How do you define a valence electron? How do you determine the valence number of an atom?
A valence electron is an electron that is in the valence shell. The valence shell is the electron shell that is furthest from the nucleus.
You can determine the valence number by finding out how many electrons the atom possesses. the amount of electrons a shell can hold varies from the amount of shells that are occupied. 2,8,8 is the number of electrons held in each shell while the three numbers refer to the number of shells. For example, Lithium, with three electrons total, has one valence electron, because the other two are used up in the first shell.