What is atomic radius?
The radius of an atom. It is defined as the distance between the center of the atom's nucleus and its valence electrons.
Atomic radius is the distance from an atom's nucleus to one of its valence electrons. Every atom has a difference atomic radius. There are certain trends for atomic radii.
When we go down a group, more electrons are present in the atom, so more electron shells are needed to hold those electrons, and so atomic radius increases down a group.
When we go from left to right from a period on the periodic table, we get to more electronegative elements, and so they will pull the valence electrons more tightly than others, therefore decreasing the atom's atomic radius. Thus, atomic radius decreases from left to right across a period.
Cations are positive ions which have lost electrons, and so may lose an electron shell, and are generally smaller than neutral elements.
Anions are ions that have gained electrons, and so may need extra electron shells to compensate the amount of electrons, and so are bigger than their neutral counterparts.
I hope this helps!