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Cations and Anions

Core Concepts

What are ions? In this tutorial, you will learn about the properties, differences, and examples of ions, cations and anions, as well as how to predict them based on their positions on the periodic table.

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Introduction to Ions, Cations and Anions

What is an Ion?

The definition of an ion is a particle, atom, or molecule with an imbalance of electrical charge. Ions are charged. They contain different numbers of protons and electrons. Ions form when atoms move into a more stable electron configuration. Ions are identified by a superscript that shows the sign and size of the electric charge – for example Ca+2. There are two types of ions: cations and anions.

A cation has a net positive electrical charge, which means it has more protons than electrons.

An anion has a net negative electrical charge, which means it has more electrons than protons.

Cations

A cation is an ion that has lost one or more electrons, giving a net positive charge. Because one or more electrons are removed to form a cation, the cation of an atom is smaller than the neutral atom.

Examples of cations include the following:

  • Calcium: Ca2+
  • Silver: Ag+
  • Aluminum: Al3+
  • Hydronium ion: H3O+
  • Ammonium: ion NH4+
  • Mercurous ion: Hg2+2

Anions

An anion is an ion that has gained one or more electrons, giving a net negative charge. Because electrons are added to form an anion, the anion of an atom is bigger than the neutral atom.

Examples of anions include the following:

  • Chlorine: Cl
  • Hydroxide: OH
  • Iodide: I
  • Dichromate: Cr2O7-2
  • Oxide anion: O-2
  • Sulfate anion: SO4-2

Predicting Cations and Anions based on the Periodic Table

Whether an atom forms a cation or an anion depends on its position on the periodic table. Group 1A and 2A of the periodic table, alkali metals and alkaline earth metals respectively, always form cations. In contrast, Group 17A, which consists of halogens, always forms anions.

Most metals (e.g., iron, lead, gold) form cations, whereas most nonmetals (e.g., oxygen, nitrogen, sulfur) form anions.

Ions: Writing Chemical Formulas

When writing the chemical formula of a compound, cation always comes before anion. For example, in NaBr, sodium is the cation, while bromine is the anion.

Examples:

  • NaCl – cation: Na+, anion: Cl
  • LiF – cation: Li+, anion: F
  • Mg(OH)2 – cation: Mg2+, anion: OH
  • K2S – cation: K+, anion: S2-
  • BeBr2 – cation: Be2+, anion: Br

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