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Molecular Shapes: Molecules with No Lone Pairs on Central Atom

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Molecular Shapes: No Lone Pairs on Central Atom
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Repulsion in an electroscope is like repulsion between electron pairs

Credit: Sylvanus P. Thompson
Source: http://commons.wikimedia.org/wiki/File:Electroscope_showing_induction.png
License: CC BY-NC 3.0

How does an electroscope work?

An electroscope is a device used to study charge.  When a positively charged object (the rod) nears the upper post, electrons flow to the top of the jar leaving the two gold leaves positively charged. The leaves repel each other since both hold positive, like charges. The VSEPR theory says that electron pairs, also a set of like charges, will repel each other such that the shape of the molecule will adjust so that the valence electron-pairs stay as far apart from each other as possible. 

Central Atom with No Lone Pairs

In order to easily understand the types of molecules possible, we will use a simple system to identify the parts of any molecule.

A = central atom in a molecule

B = atoms surrounding the central atom

Subscripts after the B will denote the number of B atoms that are bonded to the central A atom.  For example, AB 4 is a molecule with a central atom surrounded by four covalently bonded atoms.  Again, it does not matter if those bonds are single, double, or triple bonds.

AB 2 : Beryllium hydride (BeH 2 )

Beryllium hydride consists of a central beryllium atom with two single bonds to hydrogen atoms.  Recall that it violates the octet rule.

H-Be-H

According to the requirement that electron pairs maximize their distance from one another, the two bonding pairs in the BeH 2 molecules will arrange themselves on directly opposite sides of the central Be atom.  The resulting geometry is a linear molecule, shown in the Figure below in a “ball and stick” model.

Model of beryllium hydride, which is linear

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

Beryllium hydride model. [Figure2]

The bond angle from H-Be-H is 180° because of its linear geometry.

Carbon dioxide is another example of a molecule which falls under the AB 2 category.  Its Lewis structure consists of double bonds between the central carbon and the oxygen atoms (see Figure below ).

Structure of carbon dioxide, which is linear

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

Carbon dioxide bonding. [Figure3]

The repulsion between the two groups of four electrons (two pairs) is no different than the repulsion of the two groups of two electrons (one pair) in the BeH 2 molecule.  Carbon dioxide is also linear (see Figure below ).

Model of carbon dioxide, which is linear

Credit: User:Benji9072/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Carbon_dioxide_structure.png
License: CC BY-NC 3.0

Carbon dioxide. [Figure4]

AB 3 : Boron Trifluoride (BF 3 )

Boron trifluoride consists of a central boron atom with three single bonds to fluorine atoms (see  Figure   below ) .  The boron atom also has an incomplete octet.

Structure of boron trifluoride, which has a trigonal planar shape

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

Boron trifluoride bonding. [Figure5]

The geometry of the BF 3 molecule is called trigonal planar (see Figure below ).  The fluorine atoms are positioned at the vertices of an equilateral triangle.  The F-B-F angle is 120° and all four atoms lie in the same plane.

Model of boron trifluoride, which has a trigonal trigonal shape

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: http://commons.wikimedia.org/wiki/File:Boron-trifluoride-3D-balls.png
License: CC BY-NC 3.0

Boron trifluoride model. [Figure6]

AB 4 : Methane (CH 4 )

Methane is an organic compound that is the primary component of natural gas.  Its structure consists of a central carbon atom with four single bonds to hydrogen atoms (see  Figure   below ). In order to maximize their distance from one another, the four groups of bonding electrons do not lie in the same plane.  Instead, each of the hydrogen atoms lies at the corners of a geometrical shape called a tetrahedron.  The carbon atom is at the center of the tetrahedron.  Each face of a tetrahedron is an equilateral triangle.

The tetrahedral structure of methane

Credit: (Left) Pearson Scott Foresman; (Right) Ben Mills (Wikimedia: Benjah-bmm27)
Source: (Left) http://commons.wikimedia.org/wiki/File:Tetrahedron_(PSF).png; (Right) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-balls.png
License: CC BY-NC 3.0

Tetrahedral structure of methane. [Figure7]

The molecular geometry of the methane molecule is tetrahedral (see  Figure   below ). The H-C-H bond angles are 109.5°, which is larger than the 90° that they would be if the molecule was planar.  When drawing a structural formula for a molecule such as methane, it is advantageous to be able to indicate the three-dimensional character of its shape.  The structural formula below is called a perspective drawing.  The dotted line bond is to be visualized as receding into the page, while the solid triangle bond is to be visualized as coming out of the page.

Perspective model of methane

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: http://commons.wikimedia.org/wiki/File:Methane-2D-stereo.svg
License: CC BY-NC 3.0

Methane perspective model. [Figure8]

Summary

  • Electron pairs repel each other and influence bond angles and molecular shape.

Practice

Questions

Use the link below to answer the following questions:

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/vsepr.html

  1. What is the shape of PF 5 ?
  2. What C-H bod angles would we predict for methane if the molecule were planar?
  3. What molecule has the configuration of an octahedron?

Review

Questions

  1. What are the bond angles in carbon dioxide?
  2. What molecule has bond angles of 109.5 ° ?
  3. What is the geometry of the BF 3 molecule?

Image Attributions

  1. [1]^ Credit: Sylvanus P. Thompson; Source: http://commons.wikimedia.org/wiki/File:Electroscope_showing_induction.png; License: CC BY-NC 3.0
  2. [2]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Beryllium-hydride-molecule-IR-3D-balls.png; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  4. [4]^ Credit: User:Benji9072/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Carbon_dioxide_structure.png; License: CC BY-NC 3.0
  5. [5]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  6. [6]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Boron-trifluoride-3D-balls.png; License: CC BY-NC 3.0
  7. [7]^ Credit: (Left) Pearson Scott Foresman; (Right) Ben Mills (Wikimedia: Benjah-bmm27); Source: (Left) http://commons.wikimedia.org/wiki/File:Tetrahedron_(PSF).png; (Right) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-balls.png; License: CC BY-NC 3.0
  8. [8]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Methane-2D-stereo.svg; License: CC BY-NC 3.0

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