Difference Between Monatomic and Polyatomic Ions

Main Difference – Monatomic vs Polyatomic Ions

The terms monatomic and polyatomic describes the atomicity of molecules or ions. Atomicity is the number of atoms present in a molecule or an ion. This is the main difference between monatomic and polyatomic ions. Monatomic ions are composed of a single atom whereas polyatomic ions are composed of two or more atoms per ion. These ions can be either cations or anions. Both monatomic and polyatomic ions can form ionic compounds by having electrostatic attractions with the ions of opposite electrical charge.

Key Areas Covered

1. What are Monatomic Ions
      – Definition, Formation, Examples
2. What are Polyatomic Ions
      – Definition, Formation, Examples
3. What is the Difference Between Monatomic and Polyatomic Ions
      – Comparison of Key Differences

Key Terms: Anion, Atomicity, Cation, Covalent Bonding, Electrostatic Attractions, Element, Ion, Monatomic Anions, Monatomic Cations, Monatomic Ions, Polyatomic Ions

What are Monatomic Ions

Monatomic ions are ions containing single atom per ion. These can be either cations or anions. Some atoms form ions by removing valence electrons from their outermost electron shells. These ions are monatomic cations. Some ions are formed by gaining electrons in the outermost electron shell. These are known as monatomic anions. The following table shows monatomic ions formed by group 1 and group 17 elements.

Some elements can form only monatomic ions. But other elements can form polyatomic ions as well. Monatomic ions can have different electrical charges depending on the number of valence electrons they have. For example, Magnesium (Mg) is a group 2 element and has two valence electrons. By removing these two electrons, Mg can form Mg⁺² cation. Then it is a monatomic cation. But Noble gases never have monatomic ions since their atoms are very stable with a completed electronic configuration.

Figure 1: Ionic Bonding Between Two Monatomic Ions Forms a Diatomic Molecule

The bonding between two monatomic ions with opposite charges can form a diatomic molecule. Here, the cation and the anion are attracted to each other via electrostatic forces. For example, sodium chloride (NaCl) is made of sodium monoatomic cation and chloride monatomic anion.

What are Polyatomic Ions

Polyatomic ions are ions composed of several atoms per ion. They can be either diatomic ions, triatomic ions and vice versa. There is an uncountable number of known polyatomic ions. Some of these are present in the biological system; some are naturally occurring ions whereas others are synthetic ions. They are very useful polyatomic ions.

Figure 2: A Triatomic Ion

Atoms in the polyatomic ion are covalently bonded to each other. There can be sigma bonds as well as pi bonds. Sometimes, a polyatomic ion is formed as a coordination complex. Here, a central metal is surrounded by neutrally charged ligands. These ligands are bonded to the central metal ion via coordinate covalent bonds. Then the complex is considered as a complex ion. The electrical charge of this ion is the charge of the central metal ion.

Apart from that, there are hundreds of polyatomic ions. They can be either cations or anions depending on the overall charge of the ion. The following table gives some of them.

Difference Between Monatomic and Polyatomic Ions

Definition

Monatomic Ions: Monatomic ions are ions containing single atom per ion.

Polyatomic Ions: Polyatomic ions are ions composed of several atoms per ion.

Number of Atoms

Monatomic Ions: Monatomic ions are composed of single atom per ion.

Polyatomic Ions: Polyatomic ions are composed of several atoms per ion.

Formation

Monatomic Ions: Monatomic ions are formed by either losing or gaining electrons in the outermost shell of an atom.

Polyatomic Ions: Polyatomic ions are formed by bonding several atoms together via covalent bonds or coordinate covalent bonds.

Chemical Bonding

Monatomic Ions: There are no chemical bonds in monatomic ions.

Polyatomic Ions: There are covalent or coordinate covalent bonds in polyatomic ions.

Examples

Monatomic Ions: Examples for monatomic ions include Na⁺, K⁺, Cl^–, etc.

Polyatomic Ions: Examples for polyatomic ions include NH₄⁺, NO₂^–, NO₃^–, etc.

Conclusion

Monatomic ions and polyatomic ions are named according to the number of atoms present in those ions. The main difference between monatomic and polyatomic ions is that monatomic ions are composed of single atoms whereas polyatomic ions are composed of two or more atoms per ion.

References

1. “Monatomic Ions: Definition & Naming Convention.” Study.com, Available here.
2. 15 Diatomic Ions, Chemistry Course, Available here.
3. “Polyatomic ion.” Wikipedia, Wikimedia Foundation, 12 July 2017, Available here.

Image Courtesy:

1. “Chlorite-ion-3D-vdW” (Public Domain) via Commons Wikimedia
2. “IonicBondingRH11” By Rhannosh – Own work (CC BY-SA 3.0) via Commons Wikimedia