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One of the postulates of Dalton's Atomic Theory.
What is any one of 1) Atoms are composed of tiny indivisible particles called atoms, 2) Atoms of the same element are identical and different from the atoms of any other element, 3) Atoms of different elements can combine in simple whole-number ratios to form compounds, or 4) Chemical reactions are simply the rearrangement of atoms (not the changing of one type of atom into another).