Topic 4: Chemical Bonding and Structure

to achieve stability

AKA electrovalent bond • the sum of all electrostatic attractions and repulsions in the ionic compound • established by the transfer of electrons from one atom to another • to form ions with complete valence shells

• no of valence electrons • formation of ionic bonds is exothermic due to electron transfer and bond formation • big diff in electronegativity

• solid ionic compounds held in crystal lattice - ↑ lattice energy = ↑ ionic bond strength

• high m.pt/b.pt: bonds require a lot of energy to break • solids at room temp: due to strong electrostatic forces of attraction between locked ions in crystal lattice • brittle: ions of the same charge are beside each other, so the repulsive forces cause it to split • soluble in water: as water is a polar solvent, it detaches ions from the crystal lattice due to its electrostatic pull • good conductors in molten state: as ions are free to move about • low volatility

• bond formed by mutual sharing of electrons between the combining atoms

• must have 5-7 valence e-s • equal electronegativity (so no transferring of electrons occurs) • equal electron affinity to equally attract the electron pair

• bond length • bond angle • bond energy

avg. distance between centres of nuclei of 2 bonded atoms unit: picometer (pm)/angstrom (Å)

• bond multiplicity: ↑ no. of bonds (i.e. single/double/etc), ↓ bond length • atom size: ↑ size, ↑ bond length

• energy required to break one mole of covalently bonded atoms in gaseous state • measures bond strength unit: kJ/mol

• bond length: ↑ length, ↓ bond energy - size of bonded atom: ↑ size, ↓ bond energy

• type of covalent bond - both electrons in the shared pair of e-s come from the same atom

• covalent bond in which electrons are shared unequally • due to unequal electronegativity • thus bonded atoms acquire a partial positive/negative charge

Valence Shell Electron Pair Repulsion Theory • electron pairs tend to repel each other • pairs try to stay as far apart to have min. energy and max. stability • greater repulsion between non-bonded e-s • the geometry of a molecule is dependent on the no. of valence e-s around the central atom

when the molecule’s characteristic properties can be described by 2+ structures, the real structure is a resonance hybrid of the possible structures

• different forms of the same element - they have differing physical properties but similar chemical properties

• diamond • graphite • fullerene (C60) • graphene

• giant 3-D covalent tetrahedral arrangement • each C is strongly bonded to 4 other Cs (tetrahedral) • no plane of weakness • all bonds are equally strong • electrons can’t move freely • so diamond can’t conduct electricity

• cutting glass • making bores for rock drilling • grinding/polishing hard materials

• consists of 2-D hexagonal rings • each C atom is strongly bonded to 3 other Cs • weak Van der Waal forces between layers • layers can slide over each other so graphite is an excellent lubricant • graphite can also conduct electricity due to delocalized e-s, making it a good conductor

• making electrodes • lubricant • pencil lead

• perfect sphere • consisting of 60 C atoms arranged in hexagonal and pentagonal rings • soluble in suitable organic solvents • forms coloured solutions in organic solvents (varying from red to brown to magenta) • has delocalised e-s but doesn’t conduct electricity

• ferromagnetism - super conductivity