--- title: "Molecular Orbital Theory for Homonuclear Diatomics" description: "Oxygen's usual Lewis structure pairs all its electrons, yet oxygen is attracted to a magnetic field. Molecular orbital theory uses orbitals spanning both atoms to predict bond order and the number of " canonical: https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka source: https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka.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`. # Molecular Orbital Theory for Homonuclear Diatomics Oxygen's usual Lewis structure pairs all its electrons, yet oxygen is attracted to a magnetic field. Molecular orbital theory uses orbitals spanning both atoms to predict bond order and the number of unpaired electrons. Subject: Science · Area: Chemistry · Ages 18 to 19 Page: https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka ## Ready when they can - Combines two atomic orbitals into a bonding and an antibonding combination and explains the energy difference - Fills a diagram for a second-row diatomic, including the s and p mixing that reorders the levels before oxygen - Calculates bond order and explains its usual relationship to bond length and dissociation energy for comparable bonds - Uses the diagram to predict that oxygen is paramagnetic and explains why the shared-pair picture cannot ## Lesson: Two atoms sharing whole orbitals You have used atomic orbitals and shared pairs. Molecular orbitals describe electrons across a molecule. Two identical atomic orbitals with suitable overlap form bonding and antibonding combinations. Matching wave phases increase electron density between the nuclei in the bonding combination, lowering its energy. The antibonding combination has a node between the nuclei and higher energy. A node is a region where the wavefunction is zero. Take the bond as the z axis. End-on p overlap forms sigma orbitals; side-on overlap forms two equal-energy pi orbitals. A star marks antibonding. Fill lower energies first. Each orbital holds at most two electrons with opposite spins. Within an equal-energy set, place one electron in each orbital with parallel spins before pairing. This is the same filling rule you used for atomic orbitals. *(drawing: Read from lower to higher energy. These cells show order, not the sizes of the energy changes.)* B, C, N, O and F supply 3, 4, 5, 6 and 7 valence electrons per atom. The valence ladder starts sigma(2s), sigma*(2s). In B2, C2 and N2, s-p mixing puts bonding pi(2p) below sigma(2p). In O2 and F2, sigma(2p) is below pi(2p). The remaining levels are pi*(2p), then sigma*(2p). Each pi set contains two orbitals. The larger atomic 2s-to-2p gap in oxygen and fluorine reduces mixing enough to leave the sigma-first order. *(drawing: Here < means lower energy. Both cells compare bonding 2p levels, not all the levels in the molecule.)* Bond order = (bonding electrons - antibonding electrons) / 2. Subtract first, then halve. Li2 has two electrons in bonding sigma(2s) and none in antibonding levels, giving (2 - 0) / 2 = 1. Equal bonding and antibonding counts give order zero in this elementary model. Filled sigma(1s) and sigma*(1s) core levels contribute two electrons each and cancel in the subtraction. Count unpaired electrons separately to predict magnetism. For comparable bonds between the same elements, higher bond order usually means a shorter bond and a greater dissociation energy. Dissociation energy is the energy needed to separate the bonded atoms. Different elements also differ in orbital size and overlap, so bond order cannot rank every bond's length or strength. N2, O2 and F2 have orders 3, 2 and 1, and their dissociation energies follow that order. Ground-state oxygen is paramagnetic: it is attracted to an applied magnetic field. Its MO configuration leaves one electron in each of the two equal-energy pi* orbitals, giving two unpaired electrons. Oxygen is not a permanent magnet. The usual O=O Lewis structure shows all electrons paired and misses this magnetic behaviour. To predict magnetism for another diatomic, fill its complete scheme and count electrons without an opposite-spin partner. *(drawing: Each cell is one orbital. 'Up' denotes one electron; the matching directions show parallel spins.)* Add both atoms' valence electrons. Order: sigma(2s), sigma*(2s), pi(2p), sigma(2p), pi*(2p), sigma*(2p) for B2 to N2. For O2 and F2, swap bonding pi and sigma(2p). Each sigma holds 2; each pi pair holds 4. Fill low first, singly across equal-energy orbitals before pairing. Bond order is (unstarred count - starred count) / 2. Singles give paramagnetism; all paired gives diamagnetism. Include empty sets when recording the full occupation. **Recap.** Choose the level order and fill it, spreading electrons across equal-energy orbitals before pairing. Bond order is (bonding - antibonding electrons) / 2. Count unpaired electrons separately. *Lesson, questions and cards written for this dataset. OpenStax chemistry records were consulted for facts and teaching structure.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Covalent Bonding: Shared Pairs and Simple Molecules](https://lightmysky.com/learn/science/covalent-bonding-shared-pairs-and-simple-molecules-mt_fn5Dq6qUjP) - [Orbitals, Sub-Shells and Electron Configuration](https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ) ## Opens up - [Band Theory, Defects and Why Some Solids Conduct](https://lightmysky.com/learn/science/band-theory-defects-and-why-some-solids-conduct-mt_RuPWyBzGOp) - [Heteronuclear Diatomics and Polarity in Orbital Terms](https://lightmysky.com/learn/science/heteronuclear-diatomics-and-polarity-in-orbital-terms-mt_thdxFqyHJV)