The smallest part of a substance that can exist on its own is a molecule. This consists of anything between two and several thousand atoms, which are linked together by chemical bonds. Sometimes the molecule consists of identical atoms, and it is called an element. When a group of different types of atom is connected together in this way, it forms a compound.
The way in which the atoms become linked depends on the types of chemical bonds. This, in turn, determines the shape the molecule and its chemical properties.
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