What is an ionic bond?
Electrostatic attraction between oppositely charged ions
What is a giant ionic lattice?
A repeating structure of oppositely charged ions
Why do ionic compounds conduct electricity as liquids but not solids?
In solids, the ions are fixed in position, so cannot move to carry charge. As liquids, the ions are free to move, meaning there are mobile charge carriers
Why can metals conduct electricity?
Delocalised electrons act as mobile charge carriers across metallic structure
Why can’t simple molecular structures conduct electricity?
No mobile charge carriers
What two factors increase the strength of electrostatic forces?
Difference in charge
Charge density
What is charge density?
The ionic charge relative to the size of the ion
(i.e. higher charge + smaller ion = higher charge density)
Why are some ionic compounds soluble in water?
- Water molecules break down the lattice
- Leaving individual ions
- Water surrounds these ions
Why are some ionic compounds insoluble in water?
The electrostatic attraction between oppositely-charged ions is too great for the water molecules to overcome
What is relative atomic mass?
The weighted mean mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom
What is relative formula mass?
The weighted mean mass of the formula unit of a compound compared with 1/12 of the mass of a carbon-12 atom
What is relative molecular mass?
the weighted mean mass of a molecule of a compound compared with 1/12 of the mass of a carbon-12 atom
What is (average) bond enthalpy?
The amount of energy required to break a mole of a specific chemical bond OR released when bonds are formed
Name the properties of simple covalent structures
- Don’t conduct electricity (apart from H2O)
- Generally insoluble
- Low melting/boiling points
Why do simple covalent molecules have low melting/boiling points?
Molecules are held together by weak intermolecular forces that require small amounts of energy to overcome. Covalent bonds do NOT have to be broken for changes of state.
Why do some bonds (such as Br-Br) not have an average bond enthalpy?
Because it is only found in one molecule, Br2
Which are weaker: longer bonds or shorter bonds?
Longer bonds
What are the three types of intermolecular force?
- London forces
- Permanent dipole-dipole interactions
- Hydrogen bonds
What is a London force?
- Temporarily, there are more electrons on one side than the other
- Causing a partially negative charge (δ-)
- This repels nearby atoms’ electrons, forming a partially positive charge (δ+)
- This forms a chain reaction
- The opposite charges attract
Which molecules experience London forces?
All molecules experience London forces
How strong are London forces?
They’re the weakest intermolecular force
What is the trend for intermolecular forces going down groups?
Intermolecular forces get stronger going down the group, because the atoms have more electrons
What is a covalent bond?
An electrostatic attraction between a shared pair of electrons and the bonded nuclei
What are lone pairs?
Electrons that are not part of a covalent bond
