WJEC A-Level Chemistry AS Unit 1, June 2025: Question 1

1 mark · Easy difficulty · Short Answer

Draw a dot and cross diagram using outer electrons only to show the bonding in a diatomic molecule of oxygen.

Practise this question

Question

Question 1 asks: 'Using outer electrons only, draw a dot and cross diagram to show the bonding in a diatomic molecule of oxygen.' A 1-mark indicator is shown on the right with blank space below for the diagram.

Mark scheme

Show the mark scheme Mark scheme for Question 1 displays a dot and cross diagram of an O2 molecule with two overlapping circles labeled 'O'. The overlap region contains two dots and two crosses representing a double covalent bond (four shared electrons). Each oxygen atom also has four non-bonding valence electrons (two lone pairs), shown as dots for one atom and crosses for the other. 1 mark under AO1.

How to answer it

Dot and Cross Diagram: Diatomic Oxygen (O₂)

What this question tests

This question assesses your fundamental understanding of covalent bonding in simple diatomic molecules, the application of the octet rule, and your ability to accurately construct dot-and-cross diagrams showing only outer-shell electrons.

Question 1 • 1 Mark (AO1)

Drawing the Covalent Bonding in O₂

Outer-shell representation of a diatomic molecule

✅ Correct Representation

The diagram must clearly show a double covalent bond (4 shared electrons) and two lone pairs (4 unshared electrons) on each oxygen atom:

Left O Atom: 4 non-bonding dots (••   ••)
Overlap Region: 2 dots and 2 crosses ( •• ×× )
Right O Atom: 4 non-bonding crosses (××   ××)
  • Total outer electrons: 12 valence electrons in total (6 dots + 6 crosses).
  • Bonding region: Exactly 4 electrons shared between the two nuclei (2 shared pairs = double bond).
  • Octet verification: Each atom has access to 8 electrons (4 shared + 4 non-bonding).
Mark Scheme Criteria [1 mark]:
• 4 shared electrons in the overlap (2 from each atom).
• 4 non-bonding electrons on each oxygen atom.
• Outer electrons only (no inner 1s² electrons shown).

💡 Key Chemical Knowledge

  • Electronic Configuration: Oxygen has an atomic number of 8: 1s² 2s² 2p⁴ . It has 6 valence electrons (Group 16 / Group 6).
  • Covalent Bond Definition: An electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonded atoms.
  • Octet Requirement: Each oxygen needs 2 more electrons to achieve a stable noble gas configuration (like Neon, 2s² 2p⁶ ).
  • Double Bond Formation: To gain 2 electrons, each oxygen atom must contribute 2 electrons to form two shared pairs (an O=O double bond).

🧠 Step-by-Step Drawing Technique

  1. Count valence electrons: Each O has 6 outer electrons. Total = 6 + 6 = 12 electrons.
  2. Draw overlapping circles: Sketch two intersecting circles and label the centres with an "O" or place an "O" in each circle.
  3. Insert the shared pairs: Place 2 dots and 2 crosses in the intersection region (representing the double bond).
  4. Distribute the remaining electrons: Each oxygen atom has 4 remaining outer electrons. Place them in pairs around the non-overlapping part of each circle (dots on the left O, crosses on the right O).
  5. Double-check: Count each circle individually to confirm both contain exactly 8 electrons.

❌ Common Errors & Pitfalls

  • Drawing a single bond: Showing only 1 pair (1 dot, 1 cross) in the middle, leaving atoms with incomplete octets (7 electrons each).
  • Forgetting lone pairs: Drawing the 4 bonding electrons but forgetting the 4 non-bonding electrons on each atom. In dot-and-cross diagrams, all outer shell electrons must be accounted for.
  • Including inner-shell electrons: Drawing the full atom with an inner circle of 2 electrons. The question explicitly stated: "Using outer electrons only".
  • Unequal electron contribution: Putting 3 dots and 1 cross in the overlap. A standard double bond requires 2 electrons from each identical atom.

Topics

Physical Chemistry · 1.4 Bonding

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.