The valence of sodium is one. Sodium has a valence of one because it only has one electron present on its outermost shell. The outermost shell of an atom is also known as its valence shell.
Sodium has an atomic number of 11, meaning that 11 electrons orbit around the nucleus of the atom. The innermost shell holds a maximum of two electrons, while the second shell holds a maximum of 8 electrons. This means that 10 electrons occupy the first two shells, so that only one is left in the valence shell. Therefore, the valence of sodium is one.