For example, both 3p and 4s have (n+l) value that is equal to 4. If two subshells have the same (n+l) value, the electron will occupy the lower n subshell. For example, n = 3 and l = 1 refer to the 3p subshell. n and l take discrete values, like n = 0, 1, 2, … and l = 0, 1, 2, and 3. The electrons fill up the subshell according to the increasing value of (n+l). The sum of n and l determines the order of increasing energies of the subshells. They are principal quantum number n and azimuthal quantum number l. There are four quantum numbers, of which two are concerned with the subshells. Quantum numbers are used to designate the state of an electron.

Danish physicist Niels Bohr was the first to propose a quantum mechanics model of the atom in the 1920s. Several scientists have contributed to the development of the Aufbau principle. In other words, the atom builds up as electrons occupy the various subshells.

‘Aufbau’ comes from the German word Aufbauen, which means ‘building-up’. According to this principle, electrons first fill up the lower energy subshells before progressively filling up the higher ones. Aufbau principle gives a model for filling the electrons in the empty subshells of an atom in its ground state. The construction of a model of an atom begins by filling up the electrons in the lowest energy subshell. The orbits are characterized by sublevels or subshells, which are organized according to their increasing energies.

Aufbau Principle and Electron ConfigurationĪn atom consists of a nucleus around which electrons revolve in well-defined orbits.

Aufbau principle order