Assuming that the given electron configurations are in terms of the number of electrons in the different (principal) energy levels, the one which represents an atom in its excited state is
(3) 2-6-17-6
An atom in its ground state is at its lowest electronic energy while an atom in its excited state is one whose electron(s) initially at a lower energy at ground state, jumps to an orbital or shell with higher energy. Often, only the electrons in the outer energy levels and subshells are able to jump into higher energy states as the core electrons tend to be more stable.
Remembering that the subshells s, p, d, f, g, are able to hold 2,6,10,14, and 18 electrons respectively, the electronic configuration can represent atoms in their different electronic energy states. The electron energy level 1, consisting of an s-subshell can only hold 2 electrons. The energy level 2, consists of the s- and p-subshells and can therefore hold 8 (2 from s- and 6 from p-) electrons. The energy level 3 can hold 18 (2 from s-, 6 from p-, and 10 from d-), while energy level 4 can hold 32 electrons.
An excited energy state is characterized by an electron configuration with a lowery energy level (or subshell in a given energy level) containing less electrons than in ground state and a higher energy level (or subshell) containing more electrons than in ground state.
As a rule, the lower energy levels and subshells should be filled with electrons first before the higher energy levels and subshells. Thus, if a lower energy level is missing an electron, and a higher energy level or subshell is already being filled or contains an additional electron, the atom is said to be in its excited state.
Evaluating the choices above,
1) 2-8-1
Energy level 1: 2 electrons (full)
Energy level 2: 8 electrons (full)
Energy level 3: 1 valence or outermost electron
Remarks: ground state
2) 2-8-6
Energy level 1: 2 electrons (full)
Energy level 2: 8 electrons (full)
Energy level 3: 6 valence or outermost electrons
Remarks: ground state
3) 2-8-17-6
Energy level 1: 2 electrons (full)
Energy level 2: 8 electrons (full)
Energy level 3: 17 electrons (lacking 1)
Energy level 4: 6 electrons valence or outermost electrons
Remarks: excited state - because energy level 3 lacks an electron and energy level 4 is already being filled. Again, as a rule for ground state, the lower energy levels and subshells must be filled out first before electrons start filling up the higher energy levels.
4) 2-8-18-5
Energy level 1: 2 electrons (full)
Energy level 2: 8 electrons (full)
Energy level 3: 18 electrons (full)
Energy level 4: 5 electrons valence or outermost electrons
Remark: ground state (particularly the ground state of the atom in (3).
