The name indicates the mass of an atom, expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass in always very close to the mass (or nucleon) number, which indicates the number of particles within the core of an atom; this means the protons and neutrons. Each isotope of a chemical element can vary in mass. The atomic mass of an isotope indicates the number of neutrons that are present within the core of the atoms. The total atomic mass of an element is an equivalent of the mass units of its isotopes. The relative occurrence of the isotopes in nature is an important factor in the determination of the overall atomic mass of an element. In reference to a certain chemical element, the atomic mass as shown in the periodic table is the average atomic mass of all the chemical element’s stable isotopes. The average is weighted by the relative natural abundances of the element’s isotopes.
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