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The definition of an atomic number is the number of protons in the nucleus of an atom of an element. Three for lithium and 20 for calcium are each an example of an atomic number.
The atomic number is an important concept of chemistry and quantum mechanics. An element and its place within the periodic table are derived from this concept. When an atom is generally electrically neutral, the atomic number will equal the number of electrons in the atom, which can be found around the core.