Here's why:

* Electronic Configuration: Lithium has an electronic configuration of 1s² 2s¹. This means it has 2 electrons in its first shell (1s²) and 1 electron in its second shell (2s¹).

* Valence Shell: The valence shell is the outermost shell of an atom. For lithium, the second shell (2s¹) is the outermost, hence it's its valence shell.

Therefore, lithium has one electron in its valence shell.