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molecule (noun) (chemistry) smallest particle of a compound.
A molecule is the smallest unit consisting of a group of atoms, into which a substance can be divided without a change in its chemical properties. A molecule of water, for example, is the smallest particle of water which has the properties of water, namely a compound of oxygen and hydrogen.
- Water molecules collected on the outside of the glass, when ice was put in the glass.
- The molecules of the chloride collected on underside of the lid, when the substance began to evaporate.