Down a group vs across a group

[raina_bandekar_1G]

Can someone explain why atoms get bigger down a group (when adding electrons) and when going across a group they get smaller even though you are still adding electrons?

[Annabel Preston]

Atoms get bigger down a group because with each level you go down you are adding an electron shell which increases the atomic radius because there are electrons further away from the nucleus. Across a period they get smaller because even though you are adding an electron you are also adding a proton which exhibits a greater pull on the electrons and the electrons get pulled closer to the nucleus decreasing the atomic radius.

[Raquel]

Adding to the other post, simply put, moving to the right the size gets smaller because there is a greater pull of the protons towards the nucleus, as you move down there are more electrons so relatively it gets larger, but with the added protons it is able to pull them in further than the atoms on the left bottom.