Assorted chemical weights. The different amount of each chemical represents a measurement known as the mole. One mole of any sample contains the same number of molecules or atoms. The weight (shown in grams) of each molar sample is determined by the weight of the molecules (the molecular weight) and is calculated from the weights of the atoms in the molecule. Three of the chemicals are hydrated with water (H2O). The chemical formulae are, clockwise from lower left: NaCl, FeCl3.6(H2O), CuSO4.5(H2O), KI, Co(NO3)2.6(H2O) and KMnO4. The heaviest atom here is iodine (I), 8 times heavier than oxygen (O). The transition metal compounds are coloured.