Question

How to find valencey of elements whose atomic number is more than 25?

Answer 1

By figuring out their group in the Periodic Table by writing out their electronic configuration.

Answer 2

my friend,
my advice is to the whole periodic table buy their groups and periods. they have their own advantages.

if you remember the elements by their groups, every group have the same valency. so you could easily find the valency up to 118 elements. this is the easy and the best way to remember the valencies.

for example,
group 1 elements that is alkali metals have a same valency of one
group 2 elements or alkaline earth metals have a valency of 2
group 2 elements have a valency of 3 or 4
and so on....

try to remember the whole periodic table by the groups so you can easily remember all the valencies value..

you can either remember the periodic table by nemonics for periodic song

hope it helps and all the best for your exams