* Electron Configuration: Sodium has an atomic number of 11, meaning it has 11 protons and 11 electrons. Its electron configuration is 2, 8, 1.

* Outermost Shell: The outermost shell, also known as the valence shell, is the third shell in this case, containing only one electron.

* Tendency to Lose: Sodium wants to achieve a stable electron configuration like the noble gases. To do so, it readily loses its single valence electron, becoming a positively charged ion (Na+).

* Valency Definition: Valency refers to the number of electrons an atom can gain, lose, or share to form chemical bonds. Since sodium loses one electron, its valency is one.

In simpler terms, sodium only needs to lose one electron to become stable, hence its valency is one.