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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.
Atomic number describes the number of protons in one atom of that element. For example, an atom of oxygen contains 8 protons. Elements are listed in order of increasing atomic number from left to right. Each row of the periodic table is called a period and each column of the periodic table is called a group (or family).