1.3 Structures

Question 1

What are the four main types of structures?

Answer 1

Giant metallic, giant ionic, giant covalent and molecular covalent

Question 2

Give a description of a giant metallic structure

Answer 2

Found in metals, where metallic bonds hold together positive metal ions via a ‘sea of delocalised electrons’

Question 3

Give a description of a giant ionic structure

Answer 3

Found in compounds of metals with non-metals. A lattice of positive metal, and negative non-metal ions, held together by electrostatic attractions. The ions are packed close together and form a regular three dimensional lattice

Question 4

Give a description of a giant covalent structure

Answer 4

Very large covalent molecules, where atoms are joined by covalent bonds throughout the three dimensional structure

Question 5

Describe the properties of a giant ionic structure

Answer 5

1. HIGH MELTING and BOILING POINTS- NaCl has Mpt.= 801 degrees celcius and Bpt.= 1413 degrees celcius. EXPLANATION: ionic bonds are very strong and they extend in three dimensions throughout the giant structure. Therefore a lot of heat energy is needed to separate the ions and break up the lattice, so a high temperature is required.
2. BRITTLE: they shatter when hit with a hammer or bent.
   EXPLANATION: bending causes the ions to move into positions where ions of the same charge are opposite each other. This leads to repulsion between layers when they split apart.
3. ELECTRICAL INSULATORS WHEN SOLID: ions held in fixed positions so cannot move to carry current.
4. ELECTRICAL CONDUCTORS AS MOLTEN OR AQUEOUS SOLUTIONS: melting the solid or dissolving it in water allows positive and negative ions to move and carry and electric current between electrodes. The ions are charge carriers.
5. Usually SOLUBLE IN WATER but INSOLUBLE IN ORGANIC SOLVENTS e.g. tetrachloromethane.

Question 5

Give a description of a molecular covalent structure

Answer 5

a.k.a ‘covalent molecular’ or ‘simple covalent’ or ‘simple molecular’. Molecular covalent molecules have strong covalent bonds between the atoms within a molecule and weak attractions (van der Waal’s forces) between separate molecules

Question 6

Describe the properties of molecular covalent structures

Answer 6

1. VERY LOW MELTING AND BOILING POINTS: very little heat energy needed to break the very weak bonds between seperate molecules. Hence many molecular covalent substances are gases at room temperature. Remember no covalent bonds are broken when a molecular covalent substance melts.
2. BRITTLE AND SOFT IF SOLID because of the weak forces between molecules.
3. ELECTRICAL INSULATORS: their molecules are not charged.
4. INSOLUBLE IN WATER or have very low solubility.
5. SOLUBLE IN ORGANIC SOLVENTS e.g. tetrachloromethane, CCl4.

Question 7

What is the definition of an allotrope?

Answer 7

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

Question 8

Describe carbon graphite

Answer 8

Graphite consists of layers of carbon atoms joined covalently in a hexagonal repeating pattern where each carbon atom forms strong covalent bonds with three surrounding carbon atoms. These flat sheets stack on top of each other and because they are only weakly held layer to layer (van der Waal’s forces), they can slide over each other easily.

Question 9

Describe the properties of carbon graphite

Answer 9

1. VERY HIGH MELTING AND BOILING POINTS: Mpt.= 3697 degrees celcius and Bpt.= 4827 degrees celcius.
   EXPLANATION: to melt graphite three strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. HARDNESS: VERY SOFT AND SLIPPERY (use: lubricants and pencils)
   EXPLANATION: Graphite has a layered structure- one layer held to another by weak van der Waal’s forces, allowing them to slide easily over each other.
3. Good electrical conductor: Carbon is unique among non-metals in this way. The only solids that conduct electricity are metals and carbon (graphite and graphene).
   EXPLANATION: The free electrons from each of the carbon atoms in graphite are able to move throughout the structure and carry an electric current.
4. INSOLUBLE IN WATER AND ORGANIC SOLVENTS.

5 .OPAQUE AND BLACK: graphite does not allow light to pass through it.

Question 10

Describe diamond

Answer 10

In diamond each carbon atom forms strong covalent bonds with four surrounding carbon atoms in a repeating tetrahedral structure (RIGID). This tetrahedral unit is repeated millions and millions of times to form a 3D lattice (giant covalent structure).

Question 11

Describe the properties of diamond

Answer 11

1. VERY HIGH MELTING AND BOILING POINTS: Mpt.= 3550 degrees celcius and Bpt.= 4827 degrees celcius.
   EXPLANATION: to melt diamond four strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. VERY HARD: Diamond is the hardest known material- it can only be scratched by another diamond. (Use: Diamond cutting tools).
   EXPLANATION: The tetrahedral structure makes diamond very rigid. The covalent bonding between the carbon atoms is very strong. Both these factors make it very difficult to penetrate into the structure and scratch it.
3. GOOD ELECTRICAL INSULATOR: (like other non-metals).
   EXPLANATION: Notice that there are no free electrons in the outer shell of the central carbon atom in the dot and cross diagram of the repeating tetrahedral unit. All the carbon atoms outer electrons are involved in covalent bonds. Diamond therefore cannot conduct electricity because it has no free electrons to carry a current.
4. INSOLUBLE IN WATER AND ORGANIC SOLVENTS.
5. TRANSPARENT and COLOURLESS: diamond will allow light to pass through it.

Question 12

Describe carbon graphene

Answer 12

Graphene consists of a layer of carbon atoms joined covalently in a hexagonal repeating pattern where each carbon atom forms strong covalent bond with three surrounding carbon atoms. The flat sheet is a single atom thick. The carbon atoms are arranged in hexagons.

Question 13

Describe the properties of carbon graphene

Answer 13

1. VERY HIGH MELTING AND BOILING POINTS: Mpt.= 4227 degrees celcius and Bpt.= 4827 degrees celcius.
   EXPLANATION: to melt graphene three strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. HARDNESS: very strong- Graphene is 100 times stronger than the strongest steel.
   EXPLANATION:In graphene each carbon atom forms three strong covalent bonds in the layer of atoms. Graphene is only one atom thick and has a very low density.
3. GOOD ELECTRICAL CONDUCTOR: the structure of the single layer which forms graphene is similar to graphite which has many such layers, The explanation for why both these allotropes of carbon conduct electricity is the same.
   EXPLANATION: The free electrons from each of the carbon atoms in graphene are able to move throughout the single layer and carry an electric current.
4. INSOLUBLE IN WATER AND ORGANIC SOLVENTS.
5. TRANSPARENT: graphene is the thinnest material possible being only one atom thick and allows light to pass through it. (Use: Solar cells and Batteries).

Question 14

Describe metallic bonding

Answer 14

The positively charged metal ions in the regular lattice are held together by their attraction to the delocalised electrons between them. These strong forces of attraction are called metallic bonds. This type of structure is called a giant metallic because the bonding extends in three dimensions throughout the whole metal.

Question 15

Describe the properties of metals

Answer 15

1. HIGH MELTING POINTS- most metals change from a solid to liquid at a high temperature. It takes a lot of heat energy to break the strong metallic bonds which extend in three dimensions throughout the giant structure and so a high temperature is necessary.
   Example: Tungsten in light bulb filaments.
2. GOOD ELECTRICAL CONDUCTORS: the free electrons can move through the lattice and carry an electric current. Example: copper wires in electrical circuits.
3. GOOD HEAT CONDUCTORS: again, it is the free electrons that take in the heat energy which makes them move faster and spread the heat through the lattice. Example: aluminium in cooking pans.
4. MALLEABLE: many metals can be bent or hammered into new shapes without breaking. Example: aluminium in cooking foil.
5. DUCTILE: many metals can be drawn into wires. Example: copper wire.
   -Malleability and ductility are caused by metal atoms being able to slide over each other without the metallic bonds breaking (the metal ions slide over each other but are still held together by the delocalised electrons).
6. METALLIC LUSTRE- shiny: freshly cut or polished metal surfaces reflect light well. Examples: silver and gold.
7. HIGH STRENGTH: most metals have high tensile strength (difficult to pull apart) and high compressive strength (difficult to crush). This is caused by the strong metallic bonds throughout the giant metallic lattice.

Question 16

What is the definition of an alloy?

Answer 16

An alloy is a mixture of two or more elements, at least one of which is a metal, and the resulting mixture has metallic properties e.g. steel is an alloy of iron.

Question 17

Why are alloys harder then pure metals?

Answer 17

The regular structure of the metal ions is disrupted by the presence of different metal ions. As a result, it is more difficult for the layers to slide over each other, and so the alloy is harder and less malleable than the pure metal e.g. steel.

Question 18

What is the equation for calculating the percentage of gold in an alloy?

Answer 18

Number of carats/24 x100

(pure gold is 24 carats)