The ionization energy of a chemical species (i.e., an atom or molecule) is the energy required to remove electrons from gaseous atoms or ions.
The ionization energy is different for electrons of different atomic or molecular orbitals. More generally, the nth ionization energy is the energy required to strip off the nth electron after the first n-1 electrons have been removed. It is considered a measure of the tendency of an atom or ion to surrender an electron or the strength of the electron binding. The first ionization energy is the energy required to remove the 1st electron, the second ionization energy is the energy to remove the second electron, and so on and so forth.
Prediction from Electronic Structure for Elements in Different Groups or Rows
Moving left to right within a period or upward within a group, the first ionization energy generally increases. As the atomic radius decreases, it becomes harder to remove an electron that is closer to a more positively charged nucleus. The ionization energy of an element increases as one moves across a period in the periodic table because the electrons are held tighter by the higher effective nuclear charge. This is because additional electrons in the same shell do not substantially contribute to shielding each other from the nucleus, however, an increase in atomic number corresponds to an increase in the number of protons in the nucleus.
Conversely, as one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience greater shielding. They experience a weaker attraction to the positive charge of the nucleus. Ionization energy increases from left to right in a period and decreases from top to bottom in a group.
This pattern generally holds true for a few exceptions. There is a drop in the ionization energy between orbitals s and p as there the s orbitals have a paired electron set and a singular electron in the p orbital is easier to remove in energy. There is also a decrease ionization energy between the p3 and p4 sub orbitals as when progressing to p4 there is a pairing of electrons and this pairing causes spin pair repulsion meaning any elements with a p4 suborbital is slightly lower in ionization energy than those with a p3 sub orbital where there are 3 single electrons in individual sub orbitals.
Practice Questions
MCAT Official Prep (AAMC)
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Key Points
• The ionization energy is the energy required to remove an electron from its orbital around an atom to a point where it is no longer associated with that atom.
• The ionization energy of an element increases as one moves across a period in the periodic table because the electrons are held tighter by the higher effective nuclear charge.
• The ionization energy of the elements increases as one moves up a given group because the electrons are held in lower-energy orbitals, closer to the nucleus and therefore are more tightly bound (harder to remove).
• Generally, the nth ionization energy is the energy required to strip off the nth electron after the first n-1 electrons have been removed.
Key Terms
Valence electrons: The electrons in the highest occupied principal energy level of an atom.
Ionization energy: The energy needed to remove an electron from an atom or molecule to infinity.
First ionization energy: The energy needed to remove the outermost, or highest energy, electron from a neutral atom in the gas phase.
Second ionization energy: The energy it takes to remove an electron from a 1+ ion (meaning the atom has already lost one electron and now removing the second).
