COVALENT BONDING AND MOLECULAR STRUCTURE

 

      Elements which are classified as non-metals tend to form covalent bonds through sharing

electrons between other non-polar elements.  Covalent bonds are strong and very stable.  A particularly

stable structure is one in which all of the atoms have a share in eight valence electrons (with the

exception of hydrogen which can share a maximum of two electrons).  It is important for you to

understand and appreciate that atoms and molecules have three dimensional shapes, since most drawings

seen in texts and lab manuals try to show three dimensional forms, which still look flat.  A study of the

subtle differences in the three dimensional nature of molecules will be useful to help you understand why

some chemicals are “food”, others pass through the body untouched as “non-food”, and others may be

acutely poisonous.  The geometry of a molecule influences whether it is polar or non-polar.  The

geometry of a molecule is influenced by the number of valence electrons which an atom has, and whether

those electrons tend to be involved in bonding, or whether they tend to remain unshared, and non-bonding.

If the electrons are non-bonding, then they tend to try to be as far apart as possible, and so satisfy the

electron pair repulsion rule (VSEPR).

      An important characteristic of molecular compounds is that of polarity.  A bond between two

unlike atoms is always polar, however the molecule as a whole may be nonpolar it the complete

molecule is symmetrical.  A molecule is polar if it is structurally asymmetrical,  that is the

molecule is composed of two different elements, or the atoms are unevenly arranged around the central

atom.  HF would be polar since two elements are joined by a covalent bond, but the electrons are not

shared equally.  H2 would be non-polar and symmetrical, since both atoms of the molecule are of the

same element, so there is equal sharing of the electron cloud.  Polar molecules can also be made of more

than two elements and more than three atoms.  In all cases, the degree (or amount) of polarity depends on

the position of the atoms which are unevenly arranged around the central atom.

      Molecules are definitely three dimensional.  They have a characteristic shape, form, bulk, and

many of their properties result from their bulky shapes.  Unsaturated lipids are usually liquid at room

temperature because the molecules do not fit closely next to each other, hence have less molecular

attraction.  The shape and polarity of molecules, as well as its total molecular mass, determines the

melting point and boiling point of a molecule, as well as solubility properties.

      Water is a very polar molecule because it has two unshared pairs of electrons, and two hydrogen

atoms covalently bonded to an oxygen atom.  The oxygen atom is strongly electronegative so it strongly

attracts the electron clouds on the hydrogen atom.  In doing so, the hydrogen atoms have less electron

cloud and to develop a partial positive charge, noted by the symbol δ+.  The oxygen, having taken most of

the electron cloud from the hydrogens, now is indicated by δ–.  Molecules which are polar tend to

dissolve in water, whereas those which are non-polar tend to be repelled by water.  If one was to compare

molecular compounds of a similar molecular mass, their properties would be very different depending on

whether they are nonpolar, polar, or capable of dissociating, as organic acids are capable of doing.

      You will be using atomic models for this exercise.  Use the same size sticks between atoms of the

same color in order to preserve relative shapes of the molecules.  If you predict a double bond between

atoms, use two springs to demonstrate the double bond (sticks do not bend very well).  A triple bond

would require three springs.

      In constructing models of molecules, draw the Lewis dot structure first.  Then put the molecule

together.  As an example, in constructing a molecule of water, connect an oxygen atom to two hydrogen

atoms.  Note the angle between the hydrogen atoms, and examine the molecule from the front angel and

from the side.  Many texts draw the molecule from the “front” so that the hydrogen atoms appear to be

symmetrical in relation to the oxygen.  If you examine the molecule from the side, you will see that the

molecule is distinctly asymmetrical due to the unseen unshared pairs of electrons.  You might wish to

remake the molecule using a black “carbon” atom which has four evenly spaced holes.  Place two

hydrogen atoms onto the central “oxygen” as before, and then place two short sticks into the unused two

holes to represent the unshared pairs of electrons.  In this way you can fully appreciate the importance of

unshared pairs of electrons in the geometry of a molecule.

      A study of molecular geometry and polarity helps on understand how polyatomic ions “happen”.

Polyatomic ions are difficult to remember, use in reactions (at least on paper), and yet are are constantly

being used in chemical experiments.  This exercise will help to show that they are logically constructed,

and make sense on a chemical level.

 

OBJECTIVES

      1.  To acquire skills in understanding molecular bonding, organic structure, and isomerism.

      2.  To use Lewis structure and molecular models to predict polarity, molecular geometry, relative water

            solubility and hydrogen bonding capability.

      3.  To use molecular models to study classes of organic compounds.

 

MATERIALS

      Molecular model sets

 

COLOR CODE FOR MODELS

 

COLOR                       ATOM REPRESENTED

 

black                            carbon (or and atom where four bonds are needed)

yellow              hydrogen (1 bond)

red                               oxygen (2 bonds)

blue                              nitrogen or phosphorous (3 bonds)

green                chlorine (1 bond)

purple               iodine (1 bond)

orange              bromine (1 bond)

 
 

 

 

 

 

 

 

 

 

 

 


PROCEDURE

  A.  STUDY OF MOLECULAR SHAPE AND POLARITY

       1.  The formula of the molecules to be constructed is listed in the first column.

       2.  Using the formula, draw Lewis structure (include pairs of dots to indicate any unshared pairs

            of electrons).

       3.  Assemble each model using the colored balls and draw the shape in the space for 3-D shape and

            geometry.

       4.  Write the name of the geometrical shape which you predict from building the model, and tell

             whether the molecule has polarity or not (Yes or no).  Polarity is based on molecular asymmetry.

             If  the molecule looks the same from all angles, it is symmetrical and therefore nonpolar.  If one

             side is different from the rest, it is asymmetrical, and therefore is polar.

 

       USING A SIMILAR SET-UP AND SPACING TO THE EXAMPLE ON THE REPORT FORM, FOLLOWING THE

SYMBOLS OF THE MOLECULE, LEWIS STRUCTURE, 3-D SHAPE AS SEEN FROM YOUR MODEL, GEOMETRY,

AND WHETHER IT IS POLAR OR NOT.  ASK FOR HELP IF YOU ARE HAVING DIFFICULTY.

 

MOLECULES TO INCLUDE:

                CH4                         N2                            HCl                         SO42–

                H2O                         H2S                         H2CO3                     H2O2

                H3O+                       NH3                         HCO3                     C2H6

                CH3Cl                     NH4+                       CO32–                      C2H4

                CH2Cl2                    CH3OH                   SO2                         C2H2

                CHCl3                     CH2O                      SO3                         Br2

                CCl4                        CH3COOH             H2SO4                     CO2

 

NAME:  ________________________________________________________  SECTION NO.:  ______________

 

REPORT SHEET — Covalent Bonding and Molecular Structure

 

FORMULA OF

MOLECULE

LEWIS STRUCTURE

3-D SHAPE

GEOMETRY
MOLECULAR
POLARITY

example:

   CH4

               H

                |

        H—C—H

                |

               H

 

 

tetrahedral

 

nonpolar

 

   H2O

 

 

 

 

 

 

 

   H3O+

 

 

 

 

 

 

 

   C2H6

 

 

 

 

 

 

 

   C2H4

 

 

 

 

 

 

 

   C2H2

 

 

 

 

 

 

 

   CO2

 

 

 

 

 

 

 

   H2O2

 

 

 

 

 

 

 

   CH3Cl

 

 

 

 

 

 

 

   CH2Cl2

 

 

 

 

 

 

 

   CHCl3

 

 

 

 

 

 

 

FORMULA OF

MOLECULE

LEWIS STRUCTURE

3-D SHAPE

GEOMETRY
MOLECULAR
POLARITY

 

   CCl4

 

 

 

 

 

 

 

   H2S

 

 

 

 

 

 

 

   NH3

 

 

 

 

 

 

 

   NH4+

 

 

 

 

 

 

 

   Br2

 

 

 

 

 

 

 

   N2

 

 

 

 

 

 

 

   CH3OH

 

 

 

 

 

 

 

   CH2O

 

 

 

 

 

 

 

   HCl

 

 

 

 

 

 

 

   H2CO3

 

 

 

 

 

 

 

   HCO3

 

 

 

 

 

 

 

   CO32–

 

 

 

 

 

 

 

FORMULA OF

MOLECULE

LEWIS STRUCTURE

3-D SHAPE

GEOMETRY
MOLECULAR
POLARITY

 

   SO2

 

 

 

 

 

 

 

   SO3

 

 

 

 

 

 

 

   H2SO4

 

 

 

 

 

 

 

   SO42–

 

 

 

 

 

 

 

QUESTIONS:  MOLECULAR MODELS

 

1.  Explain why the model for each of the following atoms has a specific number of bonds:

 

            a.  hydrogen:  1 bond: ______________________________________________________________

 

 

            b.  oxygen:  2 bonds: _______________________________________________________________

 

 

            c.  nitrogen:  3 bonds:  _____________________________________________________________

 

 

            d.  carbon:  4 bonds:  _______________________________________________________________

 

 

2.  Discuss the shapes of SO2 and SO3.  Why is one polar and the other is not.

 

 

 

3.  Explain why and ion such as NH4+, CO32–, and SO42– can be nonpolar, but charged?

 

 

 

4.  Explain how molecules of H3O+  and NH4+ form from uncharged molecules of H2O and NH3.