2.1 Bonding, Structure and Properties

What is a compound?

A substance in which 2 or more elements are chemically combined.

What are the three types of chemical bonds?

1. Ionic - bonding in ionic compounds
2. Metallic - bonding in metals
3. Covalent - bonding in giant covalent structures and simple covalent molecules

What are the properties of ionic compounds?

1. Strong electrostatic attraction between oppositely charged ions means ionic compounds have high melting and boiling points
2. In a solid state, ionic compounds do not conduct electricity as the ions are fixed in place
3. When molten/ dissolved, ions can move around so then ionic compounds conduct electricity
4. They are brittle

What are the properties of metallic compounds?

1. The layers of ions in metals are able to slide over each other, so metals are malleable and ductile
2. The delocalised electrons can move through the metal and carry charge, so metals conduct electricity and heat
3. Metallic bonds are very strong and require large amounts of energy to be broken, giving most metals high melting and boiling points

Why do metals have high melting and boiling points?

There are strong electrostatic forces present between the positive ions and the sea of negative electrons therefore, more heat energy is required to break the metallic bonds.

What affects the strength of metallic bonds?

The number of electrons in which the atom 'delocalises' into the sea of electrons.

The more free electrons, the greater the charge on the ion and therefore, the stronger the metallic bond.

For instance, a metal with a higher charge will have a higher melting and boiling point because the electrostatic forces are stronger so takes more heat energy to break.

Why does melting point increase as you go across the period?

There are more electrons in the outer shell so more electrons become delocalised and the electrostatic bond is stronger/harder to melt

What charge do metal ions have?

Positive.

What charge do non-metals have?

Negative.

What is a simple molecular structure?

Simple molecules contain only a few atoms held together by covalent bonds.

An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.

Athough the covalent bonds holding the atoms together in a simple molecule are strong, the intermolecular forces between simple molecules are weak.

What are the properties of simple molecular covalent substances?

1. Simple molecules have weak inter-molecular forces between the molecules, so they have low melting and boiling points.
2. Do not conduct electricity because simple molecules do not have an overall charge. They also have no free electrons.
3. They are usually gases or liquids.

What is a giant covalent structure?

Giant covalent structures contain many atoms, each joined to adjacent atoms by covalent bonds.

The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved

What are the properties of giant covalent substances?

Substances with a giant covalent structure are solids with very high melting points.

Explain the bonding and structure in metallic compounds.

1. The electrons in the outer shell of metal atoms are delocalised and so they are free to move through the whole structure - 'sea' of delocalised electrons.
2. Metallic bonds form due to the electrostatic attraction between the positively charged metal ions and the negatively charged delocalised electrons.

How does ionic bonding take place?

1. Electrons in the outer shell of the metal are transferred
2. Metal atoms lose electrons to become positively charged ions
3. Non-metal atoms gain electrons to become negatively charged ions
4. The electrostatic attraction from these oppositely charged ions forms a giant ionic lattice
5. The electron transfer can be represented by a dot and cross diagram

How are covalent bonds formed?

1. When atoms share pairs of electrons, they form covalent bonds
2. The bonds between these atoms are strong
3. Dot and cross diagrams can be drawn to represent the sharing of electrons

Why do simple molecular substances have low melting and boiling points?

1. Weak inter-molecular forces between molecules - these are broken in melting or boiling, not the strong covalent bonds
2. The inter-molecular forces increase with the size of the molecules, larger molecules have higher melting and boiling points

What is meant by a lattice arrangement?

Particles are close together and in a regular arrangement.

Give examples of giant covalent structures.

Diamond and graphite.

What is an allotrope?

Different structural forms of the same element in the same physical state.

State which element graphite and diamond are an allotrope of.

Carbon.

How many carbons is each carbon bonded to in graphite?

3 carbons.
This leaves each atom with a spare electron, because carbon has 4 electrons in its outer shell. These together form a 'sea' of delocalised electrons.

How many carbons is each carbon bonded to in diamond?

4 carbons.
The outer shell is full and so there is no 'sea' of delocalised electrons.

Explain how the allotropes of carbon are formed and why their properties are different.

In diamond and graphite, the carbon atoms are joined by strong covalent bonds, but in such different arrangements that the properties of the allotropes are very different.

What are the properties of diamond with respect to bonding and structure?

1. It is very hard, has a very high melting point and does not conduct electricity
2. Each carbon is joined to 4 other carbons covalently - covalent bonds
- needs a lot of energy to be broken \= very high melting point.
3. This is the maximum number of bonds each carbon atom can make.

What are the properties of graphite with respect to bonding and structure?

1. Each carbon is covalently bonded to 3 other carbons, forming layers of hexagonal rings which have no covalent bonds between the layers
2. The layers can slide over each other due to the absence of covalent bonds between the layers, but there are weak inter-molecular forces between layers - so graphite is soft and slippery
3. One electron from each carbon atom is delocalised so it can conduct electricity

Explain the similarities between the properties of diamond and graphite in terms of bonding.

Both have high melting and boiling points because the covalent bonds take a lot if energy to break.

Explain the differences between the properties of diamond and graphite in terms of bonding.

1. Graphite conducts electricity because each atom is left with a spare electron, creating a sea of delocalised electrons.

Diamond does not conduct electricity because each carbon atom is bonded by 4 strong covalent bonds, leaving no free delocalised electrons.

2. Graphite is soft because there are weak intermolecular forces holding the layers together.

Diamond is extremely hard because the covalent bonds formed take more energy to break

What are the properties of graphene with respect to bonding and structure?

1. Its structure resembles a single layer of graphite
2. Graphene has a very high melting point due to
the very strong covalent bonds between the carbon atoms that require large amounts of energy to be broken
3. Conducts electricity due to the delocalised electrons that are free to move through its structure

What are fullerenes?

- Molecules of carbon atoms with hollow shapes
- They are based on hexagonal rings of carbon atoms, but they may also contain rings with five or seven carbon atoms

What are carbon nanotubes?

- Cylindrical fullerenes with very high length to diameter ratios
- They have very high tensile strength and conduct electricity due to the delocalised electrons present

Give uses of fullerenes and nanotubes.

Fullerenes can be used as lubricants, to deliver drugs in the body and catalysts.

Nanotubes can be used as for reinforcing materials, for example tennis rackets.

Do individual atoms have the same properties as bulk materials?

No - for example, carbon atoms on their own do not have any of the properties exhibited by any of the different structures (diamond, graphite, graphene, nanotubes or fullerenes)

State what is meant by nano science.

The study of particles in the range 1-100nm.

What unit is used to measure nanoparticles ?

Nanometres (nm).

How many nanometres are in a metre?

1 x 10^-9.

What are the properties of nanoparticles?

- 1-100 nanometers across
- Contain a few hundred atoms
- They have different properties from the 'bulk' properties which they form because of their high surface area to volume ratio

What are some uses of nanoparticles?

1. Nano-silver - kills bacteria: used in wound dressings, deodorants, or to line socks and fridges to kill bacteria causing bad smell
2. Nano-titanium dioxide - used in sunblock creams to block harmful UV rays without appearing white on the skin as the particles do not reflect visible light, also used in self-cleaning windows as they help break down dirt

What are the main properties of nano-silver?

1. Anti-bacterial
2. Anti-fungal
3. Anti-viral

Why is nano silver used in wound dressings rather than bulk silver?

Nano silver has a larger surface area so it can reach and kill more bacteria than bulk silver.

State uses of nano-silver.

1. Refrigerators
2. Deodorants
3. Sterilising sprays

State the chemical symbol for titanium dioxide

TiO2

Why is nano-sized titanium dioxide used in sun cream?

It absorbs and reflects UV light.

State uses of titanium oxide.

1. Self cleaning glass
2. Sun cream

Explain how titanium dioxide is used in self cleaning glass.

1. Dirt sticks to the titanium dioxide coating.
2. UV light breaks down the dirt (titanium dioxide catalyses the breakdown of dirt in the presence of UV light).
3. Rain washes the dirt away-titanium dioxide particles are hydrophobic so dirt is washed away instead of drying on the glass.

What is meant by hydrophobic?

Repels water.

List some future developments of nanoparticles.

1. New technology
2. Computers
3. Stronger and lighter building materials

What are the risks of nanoparticles?

- So small that they could potentially enter the bloodstream
- Relatively new material so long term effects are not known
- Can enter and potentially damage the environment

What is a smart material?

A material whose physical properties can change in response to an external stimulus e.g. temperature.

What are the properties and uses of thermochromic materials?

- Change colour when they reach a certain temperature
- Used in mugs and spoons which change colour when their contents are hot

What are the properties and uses of photochromic pigments?

- These pigments change colour when exposed to light
- Used in sunglasses which darken in bright sun

What are the properties and uses of polymer gels?

- Hydrogels absorb up to 1,000 times their volume in water
- Certain stimuli (changes in pH and temperature) can cause the water to be released
- Used in nappies, fake snow and hair gel

What are the properties and uses of shape memory alloys/polymers?

1. These materials can be bent and deformed but will return to their original shape when heated
2. Shape memory polymers are used in medical stitches and sports equipment such as gum shields
3. Shape memory alloys are used in car bodies and plates for bone fractures.

What is a polymer?

A long chain of monomers joined together.

What is a monomer?

Building blocks of polymers.