Cations and anions are the two types of ions. Ions have an imbalance of electrical charge, meaning they contain different numbers of protons and electrons. Cation have a positive electrical charge and have more protons than electrons. Anions have a negative electrical charge and have more electrons than protons. Neutrons are electrically neutral, so their number determines the isotope, but has no effect on whether a chemical species is an ion.
A Closer Look at Cations
Cations are ions that have a positive charge. The word “cation” comes from the Greek word ánō, which means “up.” Examples of cations include:
- Silver: Ag+
- Hydronium: H3O+
- Ammonium: NH4+
Because an electron is removed to form a cation, the cation of an atom can be smaller than the neutral atom. This is because removing one or more electrons might involve removing an entire electron shell.
A Closer Look at Anions
Anions are ions with a negative charge. The word “anion” comes from the Greek word káto, meaning “down.” Examples of anions include:
- Hydroxide anion: OH–
- Oxide anion: O2-
- Sulfate anion: SO42-
Electrons are added to form anions, so they may be larger than neutral atoms if another electron shell forms.
Remember Cation and Anion
There are a couple of simple mnemonics used to remember a cation is positive and an anion is negative. First, you can use the letters of the words. The “t” in “cation” is like a plus symbol. The letters in the word “anion” can stand for “A Negative Ion.” A pun to remember the difference is “CATions are PAWSitive.”
Writing Chemical Formulas
The chemical formula of a compound is always written with the cation first, followed by the anion. For example, Na is the cation and Cl is the anion in NaCl (table salt). The same convention applies to chemical names. The chemical name of table salt is sodium chloride. This works for polyatomic ions, too. Ammonium hydroxide is NH4OH, where NH4+ is the cation and OH– is the anion.
Cations and Anions on the Periodic Table
Technically, any atom or molecule can form both cations and anions. For example, a hydrogen atom usually has a +1 oxidation state, but sometimes it gains an electron and has a -1 charge! That being said, metals usually form cations, while nonmetals usually form anions. To put it another way, elements on the left side of the periodic table tend to form cations, while those on the right side form anions. Noble gases are the exception. They are sufficiently stable that they don’t form either anions or cations easily. Certain groups of the periodic table form characteristic ions:
- Alkali metals (Group 1): +1 cations
- Alkaline earth metals (Group 2): +2 cations
- Transition metals (Groups 3-12): Two or more oxidation states, usually differing by one. For example, copper forms +1 and +2 cations.
- Boron family (Group 13): +1 or +3 cations
- Carbon family (Group 14): -4 for carbon, but +2 going down the group
- Nitrogen family (Group 15): +3 or +5
- Oxygen family (Group 16): -2 for oxygen, but -2, +4, +6 going down the group
- Halogens (Group 17): -1
- Noble gases (Group 8): 0 (uncharged)
Dianions, Dications, and Zwitterions
There are special names for certain types of ions. An ion with a -2 charge is an anion that is also called a dianion. An ion with a +2 charge is a cation that is also called a dication. A neutral molecule that has an area of positive charge and an area of negative charge is called a zwitterion.
- Scerri, E. R. (2007). The periodic table, its story and its significance. Oxford University Press. ISBN 978-0-19-530573-9.