Question

How can the valency of an element be determined if its electronic configuration is known? What will be the valency of an element of atomic number 9 (nine)?

Answer 1

If the electronic configuration is known, then the valency of an element is determined by the number of valence electrons present in the outermost shell i.e. the number of valence electrons lost, gained, or shared by the element to attain the noble gas configuration.
The electronic configuration of an element of atomic number 9 is 2, 7. It requires one electron to get a stable noble gas configuration. Hence its valency is 1.