Question

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

Answer 1

atomic number 9 has electronic configuration 2,7 so it has the valency of -1.
Bcoz it can easily can 1

Answer 2

The valency of the element of atomic number 9 is 1. 
This is because in the time of electronic configuration, it is easier for fluorine (atomic number 9) to gain one electron to reach octet state i.e. stable. 

The valency  can be determined by the number of electrons present in the valence shall of that atom or the number of valence electrons it can easily lose at the time of electronic configuration. 

hope this helps