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How Is a Pair of Molecular Orbitals Formed?

Molecular orbitals are mathematical concepts about the structure and behavior of electrons in molecules. They are an extension of similar theories that attempt to explain the movement of electrons around an atomic nucleus. Single atoms and groups of atoms that comprise a molecule can hold a number of orbitals where electrons can be found. Each orbital has a defined energy level and can hold a maximum of two electrons. Molecular orbitals form from the overlap of a minimum of two atomic orbitals.

Quantum Physics
- Quantum physics is a branch of physics that evolved during the early part of the 20th century. Its basic principle states that the energy held by an electron during its movement around a nucleus is a multiple of a fixed unit called a quantum. The precise position of an electron during its movement is unknown. Scientists can only calculate the probability of finding an electron within a three-dimensional space called an orbital.
Electron Properties
- Electrons are subatomic particles with three fundamental properties: charge, mass and spin. They have a single negative charge and a mass that is minuscule compared with the main subatomic particles such as protons and neutrons. As electrons move, their charge generates a magnetic field. The angular momentum, a type of force, of this magnetic field is the electron spin. The spin has a specific direction for each electron.
Atomic Structure
- Electrons determine the structure of atoms by the manner in which they surround a nucleus. Quantum physics states that electrons occupy the lowest available energy level first. This is the ground state and is the energy level closest to the atomic nucleus. Electrons that occupy orbitals further away from the nucleus have a higher energy.
Pauli Exclusion Principle
- Austrian physicist Wolfgang Pauli proposed in 1925 that no two electrons in a given atom may occupy the same quantum state. This means a maximum of two electrons may occupy one orbital on condition that these electrons have opposite spins.
Bonding and Antibonding
- The simplest form of atomic orbital is the hydrogen atom. It consists of a nucleus with one positively charged proton and one negatively charged electron in motion outside it. When two hydrogen atoms move closer to each other, the atomic orbitals begin to overlap. The first electron creates a lower-energy though stable bond between the two protons. This is the bonding molecular orbital. If the second electron has an opposite spin to the first, it moves into the ground state orbital and creates a stable hydrogen molecule. If it has the same spin as the first electron, it moves to a higher energy level into the second of a pair of molecular orbitals, the antibonding orbital.

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