Chem - topic 2

What are ions?

• Ion - a charged particle

• Formed when atoms gain or lose electrons to become more stable

• Full outer shell = more stable

• Losing or gaining electrons requires energy

What is ionic bonding?

• Non-metal + metal

• Atoms have opposite charges

• So are attracted to eachother by electrostatic forces

• Ionic bond - really strong

What happens to the electrons in ionic bonding?

• Metal atoms lose electrons to become positively charged ions

• Non-metal atoms gain electrons to become negatively charged ions

• These oppositely charged ions are strongly attracted to one another by electrostatic forces

What is covalent bonding?

• Non-metal + non-metal

• Share a pair of electrons between the atoms

What happens to the electrons in covalent bonding?

• Non-metal atoms share pair(s) of electrons to make covalent bonds - only in outer shell

• Held together by electrostatic forces of attraction between the positively charged nuclei of the bonded atoms and the shared pair of electrons - covalent bonds are strong

What is metallic bonding?

The electrostatic attraction between positive metal ions and delocalised electrons

What is the structure ionic compounds?

• Giant ionic lattice

• Held together by strong electrostatic forces of attraction in all directions between oppositely charged ions

What are the properties of ionic compounds?

• High melting and boiling points - a large amount of energy is required to break the strong ionic bonds

• When solid, ions are held in place (not free to move) so cannot conduct electricity

• When molten or aqueous - can conduct electricity as ions are free to move and so charge can flow

Examples of simple molecular substances

• Hydrogen (H₂) - single covalent bond

• Chlorine (Cl₂) - single covalent bond

• Oxygen (O₂) - double covalent bond

• Nitrogen (N₂) - triple bond

• Methane (CH₄) - four covalent bonds

• Hydrogen Chloride (HCl) - single covalent bond

What are the properties of simple molecular substances? (small covalent molecules)

• Held together by strong covalent bonds but weak intermolecular forces

• Usually liquids or gases at room temperature - intermolecular forces are weak, so energy is not needed to break the bonds

• Very low melting and boiling points - little energy required to break weak intermolecular forces (increase with size of the molecules)

• Don't conduct electricity - no free electrons or ions and so no overall electrical charge

What is the effect of increased molecule size?

As molecule size increases:

• Strength of intermolecular forces increases

• More energy required to break them

• Melting/boiling point increases

What is a polymer?

• Long chain of monomers (repeating units)

• Joined together by strong covalent bonds

• Strong intermolecular forces

What are the properties of polymers?

• Solid at room temperature - have strong intermolecular forces of attraction

• High melting and boiling points - require a lot of energy to break the strong intermolecular forces

What are giant covalent structures?

• All atoms are bonded to each other by strong covalent bonds

• In a giant lattice structure

What are the properties of giant covalent structures?

• High melting and boiling point - hard to overcome strong covalent bonds

• Non conductors of electricity, except graphite

What is the structure of diamond?

• Giant covalent structure

• Each carbon atom forms covalent bonds to four other carbon atoms

• Arranged in giant lattice

What are the properties of diamond?

• Solid at room temperature because they have high melting and boiling points - unlike simple molecular substances (small covalent molecules), it is the strong covalent bonds that must be broken when they are melted / boiled

• Extremely hard substance - contains millions of carbon atoms joined by covalent bonds

• Cannot conduct electricity - all of the outer electrons are in covalent bonds so diamond has no free electrons to carry electrical charge

What is the structure of silicon dioxide?

• Giant covalent molecule - huge number of strong covalent bonds

• Each silicon atom is covalently bonded to 4 oxygen atoms

• Each oxygen atom is covalently bonded with 2 silicon atoms

What are the properties of silicon dioxide?

• Very hard

• High melting/boiling point

• Does not conduct electricity

What is the structure of graphite?

• Each carbon atoms forms covalent bonds to three other carbon atoms

• Arranged in layers of hexagonal rings (no covalent bonds between layers, only weak intermolecular forces)

• Each atom has one delocalised electron

What are the properties of graphite?

• Soft and slippery - carbon atoms form heaxagonal rings. The hexagonal rings of carbon atoms are arranged into layers. There are no covalent bonds between these layers so the layers can slide over eachother

• High melting and boiling point - requires lots of energy to break the strong covalent bonds

• Conducts electricity and heat - carbon atoms have four electrons in their outer shell. In graphite, the carbon atom forms covalent bonds to three other carbon atoms. Each carbon atom has one electron in its outer shell that is not in a covalent bond. This means graphite has delocalised electrons which are free to move and conduct electricity and heat

What are the uses of graphite?

• Lubricant - reducing friction between moving parts

• Pencils

• Electrolysis

What is the structure of graphene?

• One layer of graphite (one atom thick)

• Contains delocalised electrons

What are the properties of graphene?

• Strong

• Light (can be added to composite materials to improve strength without adding much weight)

• Can conduct electricity - delocalised electrons which are free to move and carry electrical charge

• High melting and boiling point

What are the uses of graphene?

Electronics and composites

What is the structure of fullerene?

• Fullerenes are molecules of carbon atoms with hollow shapes

• Carbon atoms arranged in hexagonal rings (can be with five or seven carbon atoms)

What are the uses of fullernes?

Delivering drugs to the body

Industrial catalysts (due to large surface area)

Lubricants in machines

What was the first fullerene discovered and what is its structure?

Buckministerfullerene

• Hexagonal rings of carbon atoms

• 60 carbon atoms (C60)

• Spherical shape

What is the structure of nanotubes?

Hollow carbon cylinders formed from fullerenes

What are the properties of nanotubes?

• High ratio between length and diameter of nanotube

• Can conduct both electricity and thermal energy - have delocalised electrons

• High tensile strength (don't break when stretched)

What are the uses of nanotubes? (nanotechnology)

• Electronics

• Strengthen materials without adding much weight

Advantage of 2D ball and stick model

Shows which atoms are bonded to each other

Disadvantage of 2D ball and stick model

Does not show true shape of the molecule

Advantage of 3D ball and stick model

Shows the shape of a molecule

Disadvantage of 3D ball and stick model

Does not show how it is bonded via electrons

Advantage of dot and cross diagram

Shows electrons from each atom

Disadvantage of dot and cross diagram

It is a 2D representation - don’t show the real shape of molecules

What is the structure of metals?

• Giant structure of positive ions arranged in regular layers

• Sea of delocalised electrons

• Strong forces of electrostatic attraction between positive metal ions and the sea of delocalised negative electrons

What are the properties of metals?

• High melting/boiling points - a lot of energy is required to overcome the strong electrostatic forces

• Good conductors of electricity and heat - delocalised electrons free to carry charge throughout the metal

• Soft -layers of atoms able to slide over each other

• Malleable

• Ductile

• Shiny

What is an alloy?

A mixture of metals

• In an alloy the different sizes of atoms distort the layers

• This makes it more difficult for the layers to slide over eachother

• Alloys are harder than pure metal

What is the particle theory?

• Energy needed for a change of state depends on strength of forces between particles

• Stronger forces between particles mean higher melting/boiling points

What are the limitations of the particle model?

• Model shows no forces between particles

• Assumes that all particles are solid spheres

Particles in solids

• Strong forces of attractions between particles, holding them close together in fixed positions to form regular lattice arrangement

• Particles vibrate about their positions - the hotter the solid becomes, the more they vibrate (slightly expand when heated)

• Particles don't move from their positions - keep a defined shape and volume

• Cannot flow and cannot be compressed

Particles in liquids

• Weak force of attraction between particles

• Randomly arranged and free to move, but tend to stick closely together

• Constantly moving in random directions - the hotter the liquid gets, the faster they move (expand when heated)

• Have definite volume but not definite shape - will flow

• Cannot be compressed

Particles in gases

• Very weak forces of attraction between particles

• Free to move and far apart

• Move constantly with random motion - the hotter the gas gets, the faster they move (expand or pressure increases when heated)

• Don't keep definite shape or volume - always fill space

• Can be compressed

Particles during melting (solid to liquid)

• Solid is heated → particles gain more KE and vibrate more

• At the melting point, the particles have enough energy to overcome the intermolecular forces holding them in fixed positions

• Temperature stays constant while the solid melts → solid becomes a liquid

Particles during boiling/evaporating (liquid to gas)

• Liquid is heated → particles gain KE and move faster

• At the boiling point, particles have enough energy to overcome the intermolecular forces between them

• Temperature stays constant while the liquid boils → liquid becomes a gas

Particles during condensation (gas to liquid)

• Gas cools → particles have less KE and move more slowly

• Particles no longer have enough energy to overcome intermolecular forces, so they start to stick together

• Temperature stays constant while the gas forms a liquid → gas becomes a liquid

Particles during freezing (liquid to solid)

• Liquid cools → particles have less KE so move around less

• Particles no longer have enough energy to overcome intermolecular forces, so they start to be held in place

• Temperature stays constant while the solid forms → liquid becomes a solid