To this point, we've been careful to say that a given neutral atom or charged ion is in its lowest energy state or its ground state, which are synonymous terms. We've noted that in some cases, however, that there are alternative electronic configurations. For example, the ground state of the carbon atom has two electrons in different 2p orbitals with the same spin, but they could be paired off in alternative ways as well. Alternative configurations are called excited states, and they play some extremely important roles in chemistry and biochemistry.

We've now also seen that there is an extended set of orbitals. We've discussed cases involving 1s, 2s, 2p, 3s, 3p, 3d, 4s, and 4p orbitals, and higher orbitals also exist. What's remarkable is that an extended set of orbitals is available in every atom, even in hydrogen. It is possible to excite the 1s electron of H atom into 2s, 3s, 4s, and even higher orbitals.

In the next chapters on bonding and molecules, we'll see that there are cases where atomic excited states play important roles in forming bonds for some elements. The figure below shows two states of the oxygen atom.

7.1

The most stable form of O (on the left) has one doubly occupied and two singly occupied 2p orbitals. In the other state, two of the 2p orbitals are doubly occupied, while the third one is unoccupied.