The atomic number is the number of protons in the nucleus of an atom. First, electrons are removed from or added to atoms or molecules, thus producing charged particles called ions. In the modern periodic table, the elements are listed in order of increasing atomic number. The technique is conceptually similar to the one Thomson used to determine the mass-to-charge ratio of the electron. Scientists can measure relative atomic masses very accurately, however, using an instrument called a mass spectrometer. Although the difference in mass is small, it is extremely important because it is the source of the huge amounts of energy released in nuclear reactions.īecause atoms are much too small to measure individually and do not have charges, there is no convenient way to accurately measure absolute atomic masses. For example, the ratio of the masses of 1H (hydrogen) and 2H (deuterium) is actually 0.500384, rather than 0.49979 as predicted from the numbers of neutrons and protons present. Each element is identified by the number of protons in its atoms. Once the masses of atoms were determined, the amu could be assigned an actual value:ġ amu = 1.66054 x 10 -24 grams conversely: 1 gram = 6.02214 x 10 23 amuĪlthough the masses of the electron, the proton, and the neutron are known to a high degree of precision ( Table 2.3.1), the mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons. There are 118 elements on the periodic table. Thus, the mass of the hydrogen atom ( 1H) is 1.0080 amu, and the mass of an oxygen atom ( 16O) is 15.995 amu. (Image credit: Karl Tate, contributor) Scientists. The atomic mass unit ( amu) was not standardized against hydrogen, but rather, against the 12C isotope of carbon ( amu = 12). The classic Periodic Table organizes the chemical elements according to the number of protons that each has in its atomic nucleus. As we saw earlier, it is convenient to use a reference unit when dealing with such small numbers: the atomic mass unit. We now know that a hydrogen atom has a mass of 1.6735 x 10 -24 grams, and that the oxygen atom has a mass of 2.6561 X 10 -23 grams. It has a unique arrangement of rows and columns. Thus, oxygen was assigned an atomic mass of 16. The periodic table is a way of organising the elements which is used by scientists to group elements with similar properties. Hydrogen, the lightest element, was assigned a relative mass of '1', and the other elements were assigned 'atomic masses' relative to this value for hydrogen.

Atomic table