Topic 2 Bonding & Structure
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Ice & I2 have open lattice structures. What is that?
Simple molecules held together in a lattice by intermolecular forces of attraction.
Ionic bonding
Strong electrostatic attraction between oppositely-charged ions held in a giant lattice.
Why do ionic radii decrease down a group?
More shells of electrons.
What experiment can be used to show a compound is ionic?
Electrodes in a circuit are placed in a solution of the ionic compound. The bulb lights up.
Isoelectronic
Ions with the same electron configuration as a Noble gas.
What is the trend in bond strength/enthalpy and bond length down group 7?
Bond lengths increases and bond strength/enthalpy decreases. Exception: F-F, which has the shortest bond length yet the lowest bond enthalpy.
Why is F-F an exception to the bond length & strength trend down group 7?
The lone pairs around each F atom repel, which weakens the covalent bond.
Covalent bond
Strong electrostatic attraction between 2 nuclei and the shared pair of electrons between them.
Sigma bond
Covalent bond where the p-orbitals overlap end-to-end.
Pi bond
Covalent bond that forms only after a sigma bond has formed, usually by the sideways overlap of p-orbitals.
Dative/coordinate covalent bond
Formed when one atom donates both bonding electrons.
Bond length
Sum of the atomic radii.
What determines the strength of a covalent bond?
Bond length. Longer bonds are weaker, hence the hydrolysis of the C-X bond in halogenoalkanes.
P & S in period 3
Can hold more than 8 e- in the outer shell due to the availability of energetically favourable orbitals.
Describe the bonding in graphene.
Same as in graphite. Single layer of hexagonal rings. Delocalised e- are free to move to conduct electricity.
Describe the bonding in graphite.
Each C has 3 sigma bonds & 1 delocalised e-. Planar hexagonal rings held together by London forces. Layers can slide over each other, and e- are free to move to conduct electricity.
Where can the delocalised e- move in graphite?
Electrons flow parallel to the layers. The energy gap is too big to allow perpendicular flow.
Describe the bonding in diamond.
Each C forms 4 sigma bonds. Tetrahedral. Numerous, strong covalent bonds.
Explain why some ionic compounds have higher melting points than others.
Compare charge and ionic radius. Higher charge density of the cation = greater attraction between the cation & anion.
Migration of ions experiment
Electrolysis of a green copper (II) chromate (VI) solution. Cu2+ ions migrate to the cathode and appear yellow. Chromate ions migration to the anode and appear blue.
In a negative ion, which atom gains the e-?
The more electronegative one.
Why do alcohols have higher boiling points than alkanes of the same chain length?
Alcohols have hydrogen bonding, but alkanes have only London forces.
Why does HF have hydrogen bonding but HBr does not?
F is small & electronegative, so creates a greater dipole moment in HF, meaning it can attract the lone pair on the F of another molecule.
Why do branched isomers have lower boiling points than straight-chain isomers?
Branched molecules have fewer points of contact, so weaker London forces.