One isotope of a metallic element has mass number 55 and 30 neutrons in the nucleus. The cation derived from the isotope has 18 electrons. Write the symbol for this cation

Atomic mass is the number of neutrons + number of protons.

Number of protons must be equal to the number of electrons in an atom.

In a cation, the atom lost some electrons, but the number of protons remained the same.

So,
1. Find the number of protons.
2. From that, you can guess how many electrons were removed.
3. You'll have the charge of the cation with that.

So would it be Mn^7+?

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