COVALENT COMPOUNDS MODELS LAB

(67 points)

PURPOSE:

The purpose of this lab is to give the student experience building 3-D models of some simple covalent molecules. Students will then predict the shapes of the molecules and use their prior knowledge of bond polarity to predict the polarity of the molecule.

INTRODUCTION

Covalent Bonds

A group of atoms held together by covalent bonds is called a molecule. The properties of a molecule, including its role in nature, depends primarily on its molecular structure, or shape. Molecular shape contributes toward determining a compound’s boiling point, freezing point, viscosity, solubility, types of reactions it can participate in, and a host of other physical and chemical properties.

The covalent bonds holding the atoms of a molecule together can be classified as three different types. Covalent bonds are classified by comparing the differences in electronegativities of the two bonded atoms. Electronegativity is a scale used to determine an atom’s attraction for an electron in the bonding process.

Bond Polarity

Differences in electronegativities are used to predict whether the bond is nonpolar covalent, polar covalent, or ionic. The range of electronegativity differences and bond classification is:

0.0 – 0.4nonpolar covalent

0.4 – 2.1polar covalent

2.1 – 4.0ionic

In a polar covalent bond, the electrons are more attracted to the atom with the greater electronegativity. This results in a partial negative charge on that atom. The atom with the smaller electronegativity value acquires a partial positive charge.

Molecular Polarity

Molecules composed of covalently bonded atoms may also exhibit polar or nonpolar properties. For the molecule to be polar, it must, of course have polar bonds. But the key factor in determining the polarity of a molecule is its shape, or geometry. If the polar bonds are arranged symmetrically around the central atom they will offset each other and the resulting molecule is nonpolar. However, if the arrangement of the polar bonds is not symmetrical around the central atom, the electrons will be pulled to one end of the molecule and the resulting molecule is polar.

Lewis Structures

The structures used to show bonding in covalent molecules are called Lewis structures. When bonding, atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the outer energy level.

Predicting Shapes

The model used to determine the molecular shape is referred to as the Valence Shell Electron pair Repulsion (VSEPR) model. The model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. By examining the central atom and identifying the number of atoms bonded to it and the number of unshared electron pairs surrounding it, one can determine the geometry, shape, of a molecule.

Molecular Shapes

TOTALSHAREDUNSHAREDSHAPE/EXAMPLE

PAIRSPAIRSPAIRSGEOMETRY

220linearN2 or CO2

330trigonal planarBF3

440tetrahedralCH4

431trigonal pyramidalNH3

422bentH2O

550trigonal bipyramidalNbBr5

660octahedralSF6

Models

To study covalent molecules, chemists find the use of models helpful. Colored wooden, or plastic, balls are used to represent atoms. These balls have holes drilled into them according to the number of covalent bonds they will form. The holes are bored at angles that approximate the accepted bond angles.

EQUIPMENT

Molecular model building set.

COLOR KEY (amount) / NUMBER OF HOLES / ELEMENT
Black (6) / 4 / Carbon
White (10) / 1 / Hydrogen
Red (4) / 2 / Oxygen
Green (4) / 1 / Halogen
Blue (2) / 4 / Nitrogen
Yellow (2) / 4 / Sulfur
Light brown (2) / 5 / Trigonal bipyramidal
Gray (2) / 6 / Octahedral
Medium gray links (20)
Long gray links (6)

SAFETY PRECAUTION

Do not place model pieces in your mouth – they present a choking hazard.

PROCEDURE

1. Obtain a molecular model set.

2. For each of the molecules in the data table:

  1. Build a model of the molecule.
  1. Draw the Lewis structure for each molecule.
  1. Determine the number of bonding pairs and lone pairs around each central atom.
  1. Draw a 3-D sketch of the molecule.
  1. Determine the shape of the molecule at each central atom.
  1. Determine whether the bonds between the central atom and the other atoms are nonpolar, polar, or ionic.
  1. Determine whether the molecule is polar or nonpolar.

DATA TABLE(44 points)

Formula

/ Electron
Dot Diagram /

Bond

Pairs

/ Lone
Pairs /

3D Drawing

/ Name of
Molecular Shape /
Bonds
Ionic, Polar or Nonpolar /
Molecule
Polar or Nonpolar
Cl2 / N/A / N/A
HBr / N/A / N/A
NH3 / / 3 / 1
SO3
N2 / N/A / N/A
SbI5
CCl4 /

Formula

/ Electron
Dot Diagram /

Bond

Pairs

/ Lone
Pairs /

3D Drawing

/ Name of
Molecular Shape /
Bonds
Polar or Nonpolar / Molecule
Polar or Nonpolar
CH2Cl2
CH4
TeBr6
C2H4
CH3OH
CO2
H2O

POST-LAB DISCUSSION: (23 points)

Read the entire lab and the relevant parts of your textbook (Chapter 9; pp. 259 – 267). Then answer the questions that follow.

(1 point) 1. Define a covalent bond.

(1 point) 2. Define a dipole.

(2 points) 3. List the two factors that determine whether a molecule is polar.

(5 points) 4. List the seven different molecular geometries (shapes) that were studied in this lab.

(4 points) 5. Calculate the electronegativity difference and predict the type of bond for the following examples:

a. Na―Cl

b. C―H

c. S―O

d. N—N

(5 points) 6. List five molecules in this labwhich are nonpolar because all bonds are nonpolar.

(3 points) 7. List three molecules in this labthat have polar covalent bonds but are nonpolar molecules because of symmetry.

(2 points) 8. Which two shapes appear to produce polar molecules?

OVER