Sodium, like all the group 1 alkali metals, has one valence electron.
are the outermost electrons, and are the ones involved in . Sodium has 11 electrons: its is 11, so it has 11 protons; atoms are neutral, so this means sodium also has 11 electrons.
Electrons are arranged in “shells” or energy levels. Depending on your level of Chemistry, it is probably easier to think of them as particles orbiting the nucleus. The first “shell” can have 2 electrons. The second “shell” can have up to 8 electrons. The third “shell” is a bit more complicated but let’s just say that it takes up to 8 electrons as well (for now…).
So, sodium’s 11 electrons are arranged this way: 2 electrons in the first “shell”, 8 electrons in the second “shell”; and 1 electron (the valence electron) in the third “shell”. We write this as 2.8.1. The last number is how we know the number of .
Aluminium has the electron arrangement 2.8.3. It has 3 valence electrons. Fluorine has the electron arrangement 2.7. It has 7 valence electrons. This trend only gets broken at Sc (21).
Here is a video which gives further explanation of this topic. video from: Noel Pauller