Question

when we go left to right in a periodic table the size of atom decrease.explain

Answer 1

It is because of nuclear charge.
When we go left to right the nuclear charge increases which nuclease the electrons towards the atom decreasing the size of atom.

Hope this helps.....

Answer 2

When we go left to right in a periodic table the size of atom decrease because the electrons are added to the same shell and the effective nuclear charge increases and it pulls the outermost electrons towards itself which results in the decrease in size