Shapes of molecules and intermolecular bonding Flashcards
What factors affect the shapes of molecules?
- How many lone pairs/bonded pairs of electrons there are.
- How many total electron pairs there are.
What is electron pair repulsion?
Electron pairs repel one another so they’re arranged as far apart as possible which minimises repulsion and holds the bonded atoms in a definite shape.
What does the line, dotted wedge and solid wedge represent?
- Line = bond in the plane of the paper.
- Dotted wedge = bond going into the plane of the paper.
- Solid wedge = bond coming out of the plane of the paper.
Which molecules have a 3D shape?
Molecules with 4 or more areas of electron density.
What is the order of electron pair repulsion?
- Lone pair/lone pair (strongest)
- Lone pair/bonded pair
- Bonded pair/bonded pair (weakest)
Why do lone pairs have a stronger repulsion than bonded pairs?
They’re closed to the nucleus and occupy more space than a bonded pair so they repel other electron pairs more strongly which pushes these pairs closer together, therefore decreasing the bond angle.
What shape and bond angle does CH4 have and why?
- Tetrahedral shape
- Bond angle 109.5
- Because 4 areas of electron density
- 4 bonded pairs and 0 lone pairs
- BP/BP have lowest repulsion and repel equally to be as far apart as possible
What shape and bond angle does NH3 have and why?
- Pyramidal shape
- Bond angle of 107
- Because 4 areas of electron density so tetrahedral derivative
- Between 1 lone pair and 3 bonded pairs
- Repel equally as the electrons want to be as far apart as possible due to BP/LP electron pair repulsion so bond angle decreases by 2.5 as 1 lone pair present
What shape and bond angle does H2O have and why?
- Non-linear shape
- Bond angle of 104.5
- Because 4 areas of electron density so tetrahedral derivative
- Between 2 lone pairs and 2 bonded pairs
- Repel equally as the electrons want to be as far apart as possible due to BP/LP electron pair repulsion so bond angle decreases by 5 as 2 lone pairs present
What shape and bond angle does CO2 have and why?
- Linear shape
- Bond angle of 180
- 2 areas of electron density
- 4 bonded pairs and 0 lone pairs
- BP/BP have lowest repulsion and repel equally to be as far apart as possible
What shape and bond angle does BF3 have and why?
- Trigonal planar shape
- Bond angle of 120
- 3 areas of electron density
- 3 bonded pairs and 0 lone pairs
- BP/BP have lowest repulsion and repel equally to be as far apart as possible
What shape and bond angle does SO2 have and why?
- Non-linear shape
- Bond angle of 117.5
- 3 areas of electron density so trigonal planar derivative
- 1 lone pair and 2 bonded pairs so bond angle decreases by 2.5
- Due to BP/LP electron pair repulsion, they repel equally to be as fair apart as possible
What shape and bond angle does PCl5 have and why?
- Trigonal bipyramidal shape
- Bond angles of 120 and 90
- 5 areas of electron density
- 5 bonded pairs and 0 lone pairs
- BP/BP have lowest repulsion and repel equally to be as far apart as possible
What shape and bond angle does SF6 have and why?
- Octahedral shape
- Bond angles of 90
- 6 areas of electron density
- 6 bonded pairs and 0 lone pairs
- (mirrored image)
- BP/BP have lowest repulsion and repel equally to be as far apart as possible
What shape and bond angle does the NH4+ ion have?
Same shape and bond angle as CH4 and for the same reasons.
What is electronegativity?
The ability of an atom in a covalent bond to attract a shared pair of electrons.
What factors affect electronegativity?
- Size of atomic radius
- Strength of nuclear charge
How is electronegativity measured?
With Pauling values
Why does electronegativity increase across a period?
- Number of protons increase
- Stronger nuclear charge
- Electronegativity increases
Why does electronegativity increase up a period?
- Number of shells decrease
- Distance to nucleus decreases
- Stronger attraction to nucleus
- Electronegativity increases
How do we represent partial charges within bonds?
Delta - or delta +
How can a covalent bond become ionic?
High electronegativity difference means electrons can be completely pulled onto one atom.
What is polarity?
The uneven distribution of electrons caused by the difference of electronegativities.
Why will a bond be non-polar?
- The bonded atoms are the same
OR - The bonded atoms have the same/similar electronegativity
so the bonded electron pair is shared equally between bonded atoms and will be shared roughly in the middle
When will a bond be polar?
- The bonded atoms are different
OR - The bonded atoms have different electronegativity values
What is a dipole?
The separation of opposite charges.
What factors can cause a molecule to have a permanent dipole?
- Molecular shape (symmetry)
- Electronegativity
Why may molecules with polar bonds have no overall dipole?
They would be symmetrical so the charges cancel out.
Why does H2O have a permanent dipole? (Symmetry rule)
- 2 lone pairs present so molecule is not symmetrical.
- This means that dipoles (charges) do not cancel out.
- Therefore the molecule has a permanent dipole meaning it’s polar.
Why doesn’t CO2 have a permanent dipole? (Symmetry rule)
- No lone pairs present so molecule is symmetrical.
- This means the dipoles cancel out.
- Therefore the molecule has no overall dipole so it’s non-polar.
Why is PCl5H polar?
- The surrounding atoms have different charges (H is delta positive and Cl is delta negative).
- This means the molecule is not symmetrical.
- Therefore the dipoles don’t cancel out so the molecule has a permanent dipole so it’s polar.
Why are hydrocarbons never polar?
C and H have near identical electronegativity so C—H bonds are never polar.
