Chapter 6: Shapes of molecules and intermolecular forces (6.1, 6.2)
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what are the three types of intermolecular forces (weakest to strongest)
induced dipole-dipole interactions (London forces) permanent dipole-dipole interactions hydrogen bonding
how do induced dipole-dipole interactions (London forces) come about
• movement of electrons movement of electrons produces a changing dipole in a molecule • at any instant, an instantaneous dipole will exist, but its position is constantly shifting • the instantaneous dipole induces a dipole on a neighbouring molecule (the positive charge attracts any electrons or the other way around) • the induced dipole induces further dipoles on neighbouring molecules, which attract one another
how permanent are induced dipoles
they are temporary, can appear and then disappear in an instant of time
electron-pair repulsion theory
The electron pairs surrounding a central atom determine the shape of the molecule or ion The electron pairs repel one another so that they are arranged as far apart as possible The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape Different numbers of electron pair result in different shapes lone pairs of electrons repel more than bonded pairs of electrons greater the number of electron pairs, the smaller the bond angle will be in a molecule
4 bonded pairs, 0 lone pairs
tetrahedral 109.5 degree bond angle
3 bonded pairs, 1 lone pair
pyramidal 107 degree bond angle
2 bonded pairs, 2 lone pairs
non linear 104.5 degree bond angle
2 bonding regions
linear 180 degree bond angle
3 bonded pairs, no lone pairs
trigonal planar 120 degree bond angle
6 bonded pairs, no lone pairs
octahedral 90 degree bond angle
factors that effect electronegativity
different nuclear charges (high nuclear charge means high electronegativity) atoms may be different sizes (low atomic radius means high electronegativity) one shared pair of electrons may be closer to one nucleus than the other (closer means high electronegativity)
electronegativity trends in the periodic table
electronegativity increases up and along (right) the periodic table
non-polar bonds
the bonded electron pair is shared equally between the bonded atoms a bond will be non-polar if the bonded atoms are the same or if the bonded atoms have similar electronegativity non-polar solutes are soluble in non-polar solvents (e.g. hydrocarbon liquids like hexane)
polar bonds
the bonded electron pair is attracted more to one of the bonded atoms than the other because it is more electronegative polar solutes are soluble in polar solvents (e.g. water)
What effects the strength of induced dipole-dipole interactions?
More electrons, larger instantaneous and induced dipoles, the greater the induced dipole, dipole interaction and the stronger the attractive forces between molecules.
Why is the boiling point of a polar substance larger than a non polar substance with the same number of electrons.
1) Because non-polar molecules only have london forces 2) Polar substances have London forces and permanent dipole-dipole interactions between molecules 3) Extra energy is needed to break additional dipole-dipole interactions
How do elements get their shapes?
The electron pairs repel one another so they are as far apart as possible to minimise repulsion giving them a definite shape.
How much is the bond angle of a molecule reduced with each lone pair?
2.5 degrees
What shape is a carbonate ion?
Trigonal planar
What shape is a nitrate ion?
Trigonal planar
What shape is a sulfate ion?
Tetrahedral
What shape is an ammonium ion ?
Tetrahedral
How are things charged in a polar molecule?
The more electronegative atom is negatively charged (d-) and the other atoms are positively charged (d+)
How do you know if a molecule with more than two different atoms is polar?
It depends on the shape of the molecule, the dipoles may reinforce each other making a larger dipole or they may cancel each other out.