Chemistry - 1G covalent bonding

  • Created by: Dan_23
  • Created on: 27-05-21 20:37

Covalent bonds

covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells of the atoms. An atom that shares one or more of its electrons will complete its outer shell.

Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

The slideshow shows a covalent bond being formed between a hydrogen atom and a chlorine atom, to form hydrogen chloride.

A hydrogen atom with one cross electron and a chlorine atom with 17 dot atoms.

1. A hydrogen atom with one electron and a chlorine atom with 17 electrons

After bonding, the chlorine atom is now in contact with eight electrons in its outer shell, so it is stable. The hydrogen atom is now in contact with two electrons in its outer shell, so it is also stable.

Both nuclei are strongly attracted to the shared pair of electrons in the covalent bond, so covalent bonds are very strong and require a lot of energy to break.

How many bonds?

Atoms may form multiple covalent bonds - they share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.

There is a quick way to work out how many covalent bonds an element will form. The number of covalent bonds is equal to eight minus the group number. The table gives more detail on this rule:

ElementNumber of bonds Group 4 Carbon 8 - 4 = 4 Group 5 Nitrogen 8 - 5 = 3 Group 6 Oxygen 8 - 6 = 2 Group 7 Chlorine 8 - 7 = 1

Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any.

Dot and cross models

Dot and cross models show how a pair of electrons form a covalent bond. Notice that in the diagrams, only the electrons in the outer shell of each atom are shown.

Examples of dot and cross models

Table containing dot and cross models of hydrogen, chlorine, methane, water, carbon dioxide, oxygen and hydrogen chloride.

Example of covalent bonding - carbon dioxide

One carbon atom with four cross electrons, and two oxygen atoms with six dot electrons.

1. One carbon atom with four cross electrons, and two oxygen atoms with six dot electrons

Simple molecules

Covalently bonded substances fall into two main types:

  1. simple molecules
  2. giant covalent structures

Simple molecules contain only a few atoms held together by covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.

Dot and cross model for carbon dioxide. There are two shared groups of electrons, each with two dots and two crosses.

However, although the covalent bonds holding the atoms together in a simple molecule are strong, the intermolecular forces between simple molecules are weak.

Properties of simple molecular substances

  • Low melting and boiling points - this is because little energy is needed to break the weak intermolecular forces.
  • Do notconductelectricity - this is because they do not


No comments have yet been made