Learn that one electron in its outermost

Learn how ions are formed using the octet rule.

Use the periodic table to predict the charge an atom will have when it becomes an ion. Learn whether an ion is a cation or anion and how to write the formula depending on what charge the ion has.

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How Ions Are Formed

As we’ve learned before, atoms like to be stable. They feel most stable when their outer electron shells are full. They become full when they have eight electrons in them.

This is called the octet rule, which says that atoms like to have full valence shells of eight electrons. Remember that the valence electrons are the electrons in the outermost energy shell of an atom. They get eight electrons by either borrowing some from or giving some to another atom.

Let’s look at how this works.

Atoms get eight valence electrons by giving electrons to another atom or by accepting electrons from another atom.
Ions are written with the superscript of the number of electrons they have gained or lost.

You can predict the charge of an ion by looking at its group number on the periodic table. Groups IA, IIA and IIIA all lose electrons and become positively charged. Groups VA, VIA and VIIA all gain electrons and become negatively charged.

Learning Outcomes

After this lesson, you should be able to:

  • Explain how ions are formed
  • Define cation and anion
  • Predict the charge of an ion by finding its group on the periodic table

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