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Hybrid Orbitals - sp and sp2

Geometry of molecules based on electron pairs

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Hybrid Orbitals - sp and sp2

Paired electrons are similar to Romeo and Juliet

Credit: Frank Dicksee
Source: http://commons.wikimedia.org/wiki/File:Romeo_and_Juliet_%28detail%29_by_Frank_Dicksee.png
License: CC BY-NC 3.0

How do you open the closed circle?

Romeo and Juliet were two of the great lovers of all time.  Their embrace allowed no other person to be a part of it – they only wanted to be with each other.  It took outside intervention (parents are like that!) to get them away from one another.  Paired electrons are similar to the lovers.  They do not bond covalently until they are unpaired.  Then they can become a part of a larger chemical structure.

Hybrid Orbitals – sp and sp2

sp Hybridization

A beryllium hydride (BeH2) molecule is predicted to be linear by VSEPR.  The beryllium atom contains all paired electrons and so must also undergo hybridization.  One of the 2s electrons is first promoted to the empty 2px orbital (see Figure below).

Promotion of the beryllium 2s electron

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Promotion of Be 2s electron.[Figure2]

Now the hybridization takes place only with the occupied orbitals and the result is a pair of sp hybrid orbitals.  The two remaining p orbitals (py and pz) do not hybridize and remain unoccupied (see Figure below).

Beryllium hybrid sp orbitals

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Be hybrid orbitals.[Figure3]

The geometry of the sp hybrid orbitals is linear, with the lobes of the orbitals pointing in opposite directions along one axis, arbitrarily defined as the x-axis (see Figure below).  Each can bond with a 1s orbital from a hydrogen atom to form the linear BeH2 molecule.

Image of how an s and p orbital combine to form sp orbitals

Credit: CK-12 Foundation - Jodi So, using 3D molecule by Ben Mills (Wikimedia: Benjah-bmm27)
Source: 3D molecule: http://commons.wikimedia.org/wiki/File:Beryllium-hydride-molecule-IR-3D-balls.png
License: CC BY-NC 3.0

The process of sp hybridization is the mixing of an s orbital with a single p orbital (the pxorbital by convention), to form a set of two sp hybrids. The two lobes of the sp hybrids point opposite one another to produce a linear molecule.[Figure4]

Other molecules whose electron domain geometry is linear and for whom hybridization is necessary also form sp hybrid orbitals.  Examples include CO2 and C2H2, which will be discussed in further detail later.

sp2 Hybridization

Boron trifluoride (BF3) is predicted to have a trigonal planar geometry by VSEPR.  First a paired 2s electron is promoted to the empty 2py orbital (see Figure below).

Promotion of the boron 2s electron

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Promotion of 2s electron.[Figure5]

This is followed by hybridization of the three occupied orbitals to form a set of three sp2 hybrids, leaving the 2pz orbital unhybridized (see Figure below).

Boron hybrid sp2 orbitals

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Formation of sp2 orbital.[Figure6]

The geometry of the sp2 hybrid orbitals is trigonal planar, with the lobes of the orbitals pointing towards the corners of a triangle (see Figure below).  The angle between any two of the hybrid orbital lobes is 120°.  Each can bond with a 2p orbital from a fluorine atom to form the trigonal planar BF3 molecule.

Image of how s and p orbitals combine to form sp2 orbitals

Credit: CK-12 Foundation - Jodi So, using 3D molecular structure by Ben Mills (Wikimedia: Benjah-bmm27)
Source: 3D molecule: http://commons.wikimedia.org/wiki/File:Boron-trifluoride-3D-balls.png
License: CC BY-NC 3.0

The process of sp2 hybridization is the mixing of an s orbital with a set of two p orbitals (px and py) to form a set of three sp2 hybrid orbitals. Each large lobe of the hybrid orbitals points to one corner of a planar triangle.[Figure7]

Other molecules with a trigonal planar electron domain geometry form sp2 hybrid orbitals.  Ozone (O3) is an example of a molecule whose electron domain geometry is trigonal planar, though the presence of a lone pair on the central oxygen makes the molecular geometry bent.  The hybridization of the central O atom of ozone is sp2.

Summary

Review

  1. Does the ground state beryllium atom contain any unpaired electrons?
  2. Why does one 2s electron in Be get promoted to a 2p orbital?
  3. What is the geometry of the two sp orbitals?

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Image Attributions

  1. [1]^ Credit: Frank Dicksee; Source: http://commons.wikimedia.org/wiki/File:Romeo_and_Juliet_%28detail%29_by_Frank_Dicksee.png; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  4. [4]^ Credit: CK-12 Foundation - Jodi So, using 3D molecule by Ben Mills (Wikimedia: Benjah-bmm27); Source: 3D molecule: http://commons.wikimedia.org/wiki/File:Beryllium-hydride-molecule-IR-3D-balls.png; License: CC BY-NC 3.0
  5. [5]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  6. [6]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  7. [7]^ Credit: CK-12 Foundation - Jodi So, using 3D molecular structure by Ben Mills (Wikimedia: Benjah-bmm27); Source: 3D molecule: http://commons.wikimedia.org/wiki/File:Boron-trifluoride-3D-balls.png; License: CC BY-NC 3.0

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