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- Describes a material for which the chemical formulaA represention of the elemental composition of a substance; subscripts are used to indicate the relative numbers of atoms of each kind of element present. deviates from small whole number ratios of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume.. For example, copper(I) sulfide occurs in the non-stoichiometricDescribes a material for which the chemical formula deviates from small whole number ratios of atoms. For example, copper(I) sulfide occurs in the non-stoichiometric form djurleite, which has the formula Cu1.96S instead of the stoichiometric Cu2S. form djurleite, which has the formula Cu1.96S instead of the stoichiometric Cu2S.