This graph displays the periodic trend of ionization energy with respect to atomic number.
RJHall, Wikipedia Commons
This graph shows the relationship between the electron binding energy, atomic number, and electron configuration. As you move left to right within a period, the ionization energy of an element generally increases.
Bvcrist, Creative Commons License
The ionization energy, or ionization potential, is the energy required to completely remove an electron from a gaseous atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher its ionization energy will be.
Units for Ionization Energy
Ionization energy is measured in electronvolts (eV). Sometimes the molar ionization energy is expressed, in J/mol.
First vs Subsequent Ionization Energies
The first ionization energy is the energy required to remove one electron from the parent atom. The second ionization energy is the energy required to remove a second valence electron from the univalent ion to form the divalent ion, and so on. Successive ionization energies increase. The second ionization energy is always greater than the first ionization energy.
Ionization Energy Trends in the Periodic Table
Ionization energies increase moving from left to right across a period (decreasing atomic radius). Ionization energy decreases moving down a group (increasing atomic radius). Group I elements have low ionization energies because the loss of an electron forms a stable octet. It becomes harder to remove an electron as the atomic radius decreases because the electrons are generally closer to the nucleus, which is also more positively charged.