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mvcdc eNotes educator| Certified Educator

Ionization energy is the energy needed to remove one or more electrons from a neutral atom. For instance, there is a corresponding ionization energy needed to produce Na+ from the neutral Na (sodium) atom. The energy is used to 'take away' the electron, which is stably revolving around the nucleus of the atom.

In the periodic table, there is a very noticeable trend in ionization energy. It increases as you go from left to right, and decreases as you go from top to bottom. Hence, Fluorine has a higher ionization energy than Lithium, and Oxygen has a higher ionization energy than Selenium. You might have noticed that in chemical reactions, particularly the formation of ions, fluorine usually gets a negative charge (F-) while lithium a positive charge (Li+). This is because atoms with lower ionization energies tend to have positive charges (easier to remove electrons) and those with higher IE tend to produce anions (harder to remove electrons, but easier to add another one to complete the octet as in the case of F-). This trend can be explained by various factors and related trends:

  1. Size of the atom- Electrons are attracted to the nucleus. The stronger the attraction, the harder it is to get them to be released. As the atom size increases, the electron moves farther from the nucleus, and hence is easier to remove. The larger Selenium atom has a lower ionization energy than the smaller oxygen atom.
  2. Nuclear charge- Higher nuclear charge atoms attract electrons more strongly, and have higher ionization energies as a result. In the same row, atoms to the right have higher nuclear charge (more protons). Hence, fluorine has a higher ionization energy than lithium.
  3. Shielding effect- This is related to the actual attractive force 'felt' by the valence shell electrons. The higher the number of inner shells, the lower this gets and the lower the ionization energy. Atoms towards the bottom of the periodic table have more inner shells, and hence are lower in ionization energy.

These main factors affect ionization energy and also affect each other. The key thing to remember is ionization energy is dependent on how strongly the electrons are attracted to an atom's nucleus.