The atomic radius is a measure of the size of an atom, typically defined as the distance from the nucleus to the outermost shell of electrons. It varies across the periodic table, generally increasing as you move down a group due to the addition of electron shells. Conversely, the atomic radius tends to decrease from left to right across a period because of the increasing positive charge in the nucleus, which pulls the electrons closer. Understanding atomic radius is important in chemistry as it influences various properties of elements, including reactivity and bonding. For example, smaller atoms, like those of helium, tend to form stronger bonds compared to larger atoms, such as cesium. This concept helps explain trends in the behavior of elements in the periodic table.