Difference Between Molecular Orbital and Atomic Orbital

The key difference between molecular orbital and atomic orbital is that atomic orbitals describe the locations where the probability of finding the electrons is high in an atom whereas molecular orbitals describe the probable locations of electrons in a molecule.

The bonding in molecules was understood in a new way with the new theories presented by Schrodinger, Heisenberg and Paul Dirac. When quantum mechanics came into the picture with their findings, it was discovered that an electron has both particle and wave properties. With this, Schrodinger developed equations to find the wave nature of an electron and came up with the wave equation and wave function. Wave function (Ψ) corresponds to different states for the electron.

CONTENTS

1. Overview and Key Difference
2. What is a Molecular Orbital
3. What is an Atomic Orbital
4. Side by Side Comparison – Molecular Orbital vs Atomic Orbital in Tabular Form
5. Summary

What is a Molecular Orbital?

Atoms join together to form molecules. When two atoms move closer together to form a molecule, atomic orbitals overlap and combine to become molecular orbitals. The number of newly formed molecular orbitals is equal to the number of combined atomic orbitals. Furthermore, the molecular orbital surrounds the two nuclei of the atoms, and electrons can move around both nuclei. Similar to atomic orbitals, molecular orbitals maximally contain 2 electrons, which have opposite spins.

Moreover, there are two types of molecular orbitals: bonding molecular orbitals and antibonding molecular orbitals. Bonding molecular orbitals contain electrons in the ground state while antibonding molecular orbitals contain no electrons in the ground state. Furthermore, electrons may occupy antibonding orbitals if the molecule is in the excited state.

What is an Atomic Orbital?

Max Born pointed out a physical meaning to the square of the wave function (Ψ2) after Schrodinger put forward his theory. According to Born, Ψ2 expresses the probability of finding an electron in a particular location; if Ψ2 is a large value, then the probability of finding the electron in that space is higher. Therefore, in the space, the electron probability density is large. However, if the Ψ2 is low, then the electron probability density is low. The plots of Ψ2 in x, y and z axes show these probabilities, and they take the shape of s, p, d and f orbitals. We call these atomic orbitals.

Furthermore, we define an atomic orbital as a region of space where the probability of finding an electron is large in an atom. We can characterize these orbitals by quantum numbers, and each atomic orbital can accommodate two electrons with opposite spins. For example, when we write the electron configuration, we write it as 1s2, 2s2, 2p6, 3s2. 1, 2, 3….n integer values are the quantum numbers. The superscript after the orbital name shows the number of electrons in that orbital. s orbitals are sphere-shaped, and small while P orbitals are dumbbell-shaped with two lobes. Here, one lobe is positive while the other lobe is negative. Moreover, the place where two lobes touch each other is the node. There are 3 p orbitals as x, y and z. They are arranged in space in such a way that their axes are perpendicular to each other.

There are five d orbitals and 7 f orbitals with different shapes. Therefore, the following are the total number of electrons that can reside in an orbital.

• s orbital-2 electrons
• p orbitals- 6 electrons
• d orbitals- 10 electrons
• f orbitals- 14 electrons

What is the Difference Between Molecular Orbital and Atomic Orbital?

The key difference between molecular orbital and atomic orbital is that atomic orbitals describe the locations where the probability of finding the electrons is high in an atom whereas molecular orbitals describe the probable locations of electrons in a molecule. Moreover, atomic orbitals are present in atoms while molecular orbitals are present in molecules. In addition, the combination of atomic orbitals results in the formation of molecular orbitals. Furthermore, atomic orbitals are named as s,p,d, and f while there are two types of molecular orbitals as bonding and antibonding molecular orbitals.

Summary – Molecular Orbital vs Atomic Orbital

The key difference between molecular orbital and atomic orbital is that atomic orbitals describe the locations where the probability of finding the electrons is high in an atom whereas molecular orbitals describe the probable locations of electrons in a molecule.

Reference:

1. Helmenstine, Anne Marie. “Orbital Definition and Example.” ThoughtCo, May. 7, 2019, Available here.

Image Courtesy:

1. “Space-filling model of a pi bond (bonding molecular orbital with π symmetry) generated by the overlap of..” By Ben Mills  (Public Domain) via Public Domain Files
2. “Atomic-orbital-clouds spd m0” By Geek3 – Own work; created with hydrogen-cloud in PythonThis PNG graphic was created with Python (CC BY-SA 4.0) via Commons Wikimedia