Sodium has 11 electrons. Its electron configuration (electrons in K, L, M shells) is:

Fill shells from inside: K (max 2): 2 electrons. L (max 8): 8 electrons. That's 10 so far. The 11th goes into M shell. So sodium's configuration is 2, 8, 1. The single electron in M shell is the 'valence electron' that sodium readily loses to form Na⁺ — explaining why sodium is highly reactive.