<P> The superscript 1 for a singly occupied subshell is not compulsory; for example aluminium may be written as either (Ne) 3s 3p or (Ne) 3s 3p . It is quite common to see the letters of the orbital labels (s, p, d, f) written in an italic or slanting typeface, although the International Union of Pure and Applied Chemistry (IUPAC) recommends a normal typeface (as used here). The choice of letters originates from a now - obsolete system of categorizing spectral lines as "sharp", "principal", "diffuse" and "fundamental" (or "fine"), based on their observed fine structure: their modern usage indicates orbitals with an azimuthal quantum number, l, of 0, 1, 2 or 3 respectively . After "f", the sequence continues alphabetically "g", "h", "i"... (l = 4, 5, 6 ...), skipping "j", although orbitals of these types are rarely required . </P> <P> The electron configurations of molecules are written in a similar way, except that molecular orbital labels are used instead of atomic orbital labels (see below). </P> <P> The energy associated to an electron is that of its orbital . The energy of a configuration is often approximated as the sum of the energy of each electron, neglecting the electron - electron interactions . The configuration that corresponds to the lowest electronic energy is called the ground state . Any other configuration is an excited state . </P> <P> As an example, the ground state configuration of the sodium atom is 1s 2s 2p 3s, as deduced from the Aufbau principle (see below). The first excited state is obtained by promoting a 3s electron to the 3p orbital, to obtain the 1s 2s 2p 3p configuration, abbreviated as the 3p level . Atoms can move from one configuration to another by absorbing or emitting energy . In a sodium - vapor lamp for example, sodium atoms are excited to the 3p level by an electrical discharge, and return to the ground state by emitting yellow light of wavelength 589 nm . </P>

What does ground state of an electron mean
find me the text answering this question