Molecular Orbital Definition

Abstract atom
A molecular orbital is a mathematical function that describes an electron in a molecule.

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by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated July 10, 2019

A molecular orbital is an orbital or wave function of a molecule's electron. The function may be used to calculate the probability of finding an electron within a specified space or to predict the molecule's chemical and physical properties. Robert Mulliken introduced the term "orbital" in 1932 to describe a one-electron orbital wave function.

Electrons around a molecule can be associated with more than one atom and are often expressed as a combination of atomic orbitals. Atomic orbitals within a molecule can interact if they have compatible symmetries. The number of molecular orbitals is equal to the number of atomic orbitals combined to form a molecule.