How to Determine Number of Protons and Electrons in Ions

Steps to Determine the Charge of an Ion

Carbon atom

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By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on September 03, 2019

The number of protons and electrons in an atom or molecule determines its charge and whether it is a neutral species or an ion. 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.

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Helmenstine, Anne Marie, Ph.D. "How to Determine Number of Protons and Electrons in Ions." ThoughtCo, Apr. 5, 2023, thoughtco.com/protons-and-electrons-in-ions-problem-609591. Helmenstine, Anne Marie, Ph.D. (2023, April 5). How to Determine Number of Protons and Electrons in Ions. Retrieved from https://www.thoughtco.com/protons-and-electrons-in-ions-problem-609591 Helmenstine, Anne Marie, Ph.D. "How to Determine Number of Protons and Electrons in Ions." ThoughtCo. https://www.thoughtco.com/protons-and-electrons-in-ions-problem-609591 (accessed June 9, 2023).
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