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Friday, 18 August 2017

Shapes of Molecules and Hybridization

Shapes of Molecules and Hybridization
A.      Molecular Geometry

 • Lewis structures provide us with the number and types of bonds around a central atom, as well as any NB electron pairs. They do not tell us the 3-D structure of the molecule

CH4 as drawn conveys no 3-D information (bonds appear like they are 90° apart)

• The Valence Shell Electron Pair Repulsion Theory (VSEPR), developed in part by Ron Gillespie at McMaster in 1957, allows us to predict 3-D shape. This important Canadian innovation is found worldwide in any intro chem course. • VSEPR theory has four assumptions

1. Electrons, in pairs, are placed in the valence shell of the central atom

2. Both bonding and non-bonding (NB) pairs are included

3. Electron pairs repel each other Æ maximum separation.

4. NB pairs repel more strongly than bonding pairs, because the NB pairs are attracted to only one nucleus • To be able to use VSEPR theory to predict shapes, the molecule first needs to be drawn in its Lewis structur

VSEPR theory uses the AXE notation (m and n are integers), where m + n = number of regions of electron density (sometimes also called number of charge clouds)

1.       Molecules with no NB pairs and only single bonds • We will first consider molecules that do not have multiple bonds nor NB pairs around the central atom (n = 0).

2.       • Example: BeCl2 o

Molecule is linear (180°)

Example: BF3 o Molecule is trigonal (or triangular) planar

(Example: CH4 o Molecule is tetrahedral (109.5°) 120°)

Example: PF5 o Molecule is trigonal bipyramidal (90° and 120°). There are three X atoms in a planar triangle and two axial atoms, one above and one below the central atom.

Example: SF6 o Molecule is octahedral (all 90°)

1.       sp hybridization

• The combination of one s and one p results in the formation of two sp orbitals. These two hybrid orbitals are 180° apart.


• Where we have used only one of the p orbitals, there must be two p orbitals remaining. (Recall there are three p orbitals). We’ll see later on that these leftover orbitals are used when there is multiple bonding. • The two leftover p are 90° to each other and the sp hybrids

2.       sp2 hybridization

• One s + two p = three sp2 orbitals 120° apart.

• There is one leftover p orbital remaining, since we started with three p orbitals and used two of them for hybridization.

3.       sp3 hybridization • One s + three p = four sp3 orbitals 109.5° apart.

• There are no leftover p orbitals.

4. Others: sp3 d and sp3 d2 hybridization • One s + three p + one d = five sp3 d orbitals • One s + three p + two d = six sp3 d2 orbitals

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