--- title: "Heteronuclear Diatomics and Polarity in Orbital Terms" description: "Different atoms can contribute unequally to a molecular orbital. A two-orbital model connects unequal parent energies with unequal sharing. Hydrogen fluoride and carbon monoxide show why symmetry, ove" canonical: https://lightmysky.com/learn/science/heteronuclear-diatomics-and-polarity-in-orbital-terms-mt_thdxFqyHJV source: https://lightmysky.com/learn/science/heteronuclear-diatomics-and-polarity-in-orbital-terms-mt_thdxFqyHJV.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Heteronuclear Diatomics and Polarity in Orbital Terms Different atoms can contribute unequally to a molecular orbital. A two-orbital model connects unequal parent energies with unequal sharing. Hydrogen fluoride and carbon monoxide show why symmetry, overlap and the complete orbital configuration also matter. Subject: Science · Area: Chemistry · Ages 18 to 19 Page: https://lightmysky.com/learn/science/heteronuclear-diatomics-and-polarity-in-orbital-terms-mt_thdxFqyHJV ## Ready when they can - Uses an interacting two-orbital model to explain why the lower-energy parent contributes more to the bonding combination - Explains how energy mismatch affects localisation at fixed interaction, without using it as an absolute ionic-bond cutoff - Builds and fills a simplified valence-level scheme for carbon monoxide and identifies its usual terminal donor orbital - Checks relative energy, spatial overlap and symmetry when deciding whether two orbitals can mix strongly ## Lesson: Unequal atoms share unequally Keep the filling rules from the homonuclear lesson. Now take two interacting atomic orbitals with unequal energies. In this two-orbital model, the bonding combination has a larger contribution from the lower-energy parent. The antibonding combination has more from the higher-energy parent. A contribution describes the parent's part in the molecular wavefunction; neither atom owns the orbital. Molecules with several interacting orbitals need a fuller scheme. Strong mixing needs suitable symmetry, nonzero spatial overlap and a small enough energy gap. Use the same molecular axis for both symmetry tests. With the HF bond along z, H 1s and F 2p_z have sigma symmetry and can interact. F 2p_x has pi symmetry. Its positive and negative overlap contributions with H 1s cancel, giving zero interaction for that pair in this model. Equal energies cannot remove this symmetry restriction. *(drawing: These are symmetry classifications, not orbital shapes. Spatial overlap and relative energies must also be checked.)* In HF, the interacting F 2p_z level lies below H 1s. Their bonding combination has more fluorine character. Fluorine attracts bonding density more strongly, so F is partially negative and H partially positive. A diatomic has no second bond dipole to cancel this separation. The full molecular dipole depends on all electronic and nuclear charges, not one orbital alone. Transverse F 2p_x and 2p_y remain nonbonding in the simple model. Hold the nonzero interaction fixed and increase the parent energy gap. Mixing decreases: the bonding combination becomes more localised on the lower-energy parent and the antibonding combination on the higher-energy parent. This can indicate greater ionic character, but the gap alone cannot classify every bond. Electronegativity measures attraction for bonding density. Its difference is a guide, not a fixed cutoff: HF is polar covalent despite its large difference. CO's simplified valence order is 3sigma, 4sigma, the 1pi pair, 5sigma, the 2pi* pair, then 6sigma*. Core levels are omitted. Leading numbers name orbitals, not electron counts. Each sigma holds 2 electrons; each pi pair holds 4. Fill low levels first, singly across equal-energy orbitals before pairing. The highest occupied orbital is the HOMO. A pair in CO's carbon-centred 5sigma orbital can donate to a metal in the usual terminal mode. *(drawing: Read from lower to higher energy. These are maximum capacities, not occupied counts. Each pi cell contains two equal-energy orbitals.)* CO's occupied 5sigma orbital contains a pair with strong carbon character. In the usual terminal coordination mode, one CO molecule donates this pair to one metal through carbon. This makes CO a ligand, a species that binds a metal. Other binding modes exist. The carbon-centred donor follows from CO's multiorbital structure; oxygen's lower atomic energies do not make every occupied CO orbital oxygen-centred. For a molecule with several bonds, bond-dipole contributions must be added with their directions. A vector has both size and direction. CO2 is linear, O=C=O. Its two equivalent C=O bonds give equal contributions directed toward opposite oxygen atoms, so their sum is zero. Equal polar bonds alone do not guarantee cancellation; their directions must also cancel. This geometry test is separate from the symmetry test for orbital mixing. **Recap.** Check symmetry, overlap and energies. Unequal orbital contributions can explain polar sharing. Use CO's full level scheme to find its donor pair; use bond directions to check dipole cancellation. *Lesson, questions and cards written for this dataset. Chemistry source records were consulted for facts and teaching structure.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Molecular Orbital Theory for Homonuclear Diatomics](https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka) ## Opens up - [Molecular Symmetry: Elements, Operations and Point Groups](https://lightmysky.com/learn/science/molecular-symmetry-elements-operations-and-point-groups-mt_KcofGO11tm) - [Main-Group Chemistry: What Changes Down a p-Block Group](https://lightmysky.com/learn/science/main-group-chemistry-what-changes-down-a-p-block-group-mt_wq15-pBrI5)