The Beechmont Crest Online Guide to Organic Chemistry
THE AUFBAU PRINCIPLE
According to quantum theory, electrons surrounding the nucleus of an atom are assigned to orbitals. An orbital has a characteristic energy and three-dimensional shape. (We will discuss orbital geometry in a subsequent section.) A maximum of 2 electrons may be assigned to each orbital.
There are four types of orbitals: s, p, d, and f. You can see below that each s subshell contains only one orbital, and each p subshell has three orbitals. (There are 5 orbitals in each d subshell and 7 in each f subshell.)
At Level 2 and higher, each shell has three p orbitals. Each p orbital within the same subshell has the same degree of energy. Orbitals of equal energy are called degenerate energy levels.
Electrons occupy the lowest energy level that is available to them. This is called the ground state. Electrons are most stable at the ground state level.
As the number of electrons increases, electrons fill the next lowest energy subshell that is available. This is the aufbau (“building up”) principle.
When the orbitals in a given subshell are all filled, electrons fill the next higher energy subshell. Note the aufbau principle at work below in a hydrogen and a carbon atom.
