Covalent bonds
Molecules
A A collection of two or more atoms held together by chemical bonds. consists of two or more Element that is a poor conductor of electricity and heat, and which forms acidic oxides. The smallest part of an element that can exist. joined together by A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons.. A molecule can be:
- an A substance made of one type of atom only., if its atoms are of the same non-metal
- a A substance formed by the chemical union of two or more elements., if its atoms are of different non-metals
A covalent bond is formed when a pair of electrons is shared between two atoms. These shared electrons are found in the outer shells of the atoms. In general, each atom contributes one electron to the shared pair of electrons.
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Dot and cross diagrams
A simple molecule consists of a small number of atoms joined together by covalent bonds. The bonding in these molecules can be modelled using dot and cross diagrams, in which:
- 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
For example, a hydrogen molecule forms when a hydrogen atom shares its outer electron with another hydrogen atom.
Drawing bonding structures
A simple molecule can be modelled by drawing its structure. In these structures:
- show each atom by its element symbol
- show each covalent bond as a straight line
For example, a hydrogen molecule has two H atoms and one covalent bond. The diagram shows its structure.
Some examples of models
Chemical formulae
Simple molecules contain fixed numbers of atoms. This means that the chemical formula used for a simple molecular substance is its Chemical formula showing the actual number of atoms of each element in a molecule..
A molecular formula shows the actual numbers of atoms of each element in a molecule. For example, the formula for methane is CH4. It shows that each methane molecule contains one carbon atom and four hydrogen atoms.
An explanation of covalent bonding