Use the periodic table to predict the characteristic valence-electron configuration of the halogens.
First, we have to locate the halogens in the periodic table. It is located at the group 17 (second from the right).
Valence electrons are the outermost electrons in a particular atom which is generally the electrons that participate in certain reactions. Halogens (Fluorine, Chlorine, Bromine, Iodine, and Astatine) have valence electrons of seven.
They belong to the p-group elements which mean that their valence electrons are the electrons present in the last s and p orbitals in the electronic configuration. Let us try to write down the electronic configuration of the halogens:
(Consider the bolded configurations)
F = 1s^2 2s^2 2p^5
Cl = 1s^2 2s^2 2p^6 3s^2 3p^5
Br = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^5
I = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^5
At = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 4f^14 5d^10 6p^5
All of them have the same valence electron configuration of s^2 p^5 or seven valence electrons.