How to count valence electrons of an element just by looking at ...

[scienceeasy]

On a multiple choice ? I had, electron configurations of various elements were given, and a ? was "which of the following elements {by looking at its electron config.} has one valence electron. The answer ended up being Cr, but how can you tell?

[Hawke]

Valence electrons are the number of electrons in the outer most energy level. The electron configuration for Cr is [Ar] 3d5 4s1 since 4 is the outer most level, you see it has 1 electron.  If you look at F, it has an electron configuration of 1s2 2s2 2p5. It's outer most level is 2 so if you add all the electrons in 2 , it = 7 valence electrons.
The reason Cr is not 3d4 4s2 is that the half filled d sublevel is at a lower energy state so the e moves out of the 4s in order to give it the lower energy configuration.