(i) Valency for any atom can be defined as the number of electrons required by the atom to complete its outermost shell and have a maximum number of valence electrons and become stable.
(ii) They can do this by sharing their electrons with other atoms or can share the electrons from other atoms of suitable valency.
(iii) A non-metal with 6 electrons in its outermost shell will have the valency of 2.
(iv) That is because it only needs 2 electrons to make its valence electrons 8 which is the maximum number of valence electrons in an atom. It can combine with any element with only 2 valence electrons and become stable.