Periodic Classification of Elements — Science STD 10 — Question

Valency is the number of electrons an element has to lose or gain in order to become stable and achieve noble gas configuration.
Silicon, has atomic number 14, electronic configuration 2, 8, 4. It has 4 valence electrons. It can gain or lose 4 electrons to achieve noble gas configuration and stability. Thus the valency of silicon is 4.
Chlorine, has atomic number 17, electronic configuration 2, 8, 7. It has to gain one more electron, to complete its outermost shell and achieve noble gas configuration. Thus valency is 1.