Which of the following is a reasonable ground-state electron configuration?

1s21p62d2

1s22s42p6

1s22s22p5

1s22s22d6

Answer is: 1s22s22p5.

1) Electron configuration 1s²1p⁶2d², this is not reasonable because 1p and 2d oritals do not exist.

2) Electron configuration 1s²2s⁴2p⁶, this is not reasonable because s orbitals only contain 2 electrons.

3) Electron configuration 1s²2s²2p⁵ is of an element fluorine. Fluorine (F) has atomic number 9, which means it has 9 protons and 9 electrons.

4) Electron configuration 1s²2s²2d⁶, this is not reasonable because 2d orbitals do not exist.