Video I took of the gas grill at my house catching on fire! 1  Comedy. 2  Standard YouTube License.
The sodium atom (atomic number 11), with 1 valence electron in its third energy level, would like to achieve the stability of its nearest noble gas neighbor, the neon (Ne) atom (atomic number 10), which contains 8 valence electrons in its second energy level.
Valence electrons by definition are the number of electrons in the valence shell. The atomic number of Chlorine is 17. Hence the configuration of electrons would be 2,8,7. Hence the number of valence electrons in Chlorine is 7. However if you talk about valency that would be number of electrons an atom has to gain/loose to achieve the stable state.