C6 - Shapes of molecules and intermolecular forces Flashcards
What is electron pair repulsion theory?
Electron pairs repel each other so are arranged as far apart as possible
Arrangement minimises repulsion and therefore holds bonds in definite shape
Lone pairs repel more
Electron pair repulsion in increasing order?
Bond bond
Bond lone
Lone lone
What determines the shape of a molecule
The electron pairs surrounding the central atom
What does a solid line mean in displayed formula
Bond in the plane of paper
What does a solid wedge mean in displayed formula?
Comes out of the plane of paper
What does a dashed wedge mean in displayed formula
Goes into the plane of paper
Bond angle of tetrahedral
109.5
Bond angle of pyramidal
107
Bond angle of non linear
104.5
Bond angle of linear
180
Bond angle of trigonal planar
120
Bond angle of octahedral
90
Bond angle of trigonal bipyramidal
90
120
Electron pairs of octahedral
6bp
Electron pairs of linear
2bp
Electron pairs of tetrahedral
4bp
Electron pairs of non-linear
2bp
2lp
Electron pairs of trigonal planar
3bp
Electron pairs of pyramidal
3bp
1lp
What is electronegativity
Ability of an atom to
attract electrons in a covalent bond
How is electronegativity measured?
Paulings scale
What happens to electronegativity down a group
Decreases
What happens to electronegativity across a period
Increases
What does it mean if the electronegativity difference is large?
One bonded atom has a much greater attraction for shared pair of electrons
What is a non polar bond?
Bonded electron pair shared equally between bonded atoms
When is a bond non polar
Bonded atoms are the same
Have same electronegativity value
What is a pure covalent bond
Molecule of elements bonded to themselves
What is a polar bond.
Bonded electron pair shared unequally between bonded atoms
What’s a polar molecule
Molecule with overall dipole
When is a bond polar?
Different elements with different electronegativity values
What’s a dipole
Separation of opposite charges in a polar covalent bond
Creating a small positive charge and a small negative charge
What’s a permanent dipole?
Small charge difference that does not change across a bond
What are the three main categories of intermolecular forces
Induced dipole-dipole interactions (London forces)
Permanent dipole-dipole interactions
Hydrogen bonding
Strength of intermolecular forces in increasing order
London forces
Permanent dipole-dipole interactions
Hydrogen bond
Single covalent bond
How do London forces work?
Electrons move constantly producing instantaneous dipole that is constantly moving
Instantaneous dipole induces dipole on neighbouring molecule
Induced dipole induces further dipole on neighbouring molecules which attract one another
Temporary
What does strength of London forces depend on
Size of molecule (number of protons/electrons)
How do permanent dipole to dipole interactions occur?
Interact between two permanent dipoles of different molecules
What are simple molecular lattice?
3D structure of molecules bonded together by weak intermolecular forces
What are the melting points of simple molecular substances?
Low
weak intermolecular forces
What is the solubility of non-polar simple molecular substances
Soluble in non-polar solvents as they can interact and form bonds
Interactions weaken intermolecular forces causing them to break and the compound dissolves
What happens to non-polar simple molecule in a polar substance
Little interaction, intermolecular forces are too strong to be broken
Explain solubility of polar simple molecule
Depends on strength of dipole, dissolves in polar solvents
Do simple molecules conduct electricity
No as there are no charged particles that are free to move
What is a hydrogen bond?
Strong dipole-dipole attraction between electron-deficient hydrogen atom on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O, F on a different molecule
What are anomalous properties of water
Ice is less dense that water:
-H bonds hold H2O molecules apart in open lattice structure
-Molecules held further apart in ice than water
What is water’s melting point
High
Why is water’s melting point high?
Has London forces and H bonds
H bonds require lot of energy to overcome
What are other properties of water?
High surface tension
High viscosity
