AAAS: THE PHYSICAL SETTING: THE STRUCTURE OF MATTER - The number of positive protons in the nucleus determines what an atom's electron configuration can be and so defines the element

The number of positive protons in the nucleus determines what an atom's electron configuration can be and so defines the element. In a neutral atom, the number of electrons equals the number of protons. But an atom may acquire an unbalanced charge by gaining or losing electrons.

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