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Science

Covalent bonding

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A covalent bond is a strong bond between two non-metal atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ]. It consists of a shared pair of electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ]. A covalent bond can be represented by a straight line or dot-and-cross diagram.

Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

A shared pair of electrons

You will need to understand what covalent bonding is, and to remember some of the properties of molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance. ] that are formed in this way.

A covalent bond forms when two non-metal atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] share a pair of electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ]. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.

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

After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level - so it is stablestable: Atoms are stable if their outer shell contains its maximum number of electrons.. The hydrogen atom is now in contact with two electrons in its highest energy level - so the hydrogen is also stable.

How many bonds?

Atoms may form multiple covalent bonds - that is, 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 (you can brush up on group numbers by reading through the section in AQA GCSE Science on the Periodic Table). The table below gives more detail on this rule:

 

 Group 4Group 5Group 6Group 7
ExampleCarbonNitrogenOxygenChlorine
Number of bonds8 - 4 = 48 - 5 = 38 - 6 = 28 - 7 = 1

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

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