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You need to know the charge on the atom or ion. The mass number is completely irrelevant if you have the atomic number.

The atomic number tells you the number of protons in the nucleus.

If the atom is neutral, then it has as many protons as electrons, and so the number of electrons equals the atomic number

If the atom is charged, and is thus called an ion, than take the atomic number and subtract the charge from it to give the number of electrons.

For instance, if you have a sodium ion with charge +1 (Na+), then the number of protons is equal to 11 (the atomic number), and the number of electrons is

11 - 1 = 10

If instead you had a chlorine ion with charge -1 (Cl-), then the number of protons is equal to 17 (the atomic number), and the number of electrons is

17 - -1 = 18

See the Related Questions to the left for more information about how to count the particles of an atom.

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