Ionization Energy Fundamentals

• Electrons are drawn to the positive nucleus of an atom.
• Energy is needed to overcome this attraction and remove electrons.
• The electron removed is from the outermost shell, called the valence electron.
• Ionization is the process of removing an electron from an atom.
• Ionization energy is the energy required to remove an electron from an atom

Key Definition

• First ionization energy is the energy needed to remove one mole of electrons from one mole of a gaseous atom, forming one mole of gaseous 1+ ions.

Formula for Ionization Energy

• The formula for 1st ionization energy is: X(g) -> X(g)[+] + e[-]
• The formula for 2nd ionization energy is: X+ -> X(g)[2+] + e[-]
• The formula for 3rd ionization energy is: X2+ -> X(g)[3+] + e[-]
• Ionization energy is measured in KJmol-1 (kilojoules per mole).
• Ionization values are positive because the process is endothermic, requiring energy input
• Endothermic reactions require the absorption of energy/heat.
• A larger ionization energy value means it is harder to ionize.

Factors Affecting Ionization Energy

• Charge of the Nucleus
  • Represents how many protons attract the outer electrons.
  • A higher number of protons results in a higher nuclear charge.
  • A stronger attraction/electrical force requires more energy to ionize, leading to a larger ionization energy.
• Atomic Radius
  • Indicates how far the outer electron is from the nucleus.
  • The further an electron is, the less energy is needed to ionize it.
  • Atomic radius increases down a group and decreases across a period.
• Shielding
  • Represents number of electrons between the nucleus and the outer electrons, shielding it from the nucleus's attractive forces.
  • Inner electrons shield the attractive forces of the nucleus.
  • More inner electrons result in higher shielding and lower energy needed to ionize.

Trends in Ionization Energies

• First ionization energy increases across a period.
• First ionization energy decreases down a group.
• Charge increases across a period, causing increased ionization energy.
• Atomic radius decreases across a period, as increased charge pulls electrons closer to the nucleus. Shorter distance increases the charge holding the electrons, resulting in higher ionization energy.
• Shielding has no effect across a period since the electron removed is in the same energy level and electron configuration is the same.
• Atomic radius increases down a group, valence electrons are further away from the nucleus which lowers energy needed for ionization as you travel down the group.
• Shielding increases down a group as the number of energy levels and electrons increases which lowers ionization energy.
• Overall, ionization energy decreases as the group is descended however charge increases down the group which increases ionization energy.

Successive Ionization

• Each successive ionization requires increasing energy, especially when crossing over shells.
• Ionized atoms have a positive charge, with more protons than electrons.
• It pulls on the nucleus stronger with each successive ionization.
• Removing an electron from a lower energy level (inner shell) which lies closer to the nucleus needs significantly more energy due to stronger attraction to the nucleus.

Exceptions to Periodic Trends

• Exceptions are caused by the existence of orbitals
• The reasons why the first ionization energy differs from trends:
  • Higher orbitals are easier to remove.
  • Paired electrons in orbitals cause repulsion.
• Example: Beryllium and Boron in period 2
  • Beryllium is expected to have a higher ionization energy due to its position.
  • Boron's valence electron is in the 2p subshell, while Beryllium's is in the 2s subshell.
  • Boron has a lower ionization energy than Beryllium
• Example: Nitrogen and Oxygen in period 2
  • Nitrogen has a lower nuclear charge and higher atomic radius than Oxygen.
  • Oxygen has a lower first ionization energy than Nitrogen due to pairings of electrons.
  • Orbitals hold a max of two electrons, but when full, electron repulsion makes it easier to remove.
  • Nitrogen has three electrons in its 2P sub-shell, each occupying a single orbital so it is more stable.
  • Oxygen has four electrons in its valence 2P sub-shell, with the first orbital containing the first and fourth electron paired so it causes repulsion which makes it easier to remove.

Ionization Energies as Evidence for Energy Levels

• The existence of energy levels is proven by the large gaps in successive ionization energies which correspond to the removal of electrons from energy levels closer to the nucleus and the need for more energy.
• Successive ionization energy increases due to the increased ratio of protons to electrons.
• In an ionization graph, a large gap between electrons removed indicates that the electron was removed from an energy level closer to the nucleus.
• Electrons in inner shells experience greater attraction, less shielding, and a shorter distance from the nucleus.
• Effective nuclear charge is the ratio of protons to electrons.