Atomic radius refers to the size of an atom. It is defined as one-half the distance between between two nuclei in a chemical bond.
Atomic radius is measured in picometers. 1 picometer is a trillionth of a meter. 1 pm = 10-12 meters. Atomic radii range from 32 pm, the atomic radius of helium, to 348 pm for francium.
As you go down a group in the Periodic Table, atomic radius increases. This is because, with each successive period, another energy level of electrons is being added, moving the outermost electrons further from the nucleus. Consider the first column of the Periodic Table, consisting mostly of the alkali metals (with the notable exception of the nonmetal hydrogen). Hydrogen has 1 valence electron in the first energy level n=1. Lithium has two shells of electrons. The first shell, or energy level, contains two electrons, and the second energy level n=2 contains 1 valence electron. Since the lithium’s valence electron is in the second energy level, it is further from the nucleus than hydrogen’s valence electron. Therefore, the atomic radius of lithium is greater than that of hydrogen. Moving further down the group, sodium has a total of 11 electrons. Sodium’s first two electrons are in the first energy level, n=1. Its next 8 electrons are in the second energy level, n=2. And its one valence electron is in the third energy level n=3. With three energy levels of electrons, sodium’s atomic radius is greater than lithium’s.
As you go across a period in the Periodic Table, atomic radius decreases. This seems counter intuitive because, with each successive element in the Periodic Table another proton and electron is being added. Realize, however, that atoms are mostly empty space, so even though the atomic mass generally increases from element to element, atomic size does not. Within the same period, no new energy levels are being added. With the addition of each subsequent proton in the same period, the nuclear charge — the positive charge of the nucleus — increases. This increased nuclear charge exerts a greater attractive force on the outermost electrons, according to Coulomb’s Law, drawing them closer to the nucleus.
Li 152 pm
Be 105 pm
B 85 pm
Here is a graph of atomic radius as a function of atomic number.
Notice that, between atomic numbers 3 and 9 (Lithium to Fluorine), atomic radius decreases. Lithium through fluorine represent most of the second period of the periodic table. Notice further that sodium Na, the first element of the third period, is larger in atomic radius than lithium, because atomic radius increases as you go down a group.