Atomic Radius: The atomic radius is defined and measured by one half the distance between the nuclei of identical atoms that are bonded together. It has a common pattern or trend for that apply for groups and periods. It is measured in pm. The atomic radius trend for groups(columns on the periodic table) is starting from the top to the bottom the atomic radius increases. The atomic radius will increase from top to bottom because of two reasons. The first being that the atomic radius grows bigger because the farther down the periodic table the more electrons an atom has. The atom has to adopt more energy levels to make room for the extra electrons causing the electron cloud and thus the atomic radius to grow bigger. The second reason the atomic radius grows larger the farther down a group because of the shielding factor. The shielding factor reduces the attraction between electrons on the outer energy level and the nucleus because the shielding electrons do just what they say they do. The shielding electrons get in the way and block attraction between the nucleus(proton location) and the outer energy levels(electron location). Because there is no attraction pulling the electrons and protons together, the atomic radius remains larger. The trend for atomic radius across a period is from left to right the atomic radius decreases. The atomic radius decreases because the attraction between the protons and electrons grows greater. As one moves across a periodic table the elements have an increasing amount of protons causing for a greater pull or attraction between protons and electrons thus because the electrons are drawn to the nucleus the atomic radius grows smaller.