Dot and Cross Diagrams of Important Molecules

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

Most examination questions will only require you to draw the outer shell electrons. As such, the answers to the practice questions below only illustrate the outer shell electrons and omit the inner shell electrons.

Q1. Draw a dot and cross diagram to show the bonding in hydrogen, H2.
Dot and cross diagram of hydrogen molecule, whereby there is a pair of shared electrons represented by a dot and a cross in the overlapping region of the electron shells
Dot and cross diagram of a hydrogen molecule

A hydrogen atom has 1 electron only and requires another electron to gain the electronic configuration of the noble gas helium.

Therefore, a hydrogen atom shares its only electrons with another hydrogen atom. After bonding, each hydrogen atom has 2 valence electrons.

The shared pair of electrons is considered a single covalent bond. It holds the two hydrogen atoms together to form a hydrogen molecule.

Q2. Draw a dot and cross diagram to show the bonding in oxygen, O2.
Dot and cross diagram of hydrogen molecule, whereby there are two pairs of shared electrons represented by two dots and two crosses in the overlapping region of the electron shells
Dot and cross diagram of an oxygen molecule

As oxygen is in Group VI with 6 valence electrons, each oxygen atom needs 2 more electrons to achieve the electronic configuration of the noble gas

The two oxygen atoms form a double bond, whereby each contributes 2 electrons for sharing, which are represented by the 2 pairs of dot and cross

After bonding, each oxygen atom is left with 4 non-bonding electrons

Q3. Draw a dot and cross diagram to show the bonding in water, H2O.
Dot and cross diagram of a water molecule, comprising 2 hydrogen atoms and 1 oxygen atom
  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
Q4. Draw a dot and cross diagram to show the bonding in methane, CH4.
Dot and cross diagram of a methane molecule, comprising 1 carbon atom and 4 hydrogen atoms
  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
Q5. Draw a dot and cross diagram to show the bonding in carbon dioxide, CO2.
Dot and cross diagram of a carbon dioxide molecule, comprising 1 carbon atom and 2 oxygen atoms
  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

Deducing Dot and Cross Diagrams

Q6. Draw a dot and cross diagram to show the bonding in hydrogen chloride gas, HCl.
Dot and cross diagram of a hydrogen chloride molecule, comprising 1 hydrogen atom and 1 chlorine atom
  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
Q7: 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