polyatomic molecule

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polyatomic molecule

chemistry

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July 1, 2026

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A polyatomic molecule is defined as a molecule containing more than two atoms. Molecules themselves are the smallest identifiable units of a pure substance that retain its composition and chemical properties. These molecules are formed when atoms bond together due to interactions that lower the system's total energy.12

Examples of polyatomic molecules include carbon dioxide (CO₂) and water (H₂O). In contrast, diatomic molecules consist of only two atoms, such as oxygen (O₂). Some molecules, like polymers, can contain thousands of atoms. Polyatomic ions are also charged entities composed of several atoms bonded together, forming part of ionic compounds like ammonium nitrate (NH₄NO₃).23

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molecule
Characteristics of molecules
…than two atoms are termed polyatomic molecules, e.g., carbon dioxide (CO₂) and water (H₂O). Polymer molecules may contain many thousands of component atoms.
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…than two atoms are termed polyatomic molecules, e.g., carbon dioxide (CO₂) and water (H₂O). Polymer molecules may contain many thousands of component atoms.
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thermodynamics
Heat capacity and internal energy
…molecules (such as oxygen) and polyatomic molecules (such as water) have additional rotational motions that also store thermal energy in their kinetic energy of rotation. Each additional degree of freedom contributes an additional amount R to c_V. Because diatomic molecules can rotate about two axes and polyatomic molecules can rotate…
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…molecules (such as oxygen) and polyatomic molecules (such as water) have additional rotational motions that also store thermal energy in their kinetic energy of rotation. Each additional degree of freedom contributes an additional amount R to c_V. Because diatomic molecules can rotate about two axes and polyatomic molecules can rotate…
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chemical bonding
Molecular orbitals of polyatomic species
The principal qualitative difference between MO theory and VB theory becomes obvious when the objects of study are polyatomic, rather than diatomic, species. The benzene molecule is considered again but in this case from the viewpoint of its molecular orbitals. The atomic orbitals…
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The principal qualitative difference between MO theory and VB theory becomes obvious when the objects of study are polyatomic, rather than diatomic, species. The benzene molecule is considered again but in this case from the viewpoint of its molecular orbitals. The atomic orbitals…
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spectroscopy
Microwave spectroscopy
For polyatomic molecules three moments of inertia are required to describe the rotational motion. They produce much more complex spectra, but basic relationships, analogous to those for a diatomic molecule, exist between their moments and the observed absorption lines. The 1–1,000-gigahertz range is referred to as…
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For polyatomic molecules three moments of inertia are required to describe the rotational motion. They produce much more complex spectra, but basic relationships, analogous to those for a diatomic molecule, exist between their moments and the observed absorption lines. The 1–1,000-gigahertz range is referred to as…
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gas
Thermal conductivity
The thermal conductivity of polyatomic molecules is accounted for by simply adding on a contribution for the energy carried by the internal molecular motions:where c_int is the contribution of the internal motions to the heat capacity (per molecule) and is easily found by subtracting (3k/2) from the total measured…
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The thermal conductivity of polyatomic molecules is accounted for by simply adding on a contribution for the energy carried by the internal molecular motions:where c_int is the contribution of the internal motions to the heat capacity (per molecule) and is easily found by subtracting (3k/2) from the total measured…
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