The valency of hydrogen is 1, magnesium, 2 aluminium 3 and carbon 4 . Can you see any connection between the valency of an element and the number of electrons it has in its outermost electron shell ? What would you predict the valencies of helium (He), phosphorus (P), sulphur (S) and neon (Ne) to be ?

Explanation

The valency of an element depends upon the number of electrons in the outermost shell (valence shell) of an atom of the element. The valency of an element is either equal to the number of valence electrons in an atom of the element or to the number of electrons requried to complete an octet of 8 electrons in its valence shell.
Valency of a metal=number of valence electrons.
Valency of a nonmetal =8-number of valence electrons.
Helium (He) : An atom of helium contains 2 electrons in its K shell (n=1) . This shell is also the outermost shell of helium which is completely filled with 2 electrons. Hence, valency of helium =0 .
Phosphorus (P): The electronic configuration of the P atom is 2,8,5. Thus , it has 5 valence electrons and it is a nonmetal . Hence , valency of P =8-5=3.
Sulphur (S) : The electronic configuration of an atom of S is 2,8,6. Sulphur is a nonmetal. Hence , valency of S=8-6=2
Neon (Ne): The electronic configuration of neon is 2,8. The outermost shell of neon is thus completely filled. Hence, valency of neon =0.