How is electron configuration determined for an atom?

Electron configuration for an atom is determined by the distribution of electrons in its atomic orbitals.

The electron configuration of an atom is the arrangement of electrons in the atom's atomic orbitals. This configuration is determined by the Aufbau principle, the Pauli exclusion principle, and Hund's rule. These principles guide the 'building up' of atoms by progressively adding electrons to the most energetically favourable orbitals.

The Aufbau principle states that electrons occupy the lowest energy levels first. This means that the 1s orbital, which is the closest to the nucleus and thus has the lowest energy, is filled before the 2s orbital, and so on. The principle is named after the German word Aufbau, which means 'building up', reflecting the way electrons are added to build up the atom.

The Pauli exclusion principle states that each atomic orbital can hold a maximum of two electrons, and these electrons must have opposite spins. This is due to the quantum mechanical nature of electrons, which are fermions and thus obey the exclusion principle. This principle ensures that no two electrons in an atom can have the same set of quantum numbers.

Hund's rule states that electrons will occupy empty orbitals of the same energy (degenerate orbitals) singly before beginning to pair up. This is because electrons are negatively charged and repel each other, so it is energetically more favourable for them to occupy different orbitals if possible.

The electron configuration is usually written as a series of numbers and letters, representing the energy levels, orbitals, and number of electrons in each orbital. For example, the electron configuration of oxygen (atomic number 8) is 1s² 2s² 2p⁴, indicating that there are two electrons in the 1s orbital, two in the 2s orbital, and four in the 2p orbital.

Understanding electron configuration is crucial in chemistry as it helps to explain the chemical behaviour of atoms, including how they bond with other atoms and their reactivity. It also forms the basis for the periodic table, with elements in the same group having similar electron configurations and thus similar chemical properties.