![]() The average atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. To take into account the natural isotopic composition in calculations related to the atomic mass of elements, the average atomic mass is used. Note that these are non-SI units that are accepted for use with the SI.Ībout two-thirds of the stable elements occur naturally on Earth in the form of a few stable isotopes. It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 at rest and in its nuclear and electronic ground state. The atomic mass is commonly measured using the dalton ( Da) or unified atomic mass unit ( u or sometimes amu). Small corrections are due to the binding energy of the nucleus (the so called mass defect), the small difference in mass between the proton and neutron, and the mass of the electrons bound to the atom. The atomic mass of an isotope is determined mainly by its mass number (i.e., the number of nucleons in its nucleus). The atomic number of oxygen is 8, which means that every oxygen atom has 8 protons so that the neutron numbers of these isotopes are 8, 9, and 10 respectively. Isotopes are two or more types of atoms that have the same number of protons and differ only in the number of neutrons in their nuclei.įor example, oxygen-16, oxygen-17, and oxygen-18 are three isotopes of the element oxygen with mass numbers 16, 17, and 18, respectively. It is known that most chemical elements occur in nature as a mixture of two or more isotopes. ![]()
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