Why calcium carbonate CaC03 is a pure substance but not an element ? 

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An element consists of atoms each of which have the same number of protons in their nucleus. A chemical element is identified by the number of protons it has, atoms can lose or acquire different number of electrons in an attempt to acquire a stable electron configuration. Atoms of the same element can also differ in the number of neutrons they have.

`CaCO_3` is the chemical symbol for calcium carbonate. This is a chemical compound each molecule of which is made up of one atom of calcium, one atom of carbon and 3 atoms of oxygen. Calcium, oxygen and carbon do not have the same number of protons in their nucleus.

A substance that is made up of only one type of particle, is called a pure substance. The particles that form the substance can be individual atoms or molecules consisting of many atoms. As calcium carbonate has only one type of particle it is a pure substance. It is not an element as the molecule is made up of atoms with different number of protons in the nucleus.