An atom with atomic number 9, mass number 19, and with an electronic configuration of 2, 6, 1 is in an excited state because the second shell is not completely filled while an electron is found in a shell of a higher energy level.
When an atom is in an excited state, some of its electrons jump from lower to higher energy levels. An element with atomic number 9 would normally have 2,7 as its electronic configuration.
For and an atom in a ground state, lower shells are completely filled before higher shells but in this case, an electron has already jumped to a higher shell even when the lower shell remains incompletely filled. Hence, the reason of 2,6,1 instead of 2,7.
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