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(i) The number of valence electrons present in the atoms helps in finding out the valency. Electronic configuration helps in finding the number of valence electrons in the atom. The valency of an element is determined by number of electrons lost or regained to achieve the nearest stable configuration.

(ii) Given: atomic no. of X = 9

Electronic configuration of X = 2, 7

Since, there are 7 electrons in its outermost shell and only one electron is required to complete the octet and to achieve stable gas configuration. So, the valency of element X = 1