Clearly even 12 atoms are too few because atoms themselves are so small. What we need, then, is a number that represents a convenient quantity of atoms so we can relate macroscopic quantities of substances. If a sample contains 40 g of Ca, this sample has the same number of atoms as there are in a sample of 7 g of Li. So we have established that the masses of atoms are constant with respect to each other, as long as we have the same number of each type of atom. For example, the ratio of the masses of silicon atoms to equal numbers of hydrogen atoms is always approximately 28:1, while the ratio of the masses of calcium atoms to equal numbers of lithium atoms is approximately 40:7. The same consistency is seen when ratios of the masses of other elements are compared. As long as we have equal numbers of hydrogen and oxygen atoms, the ratio of the masses will always be 16:1. If we have 100 atoms of each element, the ratio of the masses is approximately 1,600:100, which again reduces to 16:1. If we have 12 atoms of each element, the ratio of their total masses is approximately (12 × 16):(12 × 1), or 192:12, which also reduces to 16:1. If we have 2 atoms of each element, the ratio of their masses is approximately 32:2, which reduces to 16:1-the same ratio. The ratio of the mass of an oxygen atom to the mass of a hydrogen atom is therefore approximately 16:1. An oxygen atom has a mass of approximately 16 u. How can we keep track of so many atoms (and molecules) at a time? We do it by using mass rather than by counting individual atoms.Ī hydrogen atom has a mass of approximately 1 u. As stated in the introduction, we deal with billions of atoms at a time. One problem we have, however, is that it is extremely difficult, if not impossible, to organize atoms one at a time. The ratio of hydrogen atoms to oxygen atoms used to make water molecules is always 2:1, no matter how many water molecules are being made.
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