ZingPath: Periodic Trends

An atomic radius can be defined as the distance from the center of an atom out to the point where the electrons are most likely to be found. Although this may not sound like a very precise scientific definition, it is the best that chemistry and physics can do. Since electron orbitals are defined in terms of probabilities, there is no exact limit that the electron cannot go past. The size of an atomic radius is therefore an estimate based on where the outermost electrons are most likely to be found. The sizes of most atomic radii are between 0.3 and 3 angstroms.

The size of an atomic radius generally decreases as one moves from left to right across a period on the periodic table. This is because all of the elements in a period have the same number of electron orbitals. Since the number of positively charged protons increases across a period, there is a greater attraction for the negatively charged electrons that are found in the same orbital. This causes the electrons to be drawn more closely to the nucleus of the atom, thereby decreasing the radius of the atoms as you move from left to right.

The size of an atomic radius generally increases as one moves down through a group of elements. This is because the elements add an electron orbital for each step down in a group. Even though there are also more protons in each element as one steps down a group, the increase in electron orbitals offsets the attraction of the extra protons.