When electrons occupy the shells and subshells of an atom they always tend to start filling the lowest energy levels first. When all the electrons are in their lowest energy levels possible the atom is said to be in its electronic ground state.
The atom on the right is that of aluminium. It is in its electronic ground state because all the electrons are in the lowest possible energy configuration.
The electronic configuration is written as
1s2, 2s2, 2p6, 3s2, 3p1
The atom on the right is also of aluminium. But notice how not all of its electrons are in their lowest possible energy states.
The electronic arrangement can be written as
1s2, 2s2, 2p6, 3s1, 3p2
The 3p subshell should only contain one electron while the 3s should be full with two electrons.
This is known as an excited state.
