What feature of helium's energy level justifies placing its ...

[april345]

Also the question tells you to look at the table, the table just shows the valence electrons of various elements.

I don?t really understand this question. Could someone explain the question to me? I don?t necessary require the answer although some hints would be nice.

Thank You

[Millan]

The shell structure of an atom starts with an s subshell in shell 1.  The s subshell can hold 2 electrons.  Helium atoms have 2 electrons.  They can either both go in the 1s shell, or one could go into the next shell, no 2 and into its 2s subshell.  However, the 2s subshell requires more energy to fill than the remaining slot in the 1s shell, so the electrons will be paired in the 1s subshell.  The atom can be excited by external energy, however and that will excite one or both electrons into the next shell.  However, that is no a stable state.