Atomic radii across
–> As you go down the periodic table, the atomic radii increases. This is due to the fact that a new electron shell/energy level is being added, therefore the radii increases. For example, Hydrogen has one electron shell/energy level. Lithium, which is located underneath Hydrogen, has two electron shells/energy levels, and therefore has a larger atomic radius than Hydrogen.
–> As you go from left to right on the periodic table in the same period, the atomic radii decreases. This is due to the fact that there are electrons being added to the same shell/energy level, but the number of protons in the nucleus is increasing. The increase in the number of protons pulls the electrons with more force towards the nucleus, which decreases the atomic radius.
Ionic radii across the periodic table
–> As you go down the periodic table, the ionic radii increases. This is because the atoms are getting larger, and therefore their ionic radii is larger. For example, Lithium ionizes to have only one full electron shell/energy level. Sodium, which is located underneath Lithium, ionizes to have two full electron shells/energy levels.
–> As you move from left to right on the periodic table in the same period, the ionic radii decreases. This is because the metals are inclined to lose electrons to ionize, which allows for the protons in the nucleus to pull stronger on the remaining electrons and bring them closer to the nucleus. As the atom gets larger, the number of protons increases, which increases the force of attraction by the protons on the electrons.
However, once you reach the non-metals, the ionic radii increases dramatically. This is because the non-metals gain electrons to ionize. The increase in the number of electrons in the same electron shell/energy level results in an increase in force of repulsion between the electrons, causing the electrons to spread further apart. Consequently, the ionic radius is large. But as you keep moving towards the noble gases, the ionic radii in the non-metals decreases. This is because less electrons are being gained to ionize, therefore the force of repulsion between the electrons is less, and thus the ionic radii is smaller as compared to the non-metal ions preceding a specific non-metal ion.