Covalent bond: sharing electrons, bond length and bond energy

Pull two atoms apart and push them together to see a covalent bond form and break: H₂, F₂, O₂, N₂ and HCl, with shared electron pairs, lone pairs, bond order, bond length, bond energy and the potential energy curve. Free for every student.

Also searched as: covalent bond · covalent bond simulation · covalent bond class 11 · bond length and bond energy · bond order · polar covalent bond

About this experiment

Aim

To see how a covalent bond forms when two atoms share electron pairs, why there is a best distance (the bond length), and how much energy it takes to break the bond.

Theory

A covalent bond is formed when two atoms share one or more pairs of electrons. Each atom then has a more stable arrangement: hydrogen gets 2 electrons like helium, and atoms like F, O, N and Cl get 8 electrons in the outer shell (the octet rule).

When two atoms come close, the shared electrons are attracted by both nuclei and the energy falls. If they come too close, the two nuclei repel each other and the energy rises steeply. The lowest point of the energy curve gives the bond length (distance between nuclei) and its depth is the bond energy (the energy needed to separate the atoms).

The number of shared pairs is the bond order: 1 for a single bond (H–H, F–F, H–Cl), 2 for a double bond (O=O) and 3 for a triple bond (N≡N). A higher bond order means a shorter and stronger bond: N≡N (110 pm, 945 kJ/mol) is much stronger than F–F (141 pm, 159 kJ/mol).

Formulas used

• bond order = number of shared pairs
• bond energy = depth of the potential energy curve (kJ/mol)
• 1 eV per molecule = 96.5 kJ/mol
• Morse curve: U(r) = D[1 − e^(−a(r − rₑ))]² − D
• electronegativity difference > 1.7 → mainly ionic, 0.4–1.7 → polar covalent, < 0.4 → non-polar

Procedure: what to do in the virtual lab

1. Start with H₂ at the normal distance (1.00). Look at the shared pair between the two nuclei and the lowest point of the curve.
2. Pull the atoms apart slowly. Watch the energy rise and the shared electrons go back to their own atoms when the bond breaks.
3. Push the atoms together (below 1.0) and see the energy rise again because of the repulsion.
4. Compare H₂, F₂, O₂ and N₂: note the bond order, the bond length and the depth of the curve.
5. Choose HCl and look at which atom the shared pair is closer to. What do δ+ and δ− mean?

Observations to make

• The bond is strongest (lowest energy) at one particular distance, never at zero distance.
• Higher bond order gives a shorter and stronger bond: N≡N > O=O > F–F in strength.
• F₂ has a weak bond even though it is a single bond, because the lone pairs on the two small fluorine atoms repel each other.
• In HCl the shared electrons spend more time near chlorine: the molecule is polar.

What you can change in this experiment

• Molecule: H₂ (hydrogen) / F₂ (fluorine) / O₂ (oxygen) / N₂ (nitrogen)
• Distance between the nuclei (1 = normal bond length)
• Show lone pairs (on / off)

Viva questions on Covalent bond

Questions an examiner may ask in the Class 11 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. What is a covalent bond? How is it different from an ionic bond?
2. What is bond length and bond order?
3. Why is the N≡N bond so strong?
4. What is a polar covalent bond? Give an example.
5. Why does the energy rise when the nuclei are pushed very close together?

Frequently asked questions

Syllabus: Class XI Chemistry, Chapter 4 — Chemical Bonding and Molecular Structure, §4.3 (covalent bond), §4.4 (bond parameters)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: covalent bond · bond length · bond energy · bond order · lone pair · polar covalent bond · potential energy curve · chemical bonding

More from this unit

• Ionic bond: how sodium and chlorine make NaCl
• Shapes of molecules (VSEPR): CO₂, BF₃, CH₄, NH₃, H₂O, PCl₅, SF₆, XeF₄

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