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Molecular Orbital Theory and Hybridization CC-BY-NC

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As you bring two atoms together, there is an attraction between them.
However, too close and you will get a repulsion

RECALL: 1s, 2s and 2p atomic orbitals exist. When two orbitals overlap we get a sigma bond and a sigma* bond (antibond)
RECALL: Orbitals do not exist in real life, they are mathematical results which predict the probabilty of finding the electron.

To read and understand this article, you should be familliar with atomic orbitals

1Molecular Orbital Theory

  • In phase overlap results in a constructive bond
  • out of phase overlap results in a destructive bond (anti bond)
  • If you start with two orbitals you must end up with 2 molecular orbitals!
  • The terms "inphase" and "out of phase" correspond to how we overlap the orbitals. If the two molecules have the same probability curve and we overlap them it is known as in phase and vice versa.
  • Overlapping out of phase orbitals will result in a nodal plane/surface

2Hybridization

  • Bonds want to reorganize themselves so that they are as far apart from each other as possible.
  • Carbon has a completed 1s and 2s orbital and a 2p paritally filled in the first two orbitals
  • They can rehybridize to form a 4sp3 hybridized orbitals where there are now 4 orbitals of the same energy level each with one electron
  • They could also rehybridize to form a 3sp2 orbital and a 2pz orbital. (note: the 2pz orbital is of higher energy than the combined hybridized orbitals)

3Multiple Bonds

  • When two orbitals overlap and there is another orbital with a single electron, it is possible to overlap once again to form a pi and pi* bond

4Table

Type MO
4 groups 4sp3
4 single bonds 4sp3
3 single bonds and a double bond 3sp2
3 groups 3sp2
2 bonds and a triple bond sp
2 bonds sp
  • The number in front of sp corresponds to the total number of hybrid orbitals. The number after the sp refers to how much p character the orbitals have. The less p character, the more s character, the closer the electrons in the orbital will be to the nucleus.
  • Electron pairs count as substituent groups

5Shapes

Hybridization Shape Notes
4sp3 Tetrahaedral Long bond lengths and all sigma bonds (eg: Ethane)
3sp2+1p Trigonal Planar intermediate bond lengths, a sigma and a pi bond exist (eg: Ethene)
sp + 2p Linear Short bond lengths, a sigma and two pi bonds (eg: Ethyne)

5.1More examples

  • H2O, 4 groups around Oxygen (2 electron pairs and 2 bonds) --> 4sp3 orbitals

  • BF3, 3 groups around boron (3 bonds) --> 3sp2 orbitals

  • Note, boron has an empty p orbital around it (this means it can gain 2 more electrons from somewhere)