Difference Between Monatomic and Diatomic

Main Difference – Monoatomic vs Diatomic

Monatomic and diatomic are two terms used to name some chemical compounds depending on the number of atoms present in their particles. In monatomic compounds, single atoms act as particles and there are no chemical bonds between these atoms. In diatomic compounds, a particle is composed of two atoms bonded to each other. These compounds can be either homonuclear or heteronuclear depending on the types of atoms bonded. The main difference between monatomic and diatomic is that monatomic compounds are composed of single atoms whereas diatomic compounds are composed of molecules containing two atoms.

Key Areas Covered

1. What is Monoatomic
      – Definition, Explanation, Examples
2. What is Diatomic
      – Definition, Explanation, Examples
3. What is the Difference Between Monatomic and Diatomic
      – Comparison of Key Differences

Key Terms: Covalent Bond, Diatomic, Electronic Configuration, Heteronuclear, Ionic Bond, Homonuclear, Monatomic, Noble Gases

What is Monoatomic

The term monatomic is used to name substances composed of particles containing single atoms. In other words, the atoms of monatomic substances are not bonded to each other. This is because these elements are stable as single atoms. The best examples of monatomic substances are inert gases.

Inert gases or noble gases are in the group 18 of the periodic table of elements. Atoms of these elements are very stable due to their stable electronic configuration. These atoms have completely filled electron shells. Hence, they do not need to undergo any chemical reactions in order to get stabilized. These noble gases include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn).

Figure 1: Noble Gases in the Periodic Table of Elements

Furthermore, there can be monatomic particles in ionic solutions. When ionic compounds are dissolved in water, cations and anions separate from each other as aqueous ions. Then these aqueous ions act as single particles in that solution. These ions also have a completed electron configuration.

What is Diatomic

Diatomic substances are composed of molecules containing two atoms bonded to each other. These two atoms can belong to either the same element or two different elements. Depending on the type of element, diatomic substances can be either homonuclear diatomic compounds or heteronuclear diatomic compounds.

Figure 2: 3D Structure of a Diatomic Molecule

Diatomic molecules always have a linear molecular geometry since only two atoms are present in the molecule. Homonuclear diatomic molecules are composed of two atoms of the same element. For example oxygen (O₂), hydrogen (H₂), etc. Heteronuclear diatomic molecules are composed of two atoms of two different elements. For example, hydrochloric acid vapor (HCl) is composed of a hydrogen atom bonded to a chlorine atom. There can be either ionic bonds or covalent bonds between heteronuclear molecules.

There are one or more covalent bonds between the two atoms. Therefore, a diatomic molecule is formed by the electron sharing between the atoms. This is because when the two atoms are considered as single atoms, they are not stable. In order to get stabilized, a diatomic molecule is formed.

But some compounds are not considered as diatomic substances because they have a polymeric nature at room temperature; for example, MgO, SiO, etc. But when these compounds are evaporated, they form diatomic molecules.

Diatomic compounds can have single bonds, double bonds or triple bonds between the two atoms. Some examples are given below.

• H₂, Cl₂, F₂ like gaseous compounds are composed of single bonds between the two atoms.
• O₂ has a double bond between two O atoms.
• N₂ has a triple bond between two N atoms.

Difference Between Monatomic and Diatomic

Definition

Monatomic: Monatomic refer to substances composed of particles containing single atoms.

Diatomic: Diatomic refer to substances composed of molecules containing two atoms bonded to each other.

Number of Atoms

Monatomic: Monatomic compounds are composed of particles with single atoms.

Diatomic: Diatomic compounds are composed of molecules with two atoms.

Chemical Bonding

Monatomic: There are no chemical bonds between particles of monatomic compounds.

Diatomic: There are ionic or covalent bonds between two atoms of a diatomic compound.

Stability

Monatomic: Other monatomic species except noble gases are not that stable.

Diatomic: Diatomic compounds are stable.

Examples

Monatomic: Monatomic compounds include noble gases and ionic species such as aqueous Na^+, Ca²⁺, etc.

Diatomic: Diatomic compounds include H₂, O₂, HCl, N₂, etc.

Conclusion

Monatomic compounds are composed of single atoms and there are no chemical bonds between these atoms. Diatomic compounds are composed of molecules containing two atoms. Therefore there are chemical bonds between these atoms. The main difference between monatomic and diatomic compounds is the number of atoms present in those compounds.

Image Courtesy:

1. “Noblefe” By Pkleong at English Wikibooks – Transferred from en.wikibooks to Commons (Public Domain) via Commons Wikimedia 
2. “Iodine-3D-vdW” (Public Domain) via Commons Wikimedia