llhow to find valency of a compound using atomic number

see first learn about shell their are many shell like K,L,M,N,etc.... and each has capacity of its own to hold electrons so k can hold 2 and L can hold 8 and so on, so with the help of this we can find valency . valency is nothing but the remaining electrons after shells has aquired as much they can lik atomic no 11 which is sodium. 2 are held by k shell and 8 are held by L shell so remaining is 1  so its valency is 1.revise it through ncert and best of luck