What is the trend in ionic size from top to bottom of a group? Explain using main energy levels (n).

Question

What is the trend in ionic size from top to bottom of a group? Explain using main energy levels (n).

Answer 1

Going from top to bottom the atomic size gets bigger as you are adding an energy shell each time.  This energy shell shields the valence electrons on the outside from the positively charged protons inthe nucleus, therefore there is a lower attractive force between the protons and outermost electron.  As there is less attractive force, the valence electrons will sit further away from the nucleus, thereby increasing atomic radius down the group.  

Across a period the size will decrease as you are adding protons and electons with each element, increasing the attractive force between the nucleus and valence electrons and so the radius decreases