Determine Number of Protons & Electrons in Ions

Worked Chemistry Problem

An ion has a different number of electrons from the number of protons.
An ion has a different number of electrons from the number of protons. Paper Boat Creative / Getty Images

This worked chemistry problem demonstrates how to determine the number of protons and electrons in an ion. For atomic ions, the key points to keep in mind are:

  • A neutral atom has the same number of protons and electrons. This number is the atomic number of the element.
  • A positively-charged ion or cation has more protons than electrons. The proton number is the atomic number of the element, while the electron number is the atomic number minus the charge.
  • A negatively-charged ion or anion has more electrons than protons. Again, the number of protons is the atomic number. The number of electrons is the atomic number added to the charge.


Protons and Electrons 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.

Protons and Electrons in Polyatomic Ions

When you are working with polyatomic ions (ions consisting of groups of atoms), the number of electrons is greater than the sum of the atomic numbers of the atoms for an anion and less than this value for a cation.