This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Bonding 3 POLAR BONDS University of Lincoln presentation

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions… A HOMONUCLEAR BOND is a bond between two identical atoms A HETERONUCLEAR BOND is a bond between different atoms

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear & Heteronuclear bonds Homonuclear bondsHetronuclear bonds Ethane (C 2 H 6 ) Hydrazine (N 2 H 4 ) Hydrogen peroxide (H 2 O 2 )

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Determining Bond Energies Consider the 2 homonuclear diatomics H 2 and F 2 The bond energy of H–F would be expected to be the mean of the bond energies of H–H and F–F Is this right?

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Energies Bond Dissociation Energy (kJmol -1 ) XYX–XY–Y½ (X–X + Y–Y) Exptl X–Y HF * HCl * HBr * HI

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Anomalous Bond Energies MoleculeExpected Bond Energy (kJmol -1 ) Measured Bond Energy (kJmol - 1 ) EE H–F H–Cl H–Br H–I

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Why are some heteronuclear bonds much stronger than expected? ?

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License SYMMETRICAL BONDS In a HOMONUCLEAR diatomic molecule, the electrons within the bond are shared equally between the two atoms – a symmetrical bond: The electrons sit in molecular orbitals which lie EQUI-DISTANT from each atom Energy 2s σ * (2s) Li

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ASYMMETRICAL BONDS In a HETERONUCLEAR diatomic molecule, the electrons within the bond are NOT always shared equally between the two atoms – an asymmetrical bond. In an assymetrical bond, the electrons sit closer to one atom than the other, leading to a POLAR BOND: H–F –– ++ The electrons are sitting closer to the F atom

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Why does this happen?

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Electronegativity Pauling defined ELECTRONEGATIVITY as: “the power of an atom in a molecule to attract electrons to itself” This is an atomic property, but only applies when the atoms are in a bond

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The higher the electronegativity, the stronger the ‘pulling’ power of the atom within a bond O 3.4 F 4.0 N 3.0 C 2.6 Cl 3.2 H 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.8 Mg 1.3 Be 1.6 Ca 1.0 Sr 0.9 Ba 0.9 S 2.6 P 2.2 B 2.0 Si 1.9 Al(III) 1.6 Se 2.6 Br 3.0 As(III) 2.2 Ge(IV) 2.0 I 2.7 Te 2.1 Sb 2.1 Ga(III) 1.8 Sn(IV) 2.0 In(III) 1.8 At 2.2 Po 2.0 Bi 2.0 Pb(IV) 2.3 Tl(III) 2.0 Electronegativity

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License …When electrons are held tightly by an atom in a bond, due to the high electronegativity of that atom, the bond is much harder to break So, why are some heteronuclear bonds much stronger than expected?

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Examples of Polar Bonds –– ++ ++ ++ -- ++ -- The slight charges on each end of the molecule lead to electrostatic attraction between adjacent molecules – HYDROGEN BONDING

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definition… A HYDROGEN BOND is an interaction between a hydrogen atom attached to an electronegative atom, and an electronegative atom which possesses a lone pair of electrons The strongest hydrogen bonds involve the first row elements F, O or N

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License HYDROGEN BONDING () H–FH–F H–FH–F H–FH–F H–FH–F H–FH–F

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Hydrogen bonding affects the physical properties of molecules with polar bonds NH 3, H 2 O and HF all have anomalously HIGH boiling points, since extra energy is needed to break the hydrogen bonds

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Can Molecular Orbital Theory account for polar bonds? ?

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License A quick recap… ATOMIC Orbitals MOLECULAR Orbitals H + HH2H2

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License F2F2 Electronic configuration of 9F is: 1s2 2s2 2p5 (9 electrons) The F atom needs 1 more electron to give it a full valence shell (8 outer electrons)– it does this by forming a single covalent bond (in this case with another F atom) Hence, we know we have a single bond in F2: F–F FF

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License BUT we know that the F–F molecule has 18 electrons (2 x 9) How can we arrange 18 electrons in molecular orbitals and end up with only ONE bond? SOLUTION: For every bonding orbital there must be an ‘anti- bonding orbital’ An electron in a bonding orbital is cancelled out by an electron in an anti-bonding orbital

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Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y ) π(2p x ) 2s σ * (2s) σ (2s) F F Consider the MO diagram of F 2

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear MO diagrams are symmetrical. Heteronuclear MOs are asymmetrical – the energies of equivalent atomic orbitals are DIFFERENT Energy 2s σ * (2s) XY Heteronuclear Diatomic molecule MO

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2s σ * (2s) LiH Only valence orbitals shown. The 1s (H) and 2s (Li) overlap to form the  and  * molecular orbitals LiH molecule

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 2p z (F) can overlap with the 1s(H). T he orbitals that do not overlap form NON-BONDING MOs Energy 1s1s 2p σ*σ* σ 2s H F Non-bonding HF

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 1s orbital on the H overlaps with the 2p z on the F to form a  -bond. No overlap can occur between the 1s and the 2p x or 2p y, as these are pointing in the wrong direction 1s1s 2pz2pz 1s1s 2px2px HF H F Bonding Anti- Bonding

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License HF The electrons are sat closer to the F atomic orbitals than the H atomic orbitals. Therefore it is predicted that the H–F bond would be POLAR Energy 1s1s 2p σ*σ* σ 2s H F Non-bonding HF H–F ++ --

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2s2s 2p σ*σ* σ 2s Li F Non-bonding LiF Li–F ++ -- LiF

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Hence, the MO theory can predict POLAR bonds

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Summary

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What you should know… Difference between homonuclear and heteronuclear bonds Explain why some heteronuclear bonds are harder than expected to break How the presence of hydrogen bonding in molecules affects some of their physical properties, like boiling points How to draw the MO diagram of a heteronuclear diatomic molecule, and understand how bonding, anti-bonding and non-bonding orbitals are formed Use the MO diagram to determine whether the bonding is likely to be polar

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions… Homonuclear bond Heteronuclear bond Polar bond Hydrogen bond Electronegativity

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements JISC HEA Centre for Educational Research and Development School of natural and applied sciences School of Journalism SirenFM