- Shapes and IMF
- Electron pairs repel each other as strongly as possible, thus moving apart as much as possible. A lone pair repels more strongly, reducing the other bond angles by 2.5*.
- One bonded pair -- linear; two bonded pairs -- linear or non-linear (cater for each lone pair and reduce other bond angles by 2.5*).
- Three bonded pairs (no lone pair) trigonal planar (120*); three bonded pairs (one lone pair) -- pyrimidal (107*).
- Two bonded pairs (two lone pairs) -- non-linear (104.5*).
- Four bonded pairs and no lone pairs -- tetrahedral (109.5*).
- Six bonded pairs and no lone pairs -- octahedral (90*).
- Permanent dipole
- Electronegativity -- the attraction of the bonded atom for the shared pair of electrons in a covalent bond
- A bond is non-polar when the bonded atoms are the same or share the same electronegativity.
- A polar covalent bond is formed when bonding atoms have a different electronegativity.
- The bonded electron pair is shared unequally between the bonded atoms.
- A CO2 molecule is non-polar since although C=O bonds contain a dipole, the linear shape ensures that the dipoles work in opposing directions, cancelling each other out.
- O-H bonds have a permanent dipole and the non-linear shape of water gives these dipoles an overall direction. Consequently, O is seminegative while H is semipositive.
- Hydrogen bonding
- Hydrogen bonding occurs in N-H, O-H and F-H bonds due to the lone pairs and strong differences in electronegativities.
- 1. Hydrogen bonds hold water molecules apart in an open lattice structure in ice. 2. The water molecules in ice are further apart than in liquid water. 3. Solid ice is less dense than liquid water and floats.
- Hydrogen bonds are forces stronger than London forces. More energy is required to break these strong forces in comparison to only London force molecules. As a result, water has a higher melting and boiling point than these molecules.
- London forces
- The movement of electrons in an atom creates an instantaneous dipole. This induces dipoles in neighbouring molecules, which further induce dipoles in surrounding molecules, hence attracting each other.
- London forces are relatively weak forces which are easy to break; however increasing the size of the molecule or the number of electrons per atom increases the strength of the force. As a result, some substances which use London forces (such as iodine) are solids at room temperature.
- In a simple molecular lattice, molecules are attracted to each other via weak intermolecular forces, but bonded together by strong covalent bonds.
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