Question

how find valancy of any atom and compund

Answer 1

with the help of perodic table

Answer 2

In a periodic table the horizontal rows and vertical columns are periods and groups. There are 7 periods and 18 groups.
To calculate the valency of the elements there is a simple method. If you know the atomic number, you could calculate their electronic configurations.
the group 1, 2 has valency equal to their group no.
group 13and 14 has valency of their group no. minus 10 i.e. 13-10=3.
group 15 to 18 has valency 18 minus their group no. i.e. 18-16=2 and so on..
In case of group no 3 to 12 you'll have to use the configurations to calculate.