Filling of Atomic Orbitals

“The orbitals of the atom are filled in the sequence of their increasing energies in the ground state.” As a result, electrons first occupy the lowest energy orbitals available to them before progressing to higher energy orbitals. The (n+l) rule is used to calculate the energy of an orbital.

The Aufbau principle, which is based on Pauli’s exclusion principle, Hund’s rule of maximum multiplicity, and the relative energies of the orbitals, governs the filling of electrons into the orbitals of distinct atoms.

Orbitals and orbits

When a planet orbits the sun, it can be plotted on a precise route called an orbit. The electrons may have been portrayed as revolving around the nucleus in a simplistic depiction of the atom. The truth is that electrons do indeed reside in space-time regions known as orbitals.

Although the terms orbits and orbitals sound similar, they have very different meanings. It’s critical that you know the difference between the two.

Aufbau Principle

The dispersion of electrons in various molecular orbitals is referred to as a molecule’s electronic configuration. Understanding the molecule is crucial. The number of electrons in a molecule’s bonding and antibonding molecular orbitals can be calculated using the electronic configuration of the molecule or molecular ion. Electron configuration can be used to forecast an atom’s stability. The number of electrons in each shell is set. The maximum number of electrons for the nth shell is 2n2, as established by a simple formula.

When all of an atom’s orbitals are filled, it becomes the most stable and, as a result, unreactive. Furthermore, the most stable electron configuration has an energy state that is full. Noble gases are difficult to mix with other molecules because their orbital arrangements are so important. Aufbau is a German word that means “building up,” unlike many other chemical concepts. It is not a scientist’s name. The filling of electrons in an orbital during the creation of an electronic configuration is the subject of this principle.

The Aufbau Principle’s Key Features

According to the Aufbau principle, electrons inhabit the lowest-energy orbitals first. This indicates that electrons only enter higher-energy orbitals once the lower-energy orbitals are completely occupied.

The (n+l) rule can be used to predict the order in which the energy of orbitals grows, with the energy level of the orbital determined by the sum of the primary and azimuthal quantum numbers.

The smaller the (n+l) number, the lower the orbital energy. When two orbitals have the same (n+l) values, the orbital with the lower n value has the lower energy.

The electrons fill each orbital in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.

Pauli Exclusion Principle

The exclusion principle, proposed by Austrian scientist Wolfgang Pauli, limits the number of electrons that can be occupied in certain orbitals (1926). This principle states that no two electrons in the same atom can have the same set of four quantum numbers. “Only two electrons may exist in the same orbital, and these electrons must have opposite spin,” according to the Pauli exclusion principle. This means that the two electrons can have the same value of the three quantum numbers n, l, and ml, but the opposite spin quantum number. The limitation imposed on the number of electrons in an orbital by Pauli’s exclusion principle aids in determining the capacity of electrons to be present in any subshell. For example, subshell 1s has just one orbital, hence the maximum number of electrons in the 1s subshell can be two, while the maximum number of electrons in the p and d subshells can be six and ten, respectively. In a nutshell, the maximum number of electrons in a shell with the principal quantum number n equals 2n2.

Hund’s Rule

The filling of electrons into orbitals belonging to the same subshell is dealt with by this rule (that is, orbitals of equal energy, called degenerate orbitals). It asserts that electron pairing in orbitals belonging to the same subshell (p, d, or f) does not occur until each orbital in that subshell has one electron, i.e., it is singly occupied.

Because there are three p, five d, and seven f orbitals, electron pairing will begin with the entry of the fourth, sixth, and eighth electrons, respectively, in the p, d, and f orbitals. The symmetry of half-filled and fully-filled degenerate orbitals has been demonstrated to provide additional stability.

Conclusion 

The Aufbau principle is based on Pauli’s exclusion principle and Hund’s rule of maximum multiplicity. It asserts that the orbitals of atoms are filled in order of increasing energy in their ground state. The (n+l) rule is used to calculate the energy of an orbital. Pauli’s exclusion principle. This principle states that no two electrons in the same atom can have the same set of four quantum numbers.

A maximum number of electrons in the shell with principal quantum number n equals 2n2. Hund’s rule asserts that electron pairing in orbitals belonging to the same subshell (p, d, or f) does not occur until each orbital is singly occupied.