Aufbauprincipleä

The Aufbau principle, also known as the building-up principle, is a fundamental concept in quantum chemistry used to determine the electron configuration of atoms in their ground state. It states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. This means that electrons are added to orbitals in order of increasing energy.

The order of filling orbitals is not simply based on the principal quantum number (n) but also considers the azimuthal quantum number (l). The (n+l) rule, also known as Madelung's rule, is often used to predict this order. According to this rule, orbitals with a lower (n+l) value are filled first. If two orbitals have the same (n+l) value, the orbital with the lower n value is filled first. The typical filling order for atomic orbitals is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

The Aufbau principle, in conjunction with Hund's rule and the Pauli exclusion principle, allows for the systematic prediction of electron configurations for all elements. Hund's rule states that electrons will singly occupy each orbital within a subshell before any orbital is doubly occupied, and these single electrons will have parallel spins. The Pauli exclusion principle dictates that no two electrons in an atom can have the same set of four quantum numbers, meaning an atomic orbital can hold at most two electrons, and they must have opposite spins.

While the Aufbau principle is a powerful predictive tool, there are exceptions, particularly for transition metals and some heavier elements, where deviations from the predicted order occur due to subtle energy differences between orbitals. These exceptions often involve half-filled or fully-filled subshells, which contribute to increased stability.