Write true or false for the following statements:

Each isotope of an element is an impure substance.

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One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12^th) the mass of one atom of carbon-12.

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The relative atomic masses of all elements have been found with respect to an atom of oxygen-16.

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The relative number and kinds of atoms are varying in a given compound.

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An atom of each element has a definite combining capacity, called its valency.