The atomic radius of an atom is the distance between the nucleus of an atom to its outermost electron. Atomic radius is an example of a periodic trend. Periodic trends are patterns of properties that can be found on the periodic table. Periodic trends enable us to predict certain properties of elements based upon their location on the periodic table.
The Atomic Radius Trend Across Periods:
- Atomic radius tends to decrease as you move from left to right across a period (row) on the periodic table. As you move from left to right across the periodic table, the number of positively charged protons in the nucleus increases.This causes the overall positive charge of the nucleus to increase.
- At the same time, the number of negatively charged electrons also increases as you move from left to right across the periodic table. When electrons are added to elements in the same period, they are all added to the same energy level.
- As the nuclear charge increases, the attraction between the positively charged nucleus and the negtively charged electrons increases.
- The atomic radius gets smaller because the nucleus is able "pull" the electrons in tighter.
- Therefore, since O is located to the left of F in the same period on the periodic table, we can predict that O will have a larger atomic radius than F.
The Atomic Radius Trend Down Groups:
- Atomic radius tends to increase as you move from top to bottom down a group (column) on the periodic table.
- As you move down a group on the periodic table, the number of positively charged protons and negatively charged electrons increases; however, the valence electrons are added to different energy levels farther away from the nucleus. This means that the atom's negatively charged valence electrons will be less attracted to the positively charged nucleus. The farther an electron is from the nucleus, the lower the attraction of the nucleus for the electron.
- At the same time, there are more electrons present between the nucleus and the valence electrons. The presence of other electrons between the nucleus and the valence electrons, "shields" the valence electrons from the pull of the positively charged nucleus. Because of this, the positively charged nucleus has less attraction for the valence electrons.
- Therefore, since S is located below O in the same group on the periodic table, we can predict that S will have a larger atomic radius than O.
Based on our understanding of atomic radius periodic trends, and the relative location of S, O, and F on the periodic table, we can predict that the correct order of these elements based on increasing atomic radius is: F, O, S.