What does [He] stand for if it's written like [He] 2s1?

Question

As I'm doing chemistry homework, which includes looking at the electron configuration, I see that Lithium has [He] 2s1. What does it mean?

Answer 1

[He] is the ground state configuration of He which is 1s2. This is a short hand notation so we don't have to write the whole thing and just indicate the valence electrons. The complete electron configuration for Li is 1s2 2s1, so by writing [He], we do not have to write 1s2. This seems not a real advantage for Li, but if you consider an element like Cl, which has an electron configuration that is 1s2 2s2 2p6 3s2 3p5, the same information is conveyed by writing [Ne] 3s2 3p5, it indicates right away that the core electrons of Cl are the same as in Ne and that the valence electrons are those in 3s and 3p orbitals, thus, 7. As the number of electrons in an element increases this notation becomes more and more useful, since it saves you time.

Answer 2

[He] stands for 2 electrons - a complete first level (shell).
So [He]2s1 stands for 1 x 2s electron outside a helium electronic structure.
Later on in the Periodic Table this type of shorthand can save you a lot of writing!