name the limitations of the particle model
- All particles are represented as spheres
- Particles are represented solid and unbreakable
- We simplify the different forces between particles by calling them all bonds
what happens when when a liquid reaches boiling point
particles have enough energy to overcome forces
name the two main types of covalent structures
- simple molecules
- giant covalent structures
describe simple molecules
- low boiling point and melting point
- molecules held together by weak intermolecular forces
- covalent bonds between atoms
describe giant covalent structures
high boiling point and melting point due to strong covalent bonds which hold the structure together
why does carbon dioxide have a low boiling point
- carbon dioxide is a simple molecule
- there are weak intermolecular forces
- therefore a small amount of energy is needed to overcome these forces
why does diamond have a high melting point?
- diamond is a giant covalent structure
- there are strong covalent bonds between the carbon atoms
- and many bonds must be broken therefore a large amount of energy is needed to overcome the bonds.
describe the structure of diamond
each carbon atom forms four covalent bonds with other carbon atoms
describe the structure of graphite
each carbon atom forms three covalent bonds with other carbon atoms forming layers of hexagonal rings
can diamond conduct electricity? why?
no
no delocalised electrons
why is graphite soft and slippery? (3)
- layered structure of hexagonal rings
- with weak intermolecular forces between layers
- so layers can slide over eachother
can graphite conduct electricity? why? (3)
yes
-three covalent bonds per carbon atom
- so one electron per carbon atom is delocalised
- these delocalised electrons are free to move through the structure carrying charge
describe the structure of sodium chloride
sodium chloride consists of sodium and chloride ions arranged in a giant ionic lattice. the ions are held together by strong electrostatic forces of attraction between positive and negative ions
why do ionic compounds have high melting points?
high electrostatic forces of attraction between ions so high amounts of energy needed to overcome these forces
when can ionic compounds conduct electricity
ionic compounds can conduct electricity when molten or dissolved because the electrostatic forces are overcome so ions are separated so they are free to move within the structure and carry charge
describe the structure of a metal
- atoms arranged in regular structure
- positive metal ions
- with a sea of delocalised electrons
- strong electrostatic forces of attraction between these hold the structure together
why can metals conduct electricity
have delocalised electrons which can move through structure and carry current
why are metals malleable and ductile
layers slide over each other and therefore can form new shapes
what is an alloy
alloys are mixtures of two or more elements, one of which is a metal
why are alloys stronger than pure metals
- they have different sized atoms
- so layers are distorted
- can’t cannot slide over eachother as easily
name two properties of ionic substances
- dissolve in water
- high melting point
what is the name given to alloys which can be bent, but return to their original shape when heated?
shape memory alloys
