Difference Between Monatomic and Polyatomic

Monatomic vs Polyatomic
 

The number of atoms present in a particular ion or a molecule is what contributes to the difference between monatomic and polyatomic. The two words “mono” and “poly” give the general idea about the molecule; “mono” means “single” and “poly” means “many.” Monatomic refers to the ions or molecules having a single atom. Polyatomic refers to the molecules or ions having two or more atoms. There are so many physical and chemical differences between monatomic and polyatomic due to the difference in number of atoms. Generally monatomic chemical elements are less abundant compared to the existence of polyatomic molecules.

What is Monatomic?

The word monatomic derives from two words “mono” and “atomic,” giving the meaning of “single atom.”  Monatomic chemical species contain only one atom and they are stable even if they exist alone. This can be applied to gases and ions. Most of the noble gases exist as monatomic chemical species.

Monatomic ions: These ions are formed by losing (positive ions) or gaining (negative ions) electrons.

Positive ions: Na⁺, K⁺, Ca2+, Al³⁺

Negative ions: Cl^–, S^2-, Br^–, F^–

Monatomic molecules: Noble gases fall into this category and they are very stable; therefore, chemically inactive.

What is Polyatomic?

The word polyatomic derives from two words “poly” and “atomic,” means many atoms. It can be homogeneous atoms (O₂, Hg₂²⁺, O₃, O₂^2-) or a combination of heterogeneous atoms (CN^–, H₂SO₄, ClO₃^–). Most of the molecules and ions exist as polyatomic nature.

Polyatomic ions: “Molecular ions” is another name for polyatomic ions. Most of the polyatomic ions are either covalently bonded chemical species or metallic complexes.

Positive ions: NH₄⁺, H₃O⁺, PH₄⁺

Negative ions: CrO₄^2-, CO₃^2-, CH₃COO^–, SO₄^2-, NO₃^–

Polyatomic molecules: They are the molecules with two or more atoms. They do not have a positive or negative charge. In other words, these molecules are electrically neutral. (H₂SO₄, CH₃COOH, Na₂CO₃, NaCl, C₂H₄)

What is the difference between Monatomic and Polyatomic?

• Number of atoms:

• Monatomic chemical elements contain only one atom.

• Polyatomic chemical compounds have two or more atoms.

• State:

• Monatomic chemical species can be ions or inert gases.

• Some polyatomic species are ions and some are molecules.

• Properties

• Most of the monatomic ions are stable in water.

• Monatomic molecules are very stable; therefore, chemically inactive.

• Most of the polyatomic ions are either covalently bonded or metallic complexes.

• Polyatomic molecules are electrically neutral.

• Examples for Monatomic and Polyatomic:

• Examples for monatomic ions are Na⁺, Ca²⁺, K⁺, Al³⁺ and Fe³⁺.

• Examples for monatomic molecules are noble gases. They are Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Ra).

• Examples for polyatomic ions are CrO₄²-, CO₃^2-, NH₄⁺, H₃O⁺.

• Examples for polyatomic molecules are KCl, KBrO₃, C₆H₅COOH.

• Size:

• The size of the monatomic chemical species differs according to the way they form. For example, when positive ions are formed their sizes decreases and when negative ions are formed, the size increases than the original atom. Noble gases have the smallest size compared to the other elements in their period in the periodic table.

• When polyatomic chemical compounds are formed, the size of the polyatomic ion or the polyatomic molecule becomes bigger than all the original atoms in the compound. Because, two or more atoms combine to form a polyatomic ion /molecule.

• Shape:

• Generally monatomic molecules and ions are spherical in their geometry.

• The geometry of the polyatomic chemical species varies depending on the number of molecules and the lone pairs present in the molecule. As the number of atoms increases more complex structures are formed to attain stability.

Images Courtesy:

1. Argon by Greg Robson (CC BY-SA 2.0 uk)
2. Ammonium via Wikicommons (Public Domain)