Home · A-Level Chemistry 9701 · Chemical bonding · Dot-and-cross diagrams · Practice
AS · Practice questions · Dot-and-cross diagrams

Dots, crosses, checked.

Six original Cambridge-style questions on dot-and-cross diagrams: ionic compounds, simple and multiple covalent bonds, dative bonds, ions and electron counting.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Outer electrons only, then check the count.

01
Analysis
[2 marks]

Draw a dot-and-cross diagram for magnesium chloride, MgCl2. Show outer electrons only.

  • [Mg]2+ drawn with no outer electrons (or its full second shell), in square brackets. ✓
  • Two separate [Cl]− ions, each with seven dots and one cross (from Mg), in square brackets with a 1− charge. ✓
02
Analysis
[2 marks]

Draw a dot-and-cross diagram for a molecule of ammonia, NH3, and state the number of lone pairs on nitrogen.

  • Three shared pairs between N and the H atoms, each with one dot (N) and one cross (H). ✓
  • One lone pair (two dots) on N, so N has eight outer electrons; one lone pair. ✓
03
Analysis
[2 marks]

Draw a dot-and-cross diagram for ethene, C2H4.

  • Two shared pairs between the carbon atoms (a double bond). ✓
  • Each carbon also shares one pair with each of two H atoms; no lone pairs; each carbon has eight outer electrons. ✓
04
Analysis
[3 marks]

Draw a dot-and-cross diagram for the oxonium ion, H3O+, showing clearly the dative covalent bond.

  • Total outer electrons: 6 + 3 − 1 = 8, so three bonding pairs and one lone pair on O. ✓
  • Two O–H pairs with one dot and one cross each; the third O–H pair with both electrons from oxygen (the dative bond). ✓
  • Square brackets and a + charge around the whole ion. ✓
05
Analysis
[2 marks]

Draw a dot-and-cross diagram for carbon monoxide, showing that the molecule contains a dative covalent bond.

  • Three shared pairs between C and O: two with one electron from each atom, and one with both electrons from O. ✓
  • One lone pair on C and one on O, so each atom has eight outer electrons. ✓
06
Analysis
[2 marks]

A student draws the dot-and-cross diagram of the hydroxide ion, OH−, with oxygen having two lone pairs. Identify the error and give the correct number.

  • Outer electrons: 6 + 1 + 1 = 8, four pairs; one is the O–H bonding pair. ✓
  • Oxygen should have three lone pairs, one of which includes the extra electron from the charge; the ion needs brackets and a − charge. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.