Protons & Electrons in Ions

This worked chemistry problem demonstrates how to determine the number of protons and electrons in an ion.

Problem

Identify the number of protons and electrons in the Sc³⁺ ion.

Solution

Use the Periodic Table to find the atomic number of Sc (scandium). The atomic number is 21, which means that scandium has 21 protons.
While a neutral atom for scandium would have the same number of electrons as protons, the ion is shown to have a +3 charge. This means it has 3 fewer electrons than the neutral atom or 21 - 3 = 18 electrons.

Answer

The Sc³⁺ ion contains 21 protons and 18 electrons.