Monatomic Ion Definition and Examples   Recently updated !


Monatomic Ions
A monatomic ion is an ion with only one atom.

A monatomic ion is an ion made of exactly one atom. In other words, it is an atom that has an unequal number of protons and electrons. If there are more protons than electrons, the ion has a net positive charge and is a cation. If there are more electrons than protons, the ion has a net negative charge and is an anion. Typically, metals form monatomic cations, while nonmetals form monatomic anions.

Monatomic Ion vs Polyatomic Ion

A polyatomic ion contains more than one atom, even if these atoms consist of the same element. For example, O2 is a polyatomic ion (more accurately, a diatomic ion).

Note some dictionaries define “monatomic” as referring to one kind of atom rather than one atom. This can lead to confusion!

List of Monatomic Cations

Here are examples of common monatomic cations.

NameSymbol
HydrogenH+
LithiumLi+
SodiumNa+
PotassiumK+
RubidiumRb+
CesiumCs+
MagnesiumMg2+
CalciumCa2+
StrontiumSr2+
BariumBa2+
AluminumAl3+
SilverAg+
ZincZn2+
Iron(II)Fe2+
Iron(III)Fe3+
Copper(I)Cu+
Copper(II)Cu2+
Lead(II)Pb2+

Note that the alkali metals form cations with a +1 charge and alkali metals form cations with a +2 charge. While most monatomic ions are metals, there are also monatomic ions of metalloids and nonmetals.

List of Monatomic Anions

Nonmetals form most monatomic anions. Hydrogen is an example of an element that forms both a monatomic cation and a monatomic anion. Here is a list of common monatomic anions.

NameSymbol
HydrideH
FluorideF
ChlorideCl
BromideBr
IodideI
OxideO2-
SulfideS2-
NitrideN3-
PhosphideP3-

How to Name Monatomic Ions

The naming rules differ for monatomic cations and monatomic anions.

Monatomic cations go by their element names. The name often includes charge. This is important when an atom forms more than one cation. For example, when speaking, H+ is called “H plus or H plus one” or “hydrogen plus or hydrogen plus one.” The calcium monatomic ion (Ca2+) is “Ca plus two” or “calcium plus two”. When more than one charge is common, the name includes the atom’s oxidation state. For example, monatomic copper ions are copper(I) or Cu(I) and copper(II) or Cu(II).

Monatomic anions don’t usually have variable charges, so naming is simpler. The name is the first part of the element name, followed by the -ide suffix. So, H is hydride, N3- is nitride, and so on.

How Monatomic Ions Form

Atoms naturally form monatomic ions during chemical reactions, when ionic compounds melt, and when electrolytes dissociate in water. For example, NaCl forms the monatomic ions Na+ and Cl- when it dissolves in water. Atoms form this monatomic ions so they have the most stable electron configuration. Monatomic ions tend toward having half-filled or completely-filled valence electron shells.

Adding energy also forms monatomic ions. Either it breaks the bonds between atoms in a molecule or it lets electrons break free of atoms (ionizes them). These monatomic ions may be highly reactive and short-lived.

Monatomic Ion vs Monatomic Element

Yes, it’s true a monatomic ion is an ion of an element. But, the terms monatomic ion and monatomic element have different meanings.

A monatomic element is an element that consists of electrically neutral single atoms. Familiar examples of monatomic elements are noble gas atoms.

References

  • Cillispie, Charles (ed.) (1970). Dictionary of Scientific Biography (1st ed.). New York City: Charles Scribner’s Sons. ISBN 978-0-684-10112-5.
  • Knoll, Glenn F. (1999). Radiation Detection and Measurement (3rd ed.). New York: Wiley. ISBN 978-0-471-07338-3.
  • Masterton, William; Hurley, Cecile (2008). Chemistry: Principles and Reactions. Cengage Learning. ISBN 0-495-12671-3.