Atomic orbitals are regions around the nucleus where electrons have a high probability of being found. Orbitals allow the atom to be bound to other atoms to form molecules through the sharing or exchanging of electrons in higher orbitals. They consist of various energy levels and the orbitals containing the most electrons will depend on the amount of energy absorbed by the atom.
Orbitals and the Heisenberg Uncertainty Principle
Because of the complexities of quantum mechanics, it is not possible to know both the momentum and the position of a subatomic particle such as an electron. This was ascertained by the physicist Werner Heisenberg after which the principle is named. Because of Heisenberg’s Uncertainty Principle, we cannot know exactly where an electron is or where it will be in the future in relation to the nucleus of the atom to which it is bound by the electromagnetic force. It is only possible to know the most likely position of an electron within an atom. There are certain regions near the nucleus where the electron has a high probability of being located. These regions are called orbitals.
Energy States within Orbitals
All electrons have energy, without energy, they would just collapse into the nucleus. Electrons that are farther from their nucleus are higher energy than those which are closer to the nucleus. Electrons increase in energy when they absorb radiation. Because of the discontinuous nature of electromagnetic radiation, electrons do not smoothly move farther from the nucleus as they absorb more energy, rather, the electrons will jump from one discrete energy level to the next as they increase in energy. Each energy level is associated with a certain region around the nucleus. When an atom jumps to a higher energy region, it is said to be excited.
Types of Orbitals
In addition to different energy levels between orbitals, there are also orbitals of different shapes that are associated with varying energy levels. The most basic orbital is the s orbital which occupies a roughly spherical region around the nucleus. Within the s orbital are varying energy levels labeled, 1s, 2s, 3s, etc. The s orbitals of an atom are mostly lower energy than other orbitals, though not all of them. Another orbital is the p orbital. This orbital takes the shape of two separate globes bound at the nucleus of the atom. They also lack the lowest energy level. There is a 1s energy level but not a 1p energy level.
Beyond the p orbitals are the d orbitals whose probability regions resemble four lobes which create a cross or x-shape meeting at the nucleus. These orbitals are typically higher energy than s or p orbitals though this is not always the case. 4s, for example, is actually higher energy than 3d. The highest-energy and hardest to understand orbitals are f orbitals which have comparatively bizarre, many-lobed shapes.
Atomic Orbitals and the Formation of Molecules
Two atoms will come together when the attraction between one atom’s electrons and the other atom’s nucleus are attracted more strongly than the nuclei of the two atoms repel each other. Eventually either one atom will lose an electron, becoming more positively charged, and become ionically bonded or they will end up sharing electrons in which case the electron’s density clouds or wave functions overlap. For this to happen though, there must be space in the energy levels of the orbitals for new electrons to be introduced. This is how electrons are shared or exchanged, through new electrons occupying empty energy levels of atoms.
When atoms combine to form molecules, new orbitals take shape around the bound atomic nuclei. Molecular orbitals are regions where higher energy electrons are likely to be found within the molecule.
Types of Molecular Orbitals
In addition to orbitals of different shapes, molecular orbitals also consist of orbitals that either make an atom more stable or less stable. Bonding molecular orbitals are orbitals where electrons spend most of their time between the nuclei of the individual atoms. The bonding molecular orbitals encourage bonding and create stable molecules. Anti-bonding molecular orbitals are orbitals where electrons spend most of their time away from the space between the bonded nuclei. These create unstable molecules.
In a diatomic molecule, bonding molecular orbitals have lower energy than the 1s orbitals of the bonded atoms whereas anti-bonding molecular orbitals will have more energy than the 1s orbitals. If there are more electrons within the bonding orbitals than the anti-bonding orbitals, the molecule will be stable. If there are more electrons within the anti-bonding orbitals or if the number of electrons in the bonding and anti-bonding orbitals is the same, they will be unstable.
If there are more electrons in the bonding orbital than in the 1s orbitals of the combined atoms in a diatomic molecule, it essentially means that it takes less energy for the atoms to exist as a molecule then as separate atoms and matter always tends towards the lower energy option. More electrons in the anti-bonding orbital would imply that it takes less energy for the atoms to exist separately.
Molecular Orbital Shapes
Molecular orbitals also come in different shapes. One is the s orbital. Within this orbital, the electrons are most likely to be found along the axis connecting the two nuclei. Bonding s orbitals. The anti-bonding equivalent is denoted as s*. The p orbitals are orbitals where the electrons are all clustered above or below the previously mentioned axis.
Similarities between Atomic Orbitals and Molecular Orbitals
Atomic and Molecular Orbitals are both regions around atomic nuclei where electrons are most likely to occur. They also both have many shapes and consist of different energy levels where the energy levels of some orbitals are higher than energy levels of other orbitals.
Differences Between Atomic Orbitals and Molecular Orbitals
Although molecular orbitals and atomic orbitals are similar, they do also have significant differences
- Atomic orbitals are around individual nuclei whereas molecular orbitals occur around clusters of nuclei bound together by the electromagnetic force to make molecules.
- The configurations of electrons within atomic orbitals do not affect the stability of an atom whereas the electron configuration within molecular orbitals does affect the stability and existence of the molecule.
Atomic Orbital vs. Molecular Orbital
Atomic Orbitals are regions around atomic nuclei where electrons are most likely to be found. They can be of different shapes which consist of multiple energy levels. Electrons of higher energy are in orbitals farther from the nucleus. Molecular orbitals are regions around molecules where electrons are most likely to be found. In addition to consisting of different shapes and energy levels, there are also bonding orbitals which make molecules more stable and anti-bonding orbitals which can make them less stable. While atomic orbitals occur around a single nucleus, molecular orbitals occur around a cluster of nuclei within a molecule. Also, atomic orbitals have no effect on the stability of an atom whereas molecular orbitals can make a molecule stable or unstable depending on how many electrons are in the bonding or anti-bonding orbitals.
Author: Caleb Strom
Caleb Strom has a B.SC. in earth science from the University of California San Diego and is currently a graduate student in geological sciences at California State Polytechnic University Pomona. He has done scientific research in planetary science at the Scripps Institution of Oceanography and astrophysics at the Center for Astrophysics and Space Science at UC San Diego.