

✅ Mark Scheme Model Answer

• Point 1: Going down the group, the molecules increase in size (or atoms increase in size). [1 mark]
• Point 2: The forces between the molecules (intermolecular forces) increase. [1 mark]
• Point 3: Therefore, more energy is needed to break the intermolecular forces (or separate the molecules). [1 mark]

🧠 3-Step Structure for Melting/Boiling Point Questions

1. State what happens to particle/molecule size.
2. State how the intermolecular forces change strength.
3. State the amount of energy needed to overcome them.

❌ Common Misconception: Breaking Covalent Bonds

Never write that "covalent bonds are broken" when boiling or melting simple molecular substances like halogens. The strong covalent bonds inside the molecules remain completely intact; only the weak intermolecular forces between molecules are overcome.

✅ Correct Answer

Covalent [1 mark]

💡 Key Knowledge

• Both chlorine (Cl) and fluorine (F) are non-metals.
• Non-metal + non-metal bond by sharing electrons = covalent bonding.
• Metal + non-metal = ionic bonding; metal + metal = metallic bonding.

Mark scheme note: Both products are required for the 1 mark. The formulas Br₂ and NaCl are also accepted.

🧠 Why does this reaction happen?

Chlorine is more reactive than bromine. A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide salt.

❌ Common Errors

• Writing "bromide" instead of bromine for the element produced.
• Writing "sodium chlorine" instead of sodium chloride for the salt.

✅ 3-Mark Complete Explanation

• Mark 1: Going down the group, the outer shell (outer electrons) is further from the nucleus (or the atom has more shells / is larger).
• Mark 2: There is less electrostatic attraction between the nucleus and the incoming outer electron (or there is more shielding).
• Mark 3: Therefore, gaining an electron is more difficult.

🧠 How to Perfect Group 7 Explanations

Always link atomic structure to the halogen's "job":

• Halogens need to gain 1 electron to get a full outer shell.
• Distance & shielding increase → positive attraction from the nucleus weakens → harder to pull in that 1 extra electron.

❌ Common Errors & Group 1 Confusion

• Confusing Group 1 with Group 7: Students often say "easier to lose an electron". Group 7 non-metals gain electrons; Group 1 metals lose electrons.
• Vague statements: Saying "it's harder to react" without stating why (i.e. gaining an electron is more difficult).