how do we found valence electrons?

Answer 1

if you know how to do electron configuration then it'll be easier to find the valence electrons. simply saying, the valence electron is the number of electrons found at an element's outermost shell... to make it seem simpler here's an example:

Oxygen (8)
electron configuration 1s2 2s2 2p4

in this case the numbers before the letters are called the shells, the letters energy levels and the number after the letters(in this case, but is in actuality the super script as seen in the periodic table), the number of electrons. for oxygen(assuming that its stable), it is eight. as you can see, the outermost(note that i wrote outermost and not last because the outermost sublevel isn't always the last in the configuration), is 2. in the latter shell, there is 2 electrons on the s-energy level and 4 on the p, add them to get your valence electron.

in short:
the valence electron is dependent on the outermost shell and the outermost shell pertains to the number before the letter NOT the letter. add all the electrons(as seen as the superscript when in the periodic table) in the respective shell to acwuire the element's valence