Heteronuclear Diatomics and Polarity in Orbital Terms · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Unequal atoms share unequally

Science · Chemistry · ages 18-19
Name ______________________   Date ____________

  1. Which set of conditions supports strong mixing between two atomic orbitals?

    • Suitable symmetry, nonzero overlap and a small energy gap
    • Suitable symmetry, zero overlap and a small energy gap
    • Suitable symmetry, nonzero overlap and a very large energy gap

  2. In an interacting two-orbital model with unequal parent energies, which parent contributes more to the bonding combination?

    • The higher-energy parent orbital
    • The two parent orbitals equally
    • The lower-energy parent orbital

  3. In an interacting two-orbital model with unequal parent energies, the antibonding combination has more character from the higher-energy parent.

    Circle one:   True   False

  4. In a two-orbital model, hold the nonzero interaction fixed while increasing the parent energy gap. What happens to the orbital contributions?

    • More mixing and less localisation on the parent orbitals
    • More mixing and more localisation on the parent orbitals
    • Less mixing and more localisation on the parent orbitals

  5. HF's F 2p_z orbital lies below H 1s and contributes more to their bonding combination. Which partial charges follow, and what must a full molecular-dipole calculation include?

    • H partially negative, F partially positive; include all electrons and nuclei to obtain the full dipole.
    • H partially positive, F partially negative; include all electrons and nuclei to obtain the full dipole.
    • H partially positive, F partially negative; use the bonding orbital alone to obtain the full dipole.

  6. Fill CO's 10 valence electrons from low to high: 3sigma, 4sigma, 1pi pair, 5sigma, 2pi* pair, 6sigma*. Which option gives the electron count in each set and the usual terminal donor? Each pi entry counts both orbitals.

    • 2,2,4,2,0,0; 5sigma pair donates through carbon.
    • 2,2,4,2,0,0; 5sigma pair donates through oxygen.
    • 2,2,2,2,2,0; 5sigma pair donates through carbon.

  7. Linear CO2 has two equivalent polar C=O bonds but no permanent molecular dipole. Why?

    • Equal polar bonds give opposite contributions, which cancel in linear CO2.
    • Equal polar bonds give parallel contributions, which cancel in linear CO2.
    • Equal polar bonds give opposite contributions, which double in linear CO2.

  8. Take the HF bond as the z axis. A student says H 1s and F 2p_x must mix strongly if their energies are equal. What is missing?

    • Equal energies alone guarantee strong mixing, even with different sigma and pi symmetries about z.
    • H 1s is sigma and F 2p_x is pi, so their net overlap is zero.
    • The two orbitals must have equal sizes.
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Unequal atoms share unequally W1-mt_thdxFqyHJV-s1

  1. Suitable symmetry, nonzero overlap and a small energy gap · Strong mixing needs suitable symmetry and nonzero spatial overlap. A smaller energy gap favours stronger mixing for a given interaction.
  2. The lower-energy parent orbital · The bonding combination has more lower-energy-parent character in the stated two-orbital model.
  3. True · This is the antibonding composition rule for the stated two-orbital model, not a rule for every orbital in a molecule.
  4. Less mixing and more localisation on the parent orbitals · At fixed interaction, increasing the energy gap reduces mixing. Each combination becomes more like one parent; the gap alone does not classify the bond as ionic.
  5. H partially positive, F partially negative; include all electrons and nuclei to obtain the full dipole. · Fluorine attracts HF's bonding density more strongly, giving F partial negative charge and H partial positive charge. The complete dipole depends on all electronic and nuclear charges, not one orbital alone.
  6. 2,2,4,2,0,0; 5sigma pair donates through carbon. · The ten electrons fill 3sigma, 4sigma, both 1pi orbitals and 5sigma, giving set counts 2,2,4,2,0,0. The highest occupied orbital is 5sigma. Its pair has strong carbon character and donates through carbon in the usual terminal mode.
  7. Equal polar bonds give opposite contributions, which cancel in linear CO2. · The equivalent bonds in linear O=C=O give equal contributions in opposite directions. Their vector sum is zero.
  8. H 1s is sigma and F 2p_x is pi, so their net overlap is zero. · About the specified z axis, H 1s has sigma symmetry and F 2p_x has pi symmetry. Their positive and negative overlap contributions cancel.
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