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?

The valency of an element is determined by the number of electrons in its outer shell. Therefore, the number of valence electrons obtained from the electronic configuration of the element gives the valency i.e., the number of electrons lost, gained, or shared by the element to attain a noble gas configuration.

The valency of an element of atomic number 9 (Electronic configuration: 2,7) would be 1 (8-7= 1) since the number of valence electrons in its outer shell is 7 so it needs only one electron to attain the noble gas configuration.