Which element has atoms in the ground state with greatest number of valence electrons?

You need a list of elements to compare.

The procedure is to wirte the electron configuration and compare the number of electrons in the valence electron shell.

I will work with this list, to show you the procedure

(1) tin

(2) sulfur

(3) arsenic

(4) fluorine

element atomic number number of abbreviated electron

electrons configuration

tin 50 50 [Kr] 5s^2 4d^10 5p2

sulfur 16 16 [Ne] 3s^2 3p^4

arsenic 33 33 [Ar] 4s^2 3d^10 4p^3

fluorine 9 9 [He] 2s^2 2p^5

The valence electrons are those in the last shell.

So, fluorine has 2 + 5 = 7 valence electrons, while tin has 2 + 2 = 4, sulfur has 2 + 4 = 6 and aersenic has 2 + 3 = 5.

Therefore, the answer is fluorine.