Beginner Topic 133 of 1102 Bonding & Molecular Structure
Bond Length
The average distance between nuclei of two bonded atoms.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
In-depth explanation of Bond Length
Bond length is the equilibrium distance between the nuclei of two bonded atoms, where attractive and repulsive forces balance and the energy is at a minimum. It depends on the sizes of the atoms and on the number of shared electron pairs, so it decreases as bond order increases. Bond lengths are measured by X-ray or electron diffraction in the solid state and by rotational or vibrational spectroscopy in gases. For many bonds the length is roughly additive, so the covalent radii of the two atoms can be summed to estimate it. Bond length strongly affects molecular geometry and reactivity.
Going deeper: Bond Length
Key ideas: Estimate it by adding the covalent radii, then subtract a small correction for polar bonds. For the same pair of atoms length falls with order: C-O is about 143 pm, C=O is 123 pm and C-triple-O is 113 pm. Down a group, the bonds lengthen (H-F 92 pm, H-I 161 pm) and become weaker. Typical values are in picometres, 1 pm equal to 10^-12 m.
Common mistakes and connections: The common mistake is believing a shorter bond is always more reactive, or that length alone determines strength, since F-F is long yet weak because of lone pair repulsion. Another is confusing angstrom and picometre units. Connects to bond order, covalent radius, bond energy and IR spectroscopy.
Visualization of Bond Length
The minimum of the energy curve marks the equilibrium bond length, about 74 pm for H2. Pushing closer raises energy steeply because of repulsion. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The H-F bond is 92 pm long while the H-I bond is 161 pm, because iodine is much larger than fluorine.
Frequently asked questions about Bond Length
What is Bond Length?
The average distance between nuclei of two bonded atoms.
Is Bond Length a beginner, intermediate or advanced topic?
Bond Length is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Bond Length?
Study these earlier topics in Bonding & Molecular Structure first: Incomplete Octet, Odd-Electron Molecules, Coordinate Bond.
What comes after Bond Length?
The next topics in Bonding & Molecular Structure are Bond Energy and Bond Order.
Can I practise Bond Length with quizzes?
Yes. The interactive lesson includes a self-check question on Bond Length, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Bond Length fits in Bonding & Molecular Structure
Bond Length is topic 18 of 104 in Bonding & Molecular Structure.