Bond Energy

The exercise described below is an investigation into the energy of molecular bonds and the relationship between bond length and bond strength. The graphical displays of the Diatomic Explorer software will help you construct an understanding of the factors that affect the strength and stability of chemical bonds.

Introduction

In this activity, you can use the Diatomic Explorer to investigate how the electronic structure of individual atoms determines whether they will combine to form a bond as well as the type of bond formed between the two atoms. (If you are unfamiliar with using the Diatomic Explorer, see the Introduction to the Diatomic Explorer, which appears when the Diatomic Explorer is launched on the Web.)

Background

When two atoms combine to form a molecule, both attractive and repulsive forces affect the strength and stability of the bond. The optimum bond length between a pair of atoms is the distance at which there is a minimum energy value. In this activity, you will use the Diatomic Explorer to determine the optimum bond length for some diatomic molecules.


Related Topics
Procedure

1. Open the Diatomic Explorer and set the selection mode to multiple orbitals. Using the Edit menu, select nitrogen for both atom 1 and atom 2.

2. The left window displays values for the factors that determine bond energy. The bond is most stable when the value displayed in red (labeled "bond energy") is at its lowest value (most negative). You can see the effect of bond length on bond energy by change the distance between the nuclei in the molecule. To do so, click on the horizontal bar at the bottom of the right window. Change the distance by moving the mouse right or left. Find the bond length that results in the most stable bond between the two atoms in the Group V, VI, and VII homonuclear diatomic molecules in Period 2. For each element, record the optimum bond length and the minimum bond energy in the table below.

Molecule N-N O-O F-F
Minimum energy (eV)    
Bond length (Å)    
Lewis Dot formula     

3. Repeat step 2 for the homonuclear diatomic molecules in Period 3.
MoleculeP-PS-SCl-Cl
Minimum energy (eV)    
Bond length (Å)    
Lewis Dot formula    

4. Examine at least four diatomic molecules, including some with ionic and polar covalent bonds and record bond length and energy in the table below.
Molecule   
Minimum energy (eV)    
Bond length (Å)    
Lewis Dot formula    

Questions/Conclusions

1. Describe the overlap of bonding electron orbitals when the atoms are at the optimum distance from each other.

2. Why does the bond energy increase as the atoms move either closer or farther away from each other?

3. Identify the types of molecular orbitals formed in each of the molecules you investigated.

4. Based on your data, explain the relationship (if any) that exists among the following variables: size of bonding atoms, number of bonds between atoms, bond energy, bond length.