SHAPES OF MOLECULES AND INTERMOLECULAR FORCES
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The electron pair repulsion theory
Electron pairs surrounding central atom determines the molecule / ion shape Pairs repel each other so they are arranged as far as possible Arrangement minimises repulsion, so hold atoms in definite shape Different number of electron pairs = different shape
What 2 factors determines the shape of a molecule / ion
Bonding regions Lone pairs
Lone pair
Lone pair slightly closer to central atom Occupies more space than a bonded atom because they repel more strongly than a bonding pair
How does a lone pair affect the bond angle?
Reduces the bond angle by 2.5 per lone pair
What is the bond angle of a tetrahedral molecule
• 5 e. g. CH4
What is the bond angle of a pyramidal molecule
107 e.g. NH3
What is the bond angle of a non linear molecule
104.5 Non-linear
A molecule with 2 bonding regions
Bond angle : 180 linear e.g. CO2
A molecule with 3 bonding regions
Bond angle: 120 Trigonal planar e.g. BF3
A molecule with 4 bonding regions
Bond angle: 109.5 Tetrahedral e.g. CH4
A molecule with 6 bonding regions
Bond angle: 90 Octahedral e.g. SF6
Examples of tetrahedral molecules
CH4 Ammonium ions SO4^2-
Examples of trigonal planar molecules
CO3 ^2- NO3 ^-
What is meant by electronegativity
The attarcted of a bonded atom for the pair of electrons in a covalent bond
What scale is used to compare electrongetavitivties
Pauling scale
What is the most electronegative element?
Fluorine
Electronegativity trend
Electronegativity increases upwards and across towards fluorine Group 1 is the least electronegative elements
What kind of bond is formed where there is a large difference in electronegativities?
Ionic rather than Covalent
Non polar bond
Bonded electron pair shared equally between bonded atoms When atoms are the same element / have similar electrongetiavity
Pure covalent bond
Bonded atoms are the same element
Polar covalent bond
Bonded electron pair shared unequally Different atoms with different electronegativity. More electronegative has greater attraction for bonded pair electrons
Polar bonds are polarised with…
Dipoles
Permanent dipole
Dipole in a polar covalent bond which doesn’t change
What can polar bonds do? H2O and CO2
Polar bonds may reinforce one another to produce larger dipole over whole molecule or cancel out H2O: Polar OH bonds have permanent dipoles Act in opposite directions but don’t exactly oppose each other Overall O end is delta neg and H end is delta positive CO2: Non-polar C=O have permanent dipole Act in opposite directions and exactly oppose each other Overall dipoles cancel