Wider Definitions
There are exceptions to the definition above, and many solid chemical materials familiar on Earth (for example many silicate minerals) do not have simple formulas in which various elements that are chemically bonded to each other stand in exact and fixed ratios. These crystalline compounds are called "non-stoichiometric compounds". They vary in composition due to either the presence of foreign elements trapped within the crystal structure or a deficit or excess of the constituent elements. Such non-stoichiometric chemical compounds form most of the crust and mantle of the Earth.
Other compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which changes the ratio of elements by mass slightly.
Read more about this topic: Chemical Compound
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