OCR A-Level Chemistry AS Depth in chemistry (02), June 2018: Question 2

✅ Correct Answer

Two separate [Na]⁺ ions (each with either 0 or 8 electrons shown, and a + charge) AND one [S]²⁻ ion with 8 electrons in its outer shell (typically represented as 6 dots and 2 crosses, or vice versa) enclosed in square brackets with a 2– charge.

💡 Key Knowledge

• Ionic bonding involves the complete transfer of electrons from metal to non-metal.
• Sodium loses its 1 single outer electron to form a stable Na⁺ ion.
• Sulfur gains 2 electrons into its outer shell to form a stable sulfide ion, S²⁻ .

🧠 Exam Technique

• Always include clear square brackets and charges for each individual ion.
• Make sure to distinguish clearly between electron symbols (e.g., dots for sulfur's original electrons and crosses for the transferred sodium electrons).

❌ Common Errors

• Forgetting to draw two separate sodium ions (missing the coefficient 2 or drawing them joined together).
• Failing to include the correct charges ( + and 2– ).
• Drawing inner shell electrons when the question explicitly states to show outer electrons only.

✅ Correct Answer

Sodium sulfide (Na₂S): Giant structure | Solid: poor | Liquid: good
Sodium (Na): Giant structure | Solid: good | Liquid: good
Sulfur (S): Simple structure | Solid: poor | Liquid: poor

💡 Key Knowledge

• Ionic lattices (Na₂S): Contain ions fixed in position when solid (poor conductor), but ions are free to move when molten/liquid (good conductor). High melting point due to strong electrostatic forces.
• Metallic lattices (Na): Contain delocalised electrons that can move freely in both solid and liquid states (good conductor).
• Simple molecular lattices (S₈): Held together by weak intermolecular forces (London forces), leading to low melting points and poor electrical conductivity in all states as there are no mobile charged particles.

🧠 Exam Technique

• Marks are awarded by column. Ensure you look at each substance independently and verify its bonding model before filling out the row.

❌ Common Errors

• Confusing the electrical conductivity of solid ionic substances with metals. Solid ionic lattices do not conduct electricity because ions cannot move.

✅ Correct Answer

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴ (Sub-shells can also be written in filling order or numerical shell order, e.g., 3d¹⁰ placed after 4s² ).

💡 Key Knowledge

• Selenium (Se) has atomic number 34, meaning it has 34 electrons to distribute across atomic orbitals.
• It sits in Period 4, Group 6 of the Periodic Table, ending in a p⁴ configuration.

🧠 Exam Technique

• Double-check that your superscript numbers add up to the total atomic number (2+2+6+2+6+10+2+4 = 34).

❌ Common Errors

• Using noble gas shorthand like [Ar] when the question explicitly prompts starting from 1s² .
• Miscounting electrons in the 3d sub-shell (writing 3d⁸ instead of 3d¹⁰ ).

✅ Correct Answer

Gas B: H₂Se (Hydrogen selenide / Selenium hydride)
Equation: Na₂Se + 2HCl → 2NaCl + H₂Se

💡 Key Knowledge

• Group elements show chemical periodicity: elements in the same group react in analogous ways. Just as metal sulfides react with acids to form toxic hydrogen sulfide ( H₂S ), metal selenides react similarly to form hydrogen selenide ( H₂Se ).
• Relative molecular mass check: H (1.0 × 2) + Se (79.0) = 81.0, which matches the prompt.

🧠 Exam Technique

• Use the relative molecular mass clue (81.0) to confirm the formula of the hydride before writing your equation.
• Ensure stoichiometry is balanced (note the 2HCl required for every Na₂Se ). State symbols are ignored unless explicitly demanded, but formulae must be exact.

❌ Common Errors

• Failing to balance the equation correctly (forgetting the coefficient 2 in front of HCl and NaCl ).
• Writing H₂S instead of H₂Se due to misreading the element (though error carried forward applies if consistent).