How many valence electrons are in an atom of chlorine?

Valence electron:

1. Valence electrons are the electrons in the outermost shells.
2. The most stable atoms have a full valence shell of electrons.
3. They are determined by an atom's electrical configuration.
4. Atomic number of chlorine$=17$ so its electronic configuration $=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^5$
5. Chlorine has $3{\mathrm{s}}^2,3{\mathrm{p}}^5$ as its outermost shell.

Therefore, the number of valence electrons$=7$.