Here's why:

* Sodium's Electronic Configuration: Sodium has an atomic number of 11, meaning it has 11 electrons. Its electronic configuration is 2, 8, 1.

* Valence Shell: The outer most shell (the third shell in this case) only has one electron.

* Full Shell: A full valence shell for sodium would require 8 electrons in its outer shell.

Sodium is highly reactive because it wants to lose its single valence electron to achieve a stable, full outer shell like the noble gas neon (2, 8).