2.6 Shapes of Molecules and Forces
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What is the valence shell repulsion theory ?
-Valence electrons exist in pairs. -Electron pairs move away from each other to minimise electrostatic repulsion. -Electron pairs are either bonding or lone pairs.
What is the strongest bond - bond repulsion in order ?
Lone pair - Lone pair Lone pair - Bonding pair Bonding pair - Bonding pair
Linear shape
-2 electron pairs (2bp). -180 bond angle. E.g. BeCl2
Trigonal Planar
-3 electron pairs (3bp). -120 bond angle. E.g. BF3
Tetrahedral
-4 electron pairs (4bp). -109.5 bond angle. E.g. CH4
Pyramidal
-4 electron pair (3bp / 1lp). -107.0 bond angle. E.g. NH3
Non-linear
-4 electron pairs (2bp / 2lp). -104.5 bond angle. E.g. H20
Octahedral
-6 electron pairs (6bp). -90 bond angle. E.g. SF6
What are multiple bonds classed as ?
Multiple bonds, close together, are classed as one area of electron bonding region.
What is electronegativity ?
The ability of an atom to attract the shared electron pair in a covalent bond.
What are 2 factors that affect electronegativity ?
-Size of the atom. -Nuclear charges are different.
What are Pauling values ?
Used to compare electronegativity of the atoms of different elements. Large value indicates the atoms are very electronegative. -Covalent = o difference -Polar covalent = less than 1.8 difference. -Ionic = more than 1.8 difference.
What is a non polar bond ?
Bonded electron pair is shared equally between bonded atoms.
What is a polar bond ?
Bonded electron pair is shared unequally between bonded atoms. Atoms will have different electronegativity values resulting in a polar covalent bond.
What are polar molecules ?
-Contain polar bonds. -Not symmetrical. -Dipoles do not cancel but act in different directions.
What are intermolecular forces ?
Intermolecular forces are weak forces of attraction between molecules.
London Forces
Temporary dipole - induced dipole. Variation in external electrical field strength creates a temporary dipole. There is a shift in electron density. This causes a dipole in a neighbouring molecule.
What is the typical bond enthalpy for London forces ?
10 KJ mol-1
What is the permanent dipole-dipole interaction ?
Molecules with permanent dipoles interact. Creates a permanent dipole-dipole interaction between molecules.
What is a typical bond enthalpy of permanent dipole-dipole interaction ?
25 KJ mol-1
What is the melting/boiling point of simple molecular substances ?
Low melting and boiling point due to weak intermolecular forces between molecules.
Can non polar simple molecular substances dissolve in polar solvents ?
No because non-polar substance only has London forces, whilst polar solvent has London forces and permanent dipole - dipole interactions.
Can non polar simple molecular substances dissolve in non polar solvents ?
Yes because they will create London faces between each other as they have London forces between similar molecules.
Can polar simple molecular substances dissolve in non polar solvents ?
No because non-polar substance only has London forces but polar substance has permanent dipole-dipole interactions.