1.3(P2) Shapes And Forces Flashcards
What are the 5 basic shapes of molecules?
Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral
How do we work out the shape?
Identify the central atom
Determine how many … are around the central atom:
Electrons
Electron Pairs
Bonding Pairs
Lone Pairs
Then arrange the electron pairs as far apart as possible
Name the shape and suggest the bond angle
What are the possible shapes with 2 electron pairs? Bonding angles?
Linear - 2 bonding pairs
180 °
What are the possible shapes with 3 electron pairs? Bonding angles?
Trigonal Planar - 3 bonding pairs
V-shaped - 2 bonding pair, 1 lone pair
120 °
117.5 °
What are the possible shapes with 4 electron pairs? Bonding angles?
Tetrahedral - 4 bonding pairs
Trigonal Pyramidal - 3 bonding pairs, 1 lone pair
V-shaped - 2 bonding pairs, 2 lone pairs
109.5 °
107 °
104.5 °
What are the possible shapes with 5 electron pairs? Bonding angles?
Trigonal Bipyramidal - 5 bonding pairs
See-saw - 4 bonding pairs, 1 lone pair
T-shaped - 3 bonding pairs, 2 lone pairs
120 ° , 90 °
117.5 ° , 87.5 °
120 ° , 86 °
What are the possible shapes with 6 electron pairs? Bonding angles?
Octahedral - 6 bonding pairs
Square Pyramidal - 5 bonding pairs, 1 lone pair
Square Planar - 4 bonding pairs, 2 lone pairs
90 °
87.5 °
90 °
What is electronegativity?
The power of an atom to attract the electron density in a covalent bond towards itself
What are the trends of electronegativity?
Electronegativity increases left to right across a period
Electronegativity increases up a group
What is significant about noble gases?
They don’t form compounds therefore have no electronegativity
What are intermolecular forces?
Forces between all molecules in covalent bonding in liquids and solids
What are the intermolecular forces in order of strength?
Hydrogen bonding
Permanent dipole-dipole interaction
Van der Waals force
What are Van der Waals?
Weakest type of intermolecular force present in all molecules but the strongest type of force that acts on non-polar molecules
What does the strength of the van der Waals depend on?
Size of the molecules
As larger molecules have more electrons hence stronger attractions
What causes van der Waals?
Temporary partial charges on the molecules occur due to fluctuating electron density which creates a temporary induced dipole
Therefore the temporary dipole and the temporary induced dipole causes the molecules to be attracted
What is van der Waals also called?
Dispersion Forces
What are permanent dipole dipole forces?
They occur between polar molecules (molecules with an overall permanent dipole moment due to difference in electronegativity)
How do permanent dipole dipole forces occur?
A molecule has a high electron density due to greater electronegativity
This causes a permanent dipole
What can permanent dipole dipole forces be affected by?
Van der Waal forces but the permanent dipole dipole interaction is likely to be stronger
What is hydrogen bonding?
A specific type of permanent dipole dipole which only occurs between molecules containing hydrogen bound to:
Nitrogen
Oxygen
Fluorine
Why is hydrogen bonding special?
There is a greater dipole due to the large difference in electronegativity between hydrogen and N, O or F
What must you show on intermolecular force diagrams?
The covalent bonds
The delta partial charges
ALL the lone pairs
A dashed line between the element to a lone pair
What is the effect of hydrogen bonding on the boiling points of Hydrides?
Those with hydrogen bonding have higher boiling points
Big drop down the groups as van der Waals are weaker but slight incline as the go from there as the size of the molecules increase
What has the highest boiling point out of hydrides?
Water
It has the strongest attraction
It can form more hydrogen bonds
What should you mention when talking about boiling points?
Size of the molecule
Amount of electrons
Strength/type of IMF
Polarity
What is the general trend for melting/boiling points across period 3?
Na, Mg, Al - metallic
= increasing
Si - macromolecular
= small decrease
P, S, Cl - simple molecular
= large decrease
Ar - atomic
=decrease
How does metallic bonding across a period effect boiling points?
Higher charge density
Increased electron density in ‘sea’
Stronger electrostatic force of attraction
How does macromolecular bonding across a period effect boiling points?
Lots of atoms covalently bonded requires a lot of heat energy to break the attraction of the shared electrons
How does simple molecular bonding across a period effect boiling points?
Non-polar
Only Van der Waals forces
Which has the highest boiling points out of P, S and Cl? Why?
S -highest
P - middle
Cl - lowest
Usually P4 and S8
Sulphur is a larger molecule
= more electrons = stronger van der Waals
What is the difference is electronegativity between atoms cause?
Unequal distribution of electron density
Therefore the bonding pair can be slightly more attracted to one side
What do we use to show electronegativity?
Lower case delta sign with a charge
What affects electronegativity?
Nuclear charge
Atomic radius
Electron shielding
What is electron shielding?
The amount of electrons between the nucleus and outer shell which affects the attraction between the bonding pairs
When working out bond angles what does a lone pair equal?
For each lone pair
-2.5 degrees
From the original shape it is based on
