This worked chemistry problem demonstrates how to determine the number of protons and electrons in an ion.
Identify the number of protons and electrons in the Sc3+ ion.
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.
The Sc3+ ion contains 21 protons and 18 electrons.