Atomic radius facts for kids
The atomic radius is rather hard to define, because it is impossible to know where all of the electrons are at a given time.
In the periodic table of the elements, the atomic radius tends to increase as you move down a group because the number of electrons is greater, and so the radius of the circle of electrons in the element is bigger. As you move to the right across a period, the atomic radius tends to decrease due to the shielding effect.
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Atomic radius Facts for Kids. Kiddle Encyclopedia.