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Exp 10 Determination of Molecular Geometries using Lewis Dot Structures
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EXP 10. Determination of Molecular Geometries UsingLewis Dot Structure and VSEPR Models
OBJECTIVES
• Familiarize with Lewis Dot Structures, Valence Shell Electron Pair Repulsion (VSEPR) Theory
and the three-dimensional structures of covalent molecules
• Use Lewis Dot Structures and VSEPR Theory to construct molecular models to predict
shapes and polarity of small molecules and polyatomic ions
• Able to use the molecular models in predicting and explaining some properties of substances
LECTURE TOPIC REFERENCES
Review the following before performing the experiment:
Tro, N. (2017). Chemistry: A Molecular Approach. 4th Edition. Boston: Pearson. Chapter 9, Sections 9.5,
9.6, 9.7, and Chapter 10, Sections 10.2 and 10.3, 10.4, and 10.5.
CONCEPTS
In previous experiments, although you learned to identify the composition and chemical
formula, and the difference between physical and chemical properties of substances, they are
not sufficient to predict or explain the properties of most molecular compounds. According to
the basic concept of atomic theory, the physical (e.g. solubility) and chemical (e.g. reactivity
and interaction of atoms in molecules) properties of substances are dictated by the distribution
of outer-shell electrons (valence electrons) in its atoms, and the spatial arrangement of these
atoms in its structure – the molecular shape.
Molecular shape is very important in understanding the relationship between structure and
properties of molecules and polyatomic ions. Molecules and polyatomic ions are three-
dimensional aggregates of atoms. In molecules and polyatomic ions, atoms are bonded by
sharing pairs of valence electrons. You learned from your lecture that electrons repel one
another and they try to stay away from each other. The best arrangement of a given number
of electron pairs is the one that minimizes the repulsion among them. This outer-shell
electron behavior is the basis of Valence shell Electron Pair Repulsion or VSEPR model.
Repulsions occur among the regions of electron density (that is, among pair of bonding
electrons and lone pair electrons), which control the angles between bonds from a central
atom
1
, and to its surrounding atoms. The molecular shapes result from minimizing the electron pair
repulsion as predicted by VSEPR model.
Since atoms are too small to see with the human eye, it is necessary to use large models to
visualize their physical arrangements in molecules and polyatomic ions. These models allow the
study of the shapes and sizes, and spatial relationships of atoms, molecules and ions that make
up a substance. The simple way of predicting the molecular shapes requires the combination
of the Lewis Dot Structure and Ball-and-Stick” molecular models. The Lewis Dot Structure,
consisting of the chemical symbol and electron dots, provides the two-dimensional
representations of chemically active valence electrons, needed in predicting the three-
dimensional shapes of molecules and polyatomic ions. The ball-and-stick” molecular models
can be physically constructed using a kit as shown in Figure 1.
Figure 1 Molymod Inorganic/Organic Student Set
In this experiment, you will use the Lewis Dot Structure and “ball-and-stick” molecular models
to determine the shapes of molecules and polyatomic ions. You will then evaluate the
molecular shape or geometry to predict the polarity of the molecules. Molecular models will
help you understand the connection between Lewis Dot Structure and VSEPR models in
studying the interactions of atoms in molecules, and predicting the symmetry and polarity of
some molecules using the following steps:
(a) Determine the Lewis Dot Requirement (See Attachment 1 handout)
(b) Draw the Lewis Dot Structure (See Attachment 1 handout)
(c) Draw the Electronic Geometry and give the description (See Attachment
2 handout)
(d) Assemble the Model
2
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