Electron pair repulsion theory
- The electrons pair surrounding the central atom determine the shape of the molecule or ion
- the electron pairs repel each other is they are arranged as far as possible
- the arrangement of electron pairs minimises repulsion and holds the bonded atoms in definite shape
- different numbers of electron pairs result in different shapes
Why do lone pairs repel
- electron pairs repel as far away as possible
- lone pairs repel more strongly than bonded pairs
- lone pairs repel bonded pairs slightly closer together, decreasing the bond angle
How much is the bond angle decreased per lone pair
2.5
Shape of molecules
Answer structure
- The central carbon atom is bonded to 2 other atoms
- There are 2 regions of electron densities
- The two bonded regions repel each other as far as possible
- this gives the carbon dioxide molecule a linear shape
- with bond angles of 180 degrees
Electronegativity in periodic table
Increases across periods
Decreases down group
Ignoring noble gases
What are permanent dipoles
A difference in charge between the 2 atoms caused by a shift of electron density in the bond
How are permanent dipoles formed
In a covalent bond, the bonding electrons are pulled towards the more electronegative atom
This makes the bond polar
The difference in electronegativity between the bonding atoms causes a permanent dipole
- the d+ and d- on polar molecules cause weak electrostatic forces of attraction between molecules called permanent dipoles
Intermolecular forces
Definition
Forces between molecules
Weaker than covalent ionic and metallic
What are the 3 types of intermolecular forces
Induced dipole-dipole (London forces)
Permanent dipole-dipole
Hydrogen bonding (strongest)
What do induced dipole dipole forces do
They cause all atoms and molecules to be attracted to each other
How are induced dipole-dipoles formed
- electrons in charge clouds are constantly moving
- at any given point there will always be more electrons on one side of the cloud than the other, creating a temporary dipole
- this causes another temporary dipole in the other direction of a neighbouring atom
- because the electrons are constantly moving the dipoles are created and destroyed all the time
What factors affect induced dipole-dipole forces
Size
- larger molecules have larger electron clouds - stronger forces
Surface area
- they have a bigger exposed electron cloud
This causes a higher boiling point
Intermolecular forces affect on simple covalent compounds
Low melting points
- weak intermolecular forces , easily overcome by heat
Polar molecules
- compounds with hydrogen molecules can form hydrogen bonds with water and dissolve
Not conductors
- don’t have an overall charge
non polar molecules are soluble in non polar solvents
When a simple molecular compound is added to a non polar solvent intermolecular forces form between them
Interactions weaken the intermolecular forces breaking them so that it dissolves
Nonpolar simple molecules are insoluble in polar solvents
Too strong to be overcome
No
Interactions
Polar s8mple molecules can dissolve in polar substances
Attract each other
Depend on London forces
Atoms that can form hydrogen bonds
Lone pair
Oxygen
Fluorine
Nitrogen
