Here's why:

* Sodium's atomic number is 11. This means it has 11 protons and 11 electrons.

* Sodium's electron configuration is 2, 8, 1. This means:

* The first electron shell (closest to the nucleus) has 2 electrons.

* The second electron shell has 8 electrons.

* The third electron shell (outermost) has 1 electron.

Therefore, sodium has only one electron in its outermost shell, making it highly reactive and prone to losing that electron to form a positive ion.