chem assignment 5

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Experiment 6

MOLECULAR GEOMETRY

Experiment 5: MOLECULAR GEOMETRY

Purpose: To build chemical structures using molecular models and to extract information about nonbonding groups of electrons on a central atom in a molecular model structure.

Introduction

Molecular Geometry: The 3-dimensional shape of a molecule, the molecular geometry, plays an important part in how a substance reacts both physically and chemically. In your chemistry lectures you should have already learned how to write Lewis structures when given a molecular formula. This experiment will not deal with learning how to write Lewis structures. However, it is important to review a few terms before we proceed. We will be dealing with both neutral molecules (e.g. H2O) as well as polyatomic ions (e.g. NO3). The term “species” will be used to indicate both neutral molecules and polyatomic ions.

A molecular formula tells us only which elements and how many atoms of each element are present. It does not tell us how they are bonded to each other. A Lewis structure takes us one step further by showing which atom is joined to which and whether an atom has nonbonding electrons (also known as lone pairs). It is important to note that generally a Lewis structure is not meant to show the geometry. Examples below show the difference in these terms:

lone pair

H2O

2 bonding electrons

= 1 covalent bond

molecular formula Lewis structure

Once you have established the correct Lewis structure for a substance, you would be able to determine its geometry by applying the VSEPR (Valence Shell Electron Pair Repulsion) theory. The concept is based on the fact that electron pairs repel each other because they are all negatively charged. The geometry of the species would be based on how far apart electron pairs can move away from each other without breaking any bonds. For example, for water, there are four electron pairs on the oxygen atom (two pairs of bonding electrons and two lone pairs). The best spatial arrangement to keep the electron pairs apart would be in a tetrahedral geometry as shown below, with bond angles of 109.5.

In describing the molecule, however, we do not call it “tetrahedral” because electrons are very small and we consider only the arrangement of the atoms in our description. Students often confuse the two geometries: geometry of the electron pairs and the geometry of the molecule itself. By molecular geometry we are referring to the arrangement of the atoms only, although that geometry is indeed affected by the location of the lone pairs. For water, the electron pair geometry is tetrahedral and the molecular geometry is bent or angular.

To avoid the confusion over electron pair geometry versus molecular geometry it would be best to consider the number of electron groups around a central atom. A center atom is any atom that has two or more atoms bonded to it. Each atom is bonded to a center atom by one group of electrons (regardless of whether the bond is single, double, or triple). A lone pair or a single electron is also described as one group. For ethylene shown below, we would consider each carbon atom as having 3 electron groups around them (the double bond counting as one electron group).

S has 3 electron groups

Each C has

3 electron groups

ethylene

sulfur dioxide

For sulfur dioxide, the sulfur atom has 3 electron groups around it (the double bond counts as one group, the lone pair counts as another electron group, and the single bond counts as yet another).

Table 12.1 shows the relationship between the number of electron groups and molecular geometry.

Table 12.1: Relationship of the Number of Electron Groups to Molecular Geometry

# of bonding groups about central atom	# of non-bonding groups about central atom	Total # of electron groups about central atom	Sketch	Molecular Geometry & approx. Bond Angle*	Example (Lewis structures)
2	0	2		linear bond angle = 180	H–CN:
3	0	3		trigonal planar bond angle = 120
2	1	3	:	bent (or angular) bond angle 120
4	0	4	..	tetrahedral bond angle = 109.5
3	1	4	.. :	trigonal pyramidal bond angle 109.5
2	2	4		bent or angular bond angle 109.5

*A bond angle is the angle between two bonds.