Law of conservation of matter

:See also: Law of Conservation of Mass

Atoms

According to atomic theory, we could use the number of atoms, not mass, as a measure of matter.

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This way, the Law can expressed as a stoichometric balance, that is:

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The number of atoms of a particular element in the reactants must equal the number of those atoms in the products.

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If we believe that each atom has a specific mass, and that the sum of the atoms' masses is the equal to the total mass of the atoms; then this is the same as Lavoisier's law. This rule applies to classical mechanics. However, it does not apply to relativistic physics. At relativistic energies atoms cannot be regarded as "atoms" (in the Greek sense).

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