Other Laws Related to Position and Path of Electrons

Three other rules and principles in chemistry help us understand the position, path, orbits and energy levels of electrons in an atom, these are discussed in subsequent subsections:

Aufbau Principle

The Aufbau principle, also called the Aufbau rule, says that electrons go into lower-energy atomic orbitals first before higher-energy ones. “Aufbau” means “building-up” in German.

NCERT Definition of Aufbau Principle

In the ground state of the atoms, the orbitals are filled in order of their increasing energies.

The Aufbau principle helps us figure out how electrons arrange themselves in atoms or ions. For instance, the 1s subshell gets filled up before the 2s subshell.

Here’s the order in which electrons fill up the orbitals: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so forth. This filling pattern applies to every atom.

For example, using the Aufbau Principle, the electronic configuration of sulfur is written as [S] = 1s2 2s2 2p6 3s2 3p4.

Hund’s Rule of Maximum Multiplicity

Hund’s rule says that each orbital in a sublevel gets one electron before any of them get a second one. And all the electrons in these single-occupied orbitals have the same spin.

NCERT Definition of Hund’s Rule

Pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.

Hund’s rule, also called the maximum multiplicity rule, can result in atoms having multiple unpaired electrons. These unpaired electrons can spin in different directions, creating magnetic moments in various directions.

Hund’s rule applies to certain molecules that have unpaired electrons.

Pauli Exclusion Principle

The Pauli exclusion principle says that two identical particles with half-integer spins can’t be in the same quantum state within a system.

NCERT Definition of Pauli Exclusion Principle

No two electrons in an atom can have the same set of four quantum numbers.

OR

Only two electrons may exist in the same orbital and these electrons must have opposite spin.

In chemistry, the Pauli exclusion principle tells us that within the same atom, no two electrons can have all four of their quantum numbers identical. This means that at most, two electrons can occupy the same orbital, and they must have opposite spins.

The Pauli exclusion principle sets limits on how many electrons can be in a shell or a subshell.

Quantum numbers in Chemistry, are the sets of numbers that describe an electron’s orbit and movement within an atom. When the quantum numbers of all the electrons in a given atom are added together, they must satisfy the Schrodinger equation.

Quantum numbers are the set of numbers used to describe the position and energy of an electron in an atom. There are four types of quantum numbers: principal, azimuthal, magnetic, and spin. Quantum numbers represent the values of a quantum system’s conserved quantities.

Let’s learn about all the quantum numbers in detail in this article.