The excited state of an atom is one in which an electron is in a higher energy level (or sublevel) than it is in its ground state (the "normal" electron configuration). The energy level to which the electron transitions must be an allowable energy level (for example an electron cannot transition from a ls to a lp sublevel because the lp sublevel does not exist). An excited state for Na, above its ground state, is [Ne]3p'. When the electron transitions back to the ground state, it emits the yellow-orange light characteristic of Na. The energy of this photon of light is equal to the energy difference between the 3p and the orbital in which the electron is located in the ground state.
a) Draw the ground state electron configuration and orbital filling diagram for Na in its ground state.
b) Another excited state for Na is [Ne]4p'. Which of the following could describe the radiation emitted by the Na atom as it transitions back to the ground state: UV light, the same yellow orange light as described above, red light, or infrared light? Explain.

When energy is supplied to atoms of elements, the electrons within the atom absorb same and transit from lower to higher energy levels. The difference in energy between the higher and lower energy level corresponds to the energy of the photon absorbed during the transition. When this occurs, the atom is now said to be in excited state.

Since atoms are usually unstable in excited state, the atom quickly returns to ground state and emits a photon of the same energy and wavelength as it absorbed while getting excited. This is the principle behind spectrophotometry. We can measure the wavelength of light emitted as the atom returns to ground state using a spectrophotometer.

For sodium ([Ne]3s1), there are several accessible excited states such as 3p,4p,5p etc. However, 3p is the lowest energy excited state whose wavelength corresponds to the sodium D-line (589nm).

Excitation to the 4p level will involve a wavelength of 330nm which lies in the ultraviolet region of the electromagnetic spectrum, hence the answer provided.

The orbital filling diagram of the ground state of sodium is shown in the image attached. Sodium has an electronic configuration of 1s2 2s2 2p6 3s1 as shown.