Ch. 8.4 and 8.5: Ionization Energy and Electron Affinity
•
First Ionization Energy
o Helps to understand reactivity of elements (especially metals, which lose electrons)
o Energy needed to remove highest-energy electron from the atom in the gas phase:
§ energy + X → X+ + e–
o Trends in First IE
§ Opposite to atomic radius trend:
• IE increases moving up a group
and moving left to right across a
period
§ IE increases (harder to remove) as Zeff
increases across a row
• Smaller radius
• Stronger coulombic attraction to
nucleus
§ IE increases as radius decreases
• Electrons in lower energy outer orbitals
• Stronger coulombic
attraction to nucleus
o 1st Ionization Energy:
Exceptions
§ ns2 → np1 [e.g. Mg → Al] or
(n-1)d10 → np1 [e.g. Zn →
Ga]
• IE decreases since np
electron is shielded by
inner electrons, so it is
higher energy relative to
nucleus (weaker
coulombic attraction) &
easier to remove
3
§ np → np4 [ e.g. N → O]
• IE decreases due to
repulsion of paired
electrons (weaker coulombic attraction), only slight increase in Zeff
•
Additional Ionization Energies
o 2nd IE: energy + X+ → X2+ + e–
o 3rd IE: energy + X2+ → X3+ + e–, etc.
o IE increases moderately due to decreased
repulsion among remaining valence electrons,
stronger coulombic attraction to nucleus.
o IE increases dramatically (×5-×10) when
removing inner core electrons
Ionization Energy-Electron Affinity Notes.docx
•
Electron Affinity
o Negative (in most texts) of the energy change when
an electron is accepted by a gaseous atom:
§ X + e– → X–
EA=–∆H
o Generally increases moving up a group and left-toright across a period (except Noble gases) as
coulombic attraction of added electron to nucleus
increases.
§ Show reactivity trends in non-metals (gaining
electrons).
o Exceptions to EA trends
§ EA is small or ≤0 for Group 2A, 8A (e.g. Mg, Ne)
• Atoms have little tendency to gain
electron since extra electron would
enter shielded, higher energy orbital
(np1 or [n+1]s1)
§ EA is small or ≤0 for Group 5A (e.g. N,
P)
• Atoms have little tendency to gain
electron since extra electron would
enter half-occupied np3 sublevel, so
electron-electron repulsion reduces
gain in energy
§ Period 2 elements have lower EA than
Period 3 elements
• Smaller elements have higher
electron density, more repulsion of
added electron
p.2