Atomic and Ionic Radii
The chart below illustrates the trend for atoms to decrease in size from left to right across the periodic table. As the atomic number increases the effective nuclear charge decreases due to greater shielding of the nuclear charge by the inner electrons. The radii are one half of the distance between nuclei in the elemental substances.
The chart below demonstrates the general trend of the increase in size of atoms as you move down a group in the periodic table. As the principle quantum number increases the electrons are further from the nucleus. The neutral atoms are in black. The ions of these atoms are shown in red. Here you can see the decrease in size with the removal of electrons from the atoms. This is due to the greater attraction of electrons to the excess protons in the nucleus of a positive ion.
Below is a chart showing the increased size of anions compared to the neutral atoms. The excess negative charge results in a lowered attraction between nuclei and electrons.
