The Story of Atomic Mass
<p>Let’s start with how they got the atomic mass before the periodic table was created. Around the year 1811, <strong>Amedeo Avogadro</strong> compared the masses of two different gases of equal volumes under the same temperature and pressure. He noticed that the masses were different, and hypothesized that there were the same number of atoms/molecules in both containers. This way the only cause for difference in mass should be that they both had different atomic masses for the individual atom/molecule. Worldwide, all the scientists agreed with this hypothesis and used it to calculate the atomic mass of various atoms by comparing it to the mass of the same volume of hydrogen under the same temperature and pressure. Essentially it is the ratio of the masses between one container of gas (whose atomic mass is unknown) and one container containing an equal volume of hydrogen gas under the same conditions. By this method they figured the atomic mass of oxygen is 15.99 times the mass of hydrogen atom. The unit chosen for this is amu, where 1 amu is the mass of hydrogen atom and 15.99 amu is the mass of oxygen atom.</p>
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