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Atoms in nature are generally electrically neutral, as they have an equal number of protons in the nucleus and electrons orbiting the nucleus. However, within the nucleus there are other particles called neutrons, which are electrically neutral but have about the same mass as protons. There are two numbers used to characterize a nucleus:
the atomic number, which equals the number of protons in the nucleus;
the mass number, which equals the number of nucleons (protons plus neutrons) in the nucleus.
An element X is defined by the atomic number Z, while A denotes the particular isotope of that element. The usual notation for an element X is AZX. For example, there are four common isotopes of Carbon: 11 *1.5mm6C, 12 *1.5mm6C, 13 *1.5mm6C, and 14 *1.5mm6C, with 12 *1.5mm6C being the most abundant (> 98%). Notice that each isotope of a particular element has the same Z but different A.