Experiment 5 Experiment 2: Writing Lewis Dot Formulas and Building Molecular Models (original prepared by Kelemu Woldegiorgis) With added video link Objective:- To draw Lewis Dot formulas and use...

Experiment 5
Experiment 2: Writing Lewis Dot Formulas and Building Molecular Models
(original prepared by Kelemu Woldegiorgis)
With added video link
Objective:- To draw Lewis Dot formulas and use Valence Shell Electron Pair Repulsion (VSEPR) Theory to predict shapes and polarity of small molecules and polyatomic ions and construct models of compounds.

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 a
angements 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 a
angement of these atoms in the structure of the substance. Lewis Dot formulas are two dimensional representations that use the a
angement of outer shell electrons to give basic information on the three dimensional a
angement 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 a
angement 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:
A. Lewis Electron Dot Formula
1) A
angement of Molecular Skeleton Structure
Rule 1. i) For small molecules and polyatomic ions, place the element with the lowest electronegativity in the center and a
ange the more electronegative atoms around it. Note:- Hydrogen should not be used as a central atom.
    ii) 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.
2) A
angement of Electron Dots:-
i) count the total number of valence electrons from all atoms in the formula, including electrons due to negative charge, if any.
ii) a
ange atoms around the central atom; remember to apply Rule 2.
iii) two electrons are used to form a bond.
iv) complete the octets of the atoms attached to the central atom; remember that hydrogen can accommodate only 2 electrons.
v) put any remaining electrons on the central atom to satisfy its octet. These extra electrons are shown as pairs.
vi) if the central atom has less than an octet, form double or triple bonds with the su
ounding 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.
Example. Draw the Lewis Dot Formulas for PCl5, SF6, and XeF4.
i) Count valence electrons:
PCl5:     # valence e-s = 5 (from P XXXXXXXXXXfrom 5 Cl atoms) = 40 e-s total
SF6:     # valence e-s = 6 (from S XXXXXXXXXXfrom 6 F atoms) = 48 e-s total
IF5: # valence e-s = 7 (from I XXXXXXXXXXfrom 5 F atoms) = 42 e-s total
ii) A
ange atoms around central atoms; connect su
ounding atoms to central atoms with single bonds (with single pairs of electrons):
    
iii) Complete the octets of the atoms attached to the central atom; electron pairs are shown only for one of the identical atoms for simplicity.
    
iv) Put any remaining electrons on the central as lone pairs (nonbonding pairs).
    
B. Electron-Group Geometry
The VSEPR Theory states that shared (bonding) and unshared (nonbonding) electron pair domains around the central atom a
ange 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-?
     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.
_
The three electron domains in CO32- a
ange themselves so as to minimize repulsion with each other. In other words, the electron domains occupy three regions around the ca
on atom forming a trigonal planar geometry. Table 1 represents the relationship between the number of electron domains and the electronic geometry associated with them.     
Table 1. Electron domain geometries as function of the number of electron domains    
    # of electron groups
    2
    3
    4
    5
    6
    Electronic group
    Linea
    Trigonal plana
    Tetrahedral
    Trigonal bipyramidal
    Octahedral
    geometry
    
    
    
    
    
C. Molecular Geometry (Molecular Shape)
Molecular geometry refers to the relative positions of the atoms around a central atom of a molecule or polyatomic ion. Any nonbonding electrons on the central atom are not considered to predict the molecular shape. Molecular geometry of a molecule is determined by how the su
ounding atoms are a
anged around the central atom, which is in turn determined by how the electron domains are a
anged around the central atom. Table 2 summarizes the relationship between electronic and molecular geometries with associated examples.
Table 2. Relationship between the number of electron domains around a central atom of a molecule or polyatomic ion, electronic geometry, and molecular shape.
    # of e- groups
    Electronic geometry
    Molecular geometry
    Examples
    2
    Linea
    Linea
    CO2
    3
    Trigonal Planar
    Trigonal planar XXXXXXXXXXBent
    NO3- XXXXXXXXXXSO2
    4
    Tetrahedral
    Tetrahedral XXXXXXXXXXTrigonal pyramidal Bent
    CH4 XXXXXXXXXXNH3 XXXXXXXXXXH2O
    5
    Trigonal bipyramidal
    Trigonal bipyramidal Seesaw XXXXXXXXXXT-shaped XXXXXXXXXXLinea
    PCl5 XXXXXXXXXXSCl4 XXXXXXXXXXClF XXXXXXXXXXXeF2
    6
    Octahedral
    Octahedral XXXXXXXXXXSquare pyramidal XXXXXXXXXXSquare plana
    SCl6 XXXXXXXXXXBrF5 XXXXXXXXXXXeF4
Also: in your textbook (4th edition Tro) check out Table 11.1 Electron and Molecular Geometries in Chapter 11 (page XXXXXXXXXXTro 3rd edition will also have the same table but in chapter 10).
D. 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.
MATERIALS
Balls and sticks as models for atoms and bonds.
PROCEDURE
In this experiment you will predict the polarity of a series of molecules and polyatomic ions by the following procedure:
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.
After you have read the procedure, view the video
Khan Academy, Molecular Polarity, Aug 24, XXXXXXXXXXCopy the following video link into your web
owser and play the video:
https:
www.youtube.com/watch?v=TyFd94TbdXU
Lab Report
For this Lab writeup, you supply a Title Page, then finish off the lab report by completing the following:

Experiment 2: Writing Lewis Dot Formulas and Building Molecular Models
Name: _________________________________        Date: _________________
Lab Partner(s): ______________________________________________
    Molecule/ion
    # of valence e-s
    Lewis Dot Formula
    Electronic Geometry
    Molecular Geometry
    Polarity (circle one)
    e.g.
BF4-
    
32 e- s
    
    Tetrahedral
    Tetrahedral
    Pola
Nonpolar
    
HCN
    
    
    
    
    Pola
Nonpolar
    
H2S
    
    
    
    
    Pola
Nonpola
    
OF2
    
    
    
    
    Pola
Nonpolar
    
SF4
    
    
    
    
    Pola
Nonpolar
…. Table continued
    Formula
    # of valence electrons
    Lewis Dot Formula
    Electronic Geometry
    Molecular Geometry
    Polarity (Circle one)
    
IF4-
    
    
    
    
    Pola
Nonpolar
    
NO2-
    
    
    
    
    Pola
Nonpola
    
CHCl3
    
    
    
    
     Pola
Nonpolar
    
SeF6
    
    
    
    
    Pola
Nonpolar
    
NH4+
    
    
    
    
    Pola
Nonpolar
    
BrF3

    
    
    
    
    Pola
Nonpolar
3
2
I
F
F
F
F