Molecular orbital diagrams – bond order and magnetism
ChemistryChemical BondingAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Chemical Bonding
Fill the molecular orbital diagrams of the diatomic molecules H₂ to Ne₂ and the ions O₂⁺, O₂⁻, O₂²⁻ and N₂⁺ with electrons. The simulation checks the Aufbau principle, the Pauli principle and Hund's rule, then gives the bond order, the unpaired electrons and the magnetism. Test the magnetism on a Gouy balance and compare bond order with bond length and bond enthalpy.
Lesson: Chemical bonding: molecular orbital theory of diatomic molecules
What it shows
In molecular orbital theory the atomic orbitals of two atoms combine into molecular orbitals: bonding orbitals lie lower in energy, antibonding orbitals higher. Electrons fill them by the Aufbau principle, the Pauli principle and Hund's rule. The bond order is ½(N_b − N_a): the higher it is, the shorter and stronger the bond, and a bond order of 0 means no stable molecule. A molecule with unpaired electrons is paramagnetic. The theory explains why oxygen is paramagnetic, which a Lewis structure cannot.
How to use
In the MO diagram tab, choose the Species and the Energy order. With Fill set to By hand, tap a molecular orbital to place each electron; Undo removes the last one and Clear starts again. A correct ground state goes into the table. Press Switch on magnet to test the sample on the Gouy balance. The Trends tab plots bond length and bond enthalpy against bond order.
Parameters you can change
- Tab MO diagram, Trends
- Species H₂, He₂, Li₂, Be₂, B₂, C₂, N₂, O₂, F₂, Ne₂, H₂⁺, He₂⁺, N₂⁺, O₂⁺, O₂⁻, O₂²⁻
- Energy order As observed, With s–p mixing, Without s–p mixing
- Fill By hand, Automatic
Questions to explore
- Fill the diagrams of O₂⁺, O₂, O₂⁻ and O₂²⁻: how do bond order and bond length change as electrons enter π*2p?
- Why is O₂ pulled into a magnetic field when its Lewis structure has every electron paired?
- The Gouy test gives about 7.2 mg for O₂ and 2.7 mg for O₂⁺: how does this ratio reveal the unpaired electrons?
