What is the valency of vanadium,manganese,iron,cobalt,copper

Q. How to calculate valency of element having atomic number more than 20. For ex,iron,manganese,copper,etc

Q.

Which of the following boxes contains isotones? (Numbers in the box represents mass number)

Q. What is the valency of :
Manganese in $Mn{O}_2$

Q. The atomic numbers of vanadium, chromium, manganese and iron are respectively $23,24,25,26$. Which one of these may be expected to have the highest second ionisation energy?

Q. in the ground state,an element has 13 electrons in its M-shell.the element is
(1) manganese
(2) cobalt
(3) nickel
(4) iron