What is the mass number of an atom with five protons and seven neutrons

12

Explanation:

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.

Atoms can lose or gain electrons. When they do, they form charged particles called ions:

if an atom loses one or more electrons, it becomes a positively charged ion

if an atom gains one or more electrons, it becomes a negatively charged ion

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