How To Draw Dot And Cross Diagrams For Covalent Compounds

Covalent bonds - OCR 21C

A covalent bond is a shared pair of electrons. Covalent bonding results in the formation of molecules. Simple molecular substances have low melting and boiling points, and do not conduct electricity.

Dot and cross diagrams

A dot and cross diagram can model the bonding in a simple molecule :

the outer shell of each atom is drawn as a circle
circles overlap where there is a covalent bond
electrons from one atom are drawn as dots, and electrons from another atom as crosses

Example 1 - Hydrogen chloride

Write down the formula HCl. This explains which atoms you need to draw (1 × H and 1 × Cl).
Sketch a simple displayed formula to work out the order in which the atoms are connected (H-Cl).
Work out how many electrons are in the outer shell of each atom (H has 1 and Cl has 7).
Draw circles to represent the electron shell of each atom overlapping the circles where the atoms are bonded.
Add dots to represent the outer electrons of one type of atom (H).
Add crosses to represent the outer electrons of the other type of atom (Cl).
Make sure the electrons are always in pairs. A bond is a dot and a cross. Other pairs should be two dots or two crosses.

Dot and cross diagram of hydrogen chloride.

Example 2 - Water

The formula is H ₂ O so the dot and cross diagram includes two H atoms and one O atom.
H has 1 outer electron. O has 6 outer electrons.
The H circles must each overlap the O circle.

Question: Draw the dot and cross diagram for ammonia, NH ₃ . Nitrogen has five outer electrons because it is in group 5.

Question: Draw the dot and cross diagram for methane, CH ₄ . Carbon has four outer electrons.

Double and triple bonds

Some molecules contain a double bond , which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms.

Table of oxygen and carbon dioxide dot and cross diagrams

Nitrogen molecules consist of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons.