Shapes of molecules and ions (6) Flashcards
Electron repulsion
When you have a pair of electrons they will repel each other, but will also repel other electron pairs in the molecule so the bonded pairs will position themselves as far apart as possible which gives the molecule a specific shape
Which repels more, bonded or lone pairs?
Lone pairs will repel a lot more than a bonded pair so they will push the bonded pairs further apart from it and they will be moved closer together
Tetrahedral
4 bonded pairs, 0 lone pairs
109.5 angle
Trigonal planar
3 bonded pairs 0 lone pairs
120 angle
Linear
2 bonded pairs, 0 lone pairs
180 angle
Pyramidal
3 bonded pairs, 1 lone pair
107 angle
Non-Linear
2 bonded pairs, 2 lone pairs
104.5 angle
Trigonal Bipyramidal
5 bonded pairs, 0 lone pairs
120 and 90 angles
Octahedral
6 bonded pairs, 0 lone pairs
90 angle
T-shaped
3 bonded pairs, 2 lone
<90 angle
Umbrella
5 bonded pair, 1 lone pair
90, <90 angle
What is electronegativity?
The ability of an nuclei of an atom in a covalent bond to attract the shared pair of electrons
What affects the electronegativity of an atom?
How big the nuclear charge is and how big the atomic radius is
How is electronegativity measured?
The Pauling Scale
What are the most electronegative elements?
Fluorine, nitrogen, oxygen and chlorine
How do you know if it’s an ionic or covalent bond using the Pauling scale?
If it’s 0 its a pure covalent which means atoms of the same element are bonded together.
If it’s 0-1.8 its a polar covalent which means the shared pair of electrons are closer to one atom than the other
If it’s more than 1.8 it’s ionic because the shared pair of electrons is a lot closer to one of the atoms than the other until they become accepted by it to form ions.
When will a bond be non-polar?
When the two atoms in a covalent bond are the same or have very similar electronegativities
When will a bond be polar?
When both of the atoms have different electronegativity values which means the electronegative atom will have a stronger attraction to the shared pair of electrons and so will create a partially positive and negative dipole.
Is water a polar molecule, why?
Water is a polar molecule because O-H is a polar bond. The oxygen is far more electronegative than hydrogen so attracts the pair of electrons and becomes partially negative while hydrogen becomes partially positive. Because water is a non-linear molecule, the two polar bonds don’t cancel.
Is CO2 a polar molecule, why?
CO2 is not a polar molecule. This is because even though C=O is polar, the molecule is linear so the dipoles act in opposite directions and get cancelled out.
Why are ionic lattices soluble in polar solvents?
Because the partial charges on the dipole can attract the ions in the lattice and break it apart. E.g NaCl will dissolve in water because the partially positive H will attract the Cl- and the partially negative O will attract the Na+
What is an intermolecular force?
Weak interactions between dipoles of different molecules. These could be London forces, permanent dipole-dipole interactions and hydrogen bonding
What are London forces?
They are induced dipole-dipole interactions and weak intermolecular forces that occur between all molecules
How are London forces created?
Electrons are constantly moving around so instantaneous dipoles are being created in molecules. This then triggers instantaneous dipoles in other molecules and they attract each other. In the next instant, the dipole may disappear.
How strong are London forces?
The more electrons there are in the molecules that are interacting, the more instantaneous dipoles are formed, the more induced dipole and the stronger the intermolecular forces will be. They are very easily broken however so don’t have a high boiling point compared to any other bond.
What are van der Waals forces?
Something that describes both induced and permanent dipole-dipole interactions. This isn’t used anymore as we have refined it.
Permanent dipole-dipole interactions
The intermolecular forces between a partially positive dipole on one molecule and a partially negative dipole on another molecule.
What is a simple molecular substance?
A substance made up of simple molecules with small numbers of atoms like H2O and CO2
What do simple molecular substances look like in solid state?
They exist as simple molecular lattices which have weak intermolecular forces between molecules that can be broken easily and strong forces between the atoms in the molecules.
What is the melting and boiling point of a simple molecular substance?
It has a low melting and boiling point because it doesn’t take a lot of energy to overcome the weak intermolecular forces.
What happens when a simple molecular substance is added to a non-polar solvent?
If the simple molecular substance is non-polar, it will dissolve in the non-polar solvent because there will be intermolecular forces formed between the solvent and substance which will weaken the intermolecular forces in holding the lattice together, so the simple molecular substance will dissolve. If the simple molecular substance is polar, it will not interact with the solvent so won’t dissolve in it
What happens when a simple molecular substance is added to a polar solvent?
It will only dissolve in the solvent if it is polar as the solute and solvent molecules attract each other so the intermolecular forces will break. The solubility always depends on how strong the dipole is.
Can simple molecular solvents carry electricity?
No, because there are no mobile charges that can move around to conduct electricity.
What is hydrogen bonding?
A hydrogen bond is an electrostatic attraction between partially negative oxygen on one molecule and partially positive hydrogen on another molecule. They are the strongest type of intermolecular force.
What are some anomalous properties of water?
- Ice is less dense than water
- Water has a very high specific heat capacity
Why is ice less dense than water?
Because when water freezes, the water molecule forms 4 hydrogen bonds and the molecules extend outwards and further apart which creates large tetrahedral holes in the structure. This makes ice a lot less dense than water so it can float on top.
Why does water have such high specific heat capacity?
Because it has hydrogen bonds between the water molecules which require energy to break and water also have London forces between them which requires additional energy to overcome.
