chem exp 2

Experiment 2. Molecular Geometry and Polarity

Objective:-

To draw Lewis structures and use the Valence Shell Electron Pair Repulsion

(VSEPR) theory to predict the molecular geometries of molecules and polyatomic

ions.

To predict polarity of molecules and ions.

BACKGROUND

Large-sized models of molecules are used to represent particles that are too

small to see with the human eye. These macro-sized models are useful for visualizing

the physical arrangements of atoms in molecules and polyatomic ions and aid in

understanding properties, such as the polarity of some small molecules and the

reactivity and interaction of atoms in molecules. Molecular models are ball and stick

sets in which each ball of a different color represents atoms of a different element.

A basic concept of the atomic theory is that the chemical and physical properties

of a substance are determined by the distribution of outermost shell electrons in its

atoms and by the spatial arrangement of these atoms in the structure of the substance.

Lewis Dot formulas are two dimensional representations that use the arrangement of

outer shell electrons to give basic information on the three dimensional arrangement of

atoms in molecules and polyatomic ions.

Experimental techniques such as x-ray or neutron diffraction in crystals, infrared,

Raman and microwave spectroscopy, and dipole measurements provide information on

the relative positions or geometric arrangement of atoms in real molecules and in

polyatomic ions. Experimental data on shapes and polarity agree very closely with

shapes and polarity predicted from models for simple molecules and polyatomic ions.

The following rules and procedures are given as a guide in drawing Lewis

Electron Dot Formulas.

Drawing Lewis Structures

Rule 1. For small molecules and polyatomic ions, place the element with the lowest

electronegativity in the center and arrange the more electronegative atoms around it.

Note:- Hydrogen should not be used as a central atom.

2

Oxygen atoms do not bond to each other except in O2 (dioxygen), O3 (ozone),

O22- (peroxide ion), and O2- (superperoxide ion).

Rule 2. In oxyacids such as HNO3 and H2SO4, hydrogen atoms are usually bonded to

oxygen atoms which in turn are bonded to the central atom.

Arrangement of Electron Dots:-

Count the total number of valence electrons from all atoms in the formula,

including electrons due to negative charge, if any.

Arrange atoms around the central atom; remember to apply Rule 2.

Two electrons are used to form a bond.

Complete the octets of the atoms attached to the central atom; remember that

hydrogen can accommodate only 2 electrons.

Put any remaining electrons on the central atom to satisfy its octet. These extra

electrons are shown as pairs.

If the central atom has less than an octet, form double or triple bonds with the

surrounding atoms.

There are compounds which are exceptions to the octet rule. For instance, in

BF3 the central atom has less than 8 electrons. Such species are called electron-

deficient molecules. On the other hand, the central atoms in PCl5, SF6, IF5, etc.

Elements in the third row of the periodic table and beyond often exhibit expanded octets

of up to 12 electrons.

Electron-Domain Geometry (Electronic Geometry)

The VSEPR Theory states that shared (bonding) and unshared (nonbonding) electron

pair domains around the central atom arrange themselves as far apart as possible. In

other words, electron domains will orient themselves so as to minimize the repulsion

between them. Electron pairs used to form multiple bonds (i.e. double or triple bonds)

are counted as one electron domain. Electron pairs used to form single bonds are

counted as electron domains.

Example. What is the number of electron domains around the central atom in CO32-?

3

O

C

O

O

2-

The three electron domains in CO32- arrange themselves so as to minimize repulsion

with each other. In other words, the electron domains occupy three regions around the

carbon atom forming a trigonal planar geometry.

Molecular Geometry (Molecular Shape)

Molecular geometry refers to the relative positions of the atoms around a central atom of

a molecule or polyatomic ion. Molecular geometry of a molecule is determined by how

the surrounding atoms are arranged around the central atom, which is in turn determined

by how the electron domains are arranged around the central atom. The following link

can be used to determine the electronic and molecular geometries of simple molecules:

https://billvining.com/mmlib_sims/#gen_8_2.

Molecular Polarity (Dipole Moment)

In polyatomic molecules/ions, the presence of polar bonds may or may not result in a

polar molecule, depending on the molecular geometry. If the molecular geometry of a

molecule/polyatomic ion is completely symmetrical, the molecule/polyatomic ion is

nonpolar. In other words, in a totally symmetric molecule individual bond dipoles cancel

each other completely (i.e. the net dipole moment is zero). If the molecular geometry is

not totally symmetric, the molecule has a net dipole moment and hence is polar. Polarity

influences both physical and chemical properties of molecules. A molecule is nonpolar

regardless of its geometry, if it does not contain polar bonds. An individual bond is polar

if the two bonding atoms have sufficiently different electronegativities.

_

There are three electron domains around C atom:

i.e. two single bonds, counted as two electron domains

and one double bond counted as one electron domain.

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Name: _______________________ Date: _______________

CHM 114

PROCEDURE

i) Watch the lab recording posted on Canvas along with the lab handout.

Predict the molecular geometry and polarity of the molecules/ions listed in

Table 3.

ii) Draw the Lewis structures of the molecules/ions in the space provided in the

table and state the molecular geometry (shape).

iii) Check if the geometries you predicted are correct using the following

simulation: https://billvining.com/mmlib_sims/#gen_8_2.

iv) Predict the polarity of the molecules/ions based on their geometries.

Table 1. Lewis structure, molecular geometry and polarity of selected molecules

and ions.

Molecule

/Ion

Lewis Structure

Name of

Molecular

Geometry

Polar or

Nonpolar?

XeF4

PCl3

Molecular geometry and polarity

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Table 1 continued ………..

IF4-

OF2

SF4

SeF6