Answer Hi Janet, Good question, thanks. The atomic radius is the size of the atom. Moving down a column in the periodic table will produce larger atoms. The atomic number, which describes how many protons are in an element, also increases as you move down a column. However, the two numbers aren’t the same. The melting point essentially measures how well two adjacent molecules can stick together. If they stick together very rigidly, the substance is a solid. If they stick together moderately well it is a liquid. And if they don’t stick together well it is a gas. Atoms with a smaller radius can get closer together and stick together much better than an atom with a large radius. Therefore, it is generally much easier to melt the elements with larger diameters. As the radius increases, the melting point decreases.