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Chemical Bonding

High schoolChemistryIllustrative covalent Morse potentials and an isolated ionic Coulomb-plus-repulsion pair

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About this experiment

Stretch and compress a bond while measuring potential energy and restoring force. Compare a shared-electron covalent sketch with an ionic pair, then relate a potential minimum to equilibrium separation.

Learning goals

  • Identify attractive and repulsive sides of a bond potential.
  • Relate equilibrium length to zero net radial force.
  • Distinguish covalent sharing, ionic localisation and a macroscopic crystal lattice.

How to use it

  1. 1Drag the orange atom away from the equilibrium position and record the inward force.
  2. 2Compress the bond and observe the steep repulsive energy rise.
  3. 3Probe the potential chart at its minimum and at large separation.
  4. 4Switch to the ionic pair and compare the longer-range attraction with a covalent example.

The model

The Morse energy approaches zero at infinite separation and has its minimum at the equilibrium length.
Positive force increases the separation of the right atom.
The repulsion coefficient places the isolated unit-charge ion pair minimum at the chosen length.

Assumptions

  • Parameters are illustrative approximations, not fits to vibrational spectra or thermochemical dissociation measurements.
  • The ionic example represents one isolated Na-positive and Cl-negative pair. It does not include crystal Madelung interactions or electron-transfer energetics.
  • Valence dots are schematic. Scanning imposes the separation; it does not integrate molecular vibration dynamics.

Try this in class

  1. Q1Why does a bound pair not collapse to zero distance?
  2. Q2How do the signs of potential slope and radial force compare?

Frequently asked questions

Does the displayed energy include the energy to make the ions?

No. The ionic potential begins with pre-existing singly charged ions. Their preparation and electron transfer are outside the model.

Why is the force zero at the bottom of the well?

At a smooth potential minimum the energy derivative vanishes. A small displacement then produces a restoring force in this model.