An Etymological Dictionary of Astronomy and Astrophysics
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The number of → protons in an → atomic nucleus (symbol Z). Same as → Z-number. The atomic number is written as a subscript to the left of the → chemical element name. For example, the most common isotope of oxygen is shown as ₈¹⁶O, which has 8 → protons and its → mass number (A) is 16.

→ atomic; → number.