Cation vs Anion: Definition, Chart and the Periodic Table

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Introduction to ions. Credit: Khan Academy via YouTube

If an atom, or atoms, has a balanced number of electrons (negative charge) and protons (positive charge) they are neutral overall. However, if they are not balanced, they will be charged. These charged species are called ions.

What is a cation?

A cation has more protons than electrons, consequently giving it a net positive charge. For a cation to form, one or more electrons must be lost, typically pulled away by atoms with a stronger affinity for them. The number of electrons lost, and so the charge of the ion, is indicated after the chemical symbol, e.g. silver (Ag) loses one electron to become Ag⁺, whilst zinc (Zn) loses two electrons to become Zn²⁺.

Figure 1: The generation of a cation. Credit: Technology Networks. 

What is an anion?

An anion has more electrons than protons, consequently giving it a net negative charge. For an anion to form, one or more electrons must be gained, typically pulled away from other atoms with a weaker affinity for them. The number of electrons gained, and so the charge of the ion, is indicated after the chemical symbol, e.g. chlorine (Cl) gains one electron to become Cl^-, whilst oxygen (O) gains two electrons to become O^2-.

Figure 2: The generation of an anion. Credit: Technology Networks.

Cation vs anion chart

The main differences between cations and anions are summarized in the table below.

Metallic atoms hold some of their electrons relatively loosely. Consequently, they tend to lose electrons and form cations. Conversely, most nonmetallic atoms attract electrons more strongly than metallic atoms, and so gain electrons to form anions. Therefore, when atoms from a metallic and a nonmetallic element combine, the nonmetallic atoms tend to draw one or more electrons away from the metallic atoms to form ions. These oppositely charged ions then attract one other to form ionic bonds and produce ionic compounds with no overall net charge. Examples include calcium chloride (CaCl₂), potassium iodide (KI) and magnesium oxide (MgO).

Cation vs anion periodic table

It can be possible to predict whether an atom will form a cation or an anion based on its position on the periodic table. Halogens always form anions, alkali metals and alkaline earth metals always form cations. Most other metals form cations (e.g. iron, silver, nickel), whilst most other nonmetals typically form anions (e.g. oxygen, carbon, sulfur). However, some elements are capable of forming both cations and anions given the right conditions. One example is hydrogen, which may gain (H^-) or lose (H⁺) an electron, forming hydride compounds such as ZnH₂ (where it is an anion) and hydron compounds such as H₂O (where it is a cation).

Figure 3: The periodic table. Credit: Technology Networks.

Cation vs anion size

Cations and anions come in many sizes across the periodic table.

Utilizing ionic properties

Ionic properties can be exploited by chemists for a range of purposes. Ion-exchange chromatography for example relies on the affinity of the molecules being separated for the stationary phase based on their charge properties to enable separation.

Figure 4: A zinc-carbon dry cell battery (left) and alkaline battery (right). Credit: Technology Networks

In rechargeable batteries, such as many lithium-ion batteries, this chemical process is reversible and the internal structure different which allows the batteries to be recharged.

Due to the ionic properties of salt water, scientists are now striving to exploit the ionic electricity-generating potential of salinity gradients where salt water and fresh water mix as a green source of energy generation for the future.