Exam Questions

Dot and Cross Diagrams of Simple Molecules

Ah Cher
By Ah Cher
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Dot the i's and cross the t's of your dot-and-cross diagrams

Common Exam Questions on Dot and Cross Diagrams

Practise how to draw dot and cross diagrams for molecules that are commonly featured in schools exams and at the O Level.

Q1. Draw a dot and cross diagram to show the bonding in water, H2O.

Dot and cross diagram of water
  1. Place hydrogen at the ends and oxygen at the centre, as hydrogen can only form 1 bond and hence cannot be the central connector
  2. Each hydrogen forms a single bond with oxygen, which is represented by a pair of dot and cross
  3. After bonding, hydrogen has no other electron, while oxygen has 4 non-bonding electrons
Q2. Draw a dot and cross diagram to show the bonding in methane, CH4.

Dot and cross diagram of methane
  1. Like in water, place hydrogen at the ends and carbon at the centre
  2. Each hydrogen forms a single bond with carbon, which is represented by a pair of dot and cross
  3. After bonding, hydrogen and carbon alike have no other electron
Q3. Draw a dot and cross diagram to show the bonding in hydrogen chloride gas, HCl.

Dot and cross diagram of hydrogen chloride
  1. Each hydrogen forms a single bond with chlorine, which is represented by a pair of dot and cross
  2. After bonding, hydrogen has no other electron, while chlorine has 6 non-bonding electrons
Q4. Draw a dot and cross diagram to show the bonding in carbon dioxide, CO2.

Dot and cross diagram of carbon dioxide
  1. Place carbon at the centre and oxygen at the ends
  2. Each oxygen forms double bond with carbon, which is represented by 2 pairs of dot and cross
  3. After bonding, carbon has no other electron, while oxygen has 4 non-bonding electrons
Q5: Challenging Question from Prelim Paper

A non-metallic element Z in the third period forms a bromide with the formula ZBr3. Using the formula, infer the bonding in ZBr3 by drawing a dot-and-cross diagram showing the valence electron only.

Dot and cross diagram of ZBr3, which is likely to be phosphorous bromide, PBr3

Before picking up your pencil to draw, consider if the bond is covalent or ionic. Because both Z and bromine are non-metallic elements, they form a covalent bond together.

  1. Place Z at the centre and bromine at the ends
  2. Since bromine has a valency of 1, each bromine atom can only form a single bond with Z, which is represented by a pair of dot and cross
  3. After bonding, Z only has 6 electrons. Assuming that Z would have achieved the electronic configuration of a noble gas, we infer that Z must have 2 more non-bonding valence electrons.

Therefore, Z is from Group VI, as it has 5 valence electrons. 3 are used in covalent bonding, while 2 remain as non-bonding electrons in ZBr3.

Marking Points

  • Correct number of shared electrons and non-bonding electrons
  • Correct relative size of valence shell (like how the valence shell of chlorine is bigger than that of hydrogen)
  • Legend included
Notes on covalent bonding

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