Question

why dose the size of the atom increase down the group

Answer 1

Here is ur answer -
The size of the atom increases on moving down the group because the number of electrons increases.
Hope this helps :-)

Answer 2

This is caused by the increase in the number of protons and electrons across a period. One proton has a greater effect than one electron; thus, electrons are pulled towards the nucleus, resulting in a smaller radius.Atomic radius increases from top to bottom within a group. This is caused by electron shielding.