1.5 Solid structures Flashcards
Describe ionic structures.
Giant lattices of positive and negative ions.
Structures are made of the same base unit repeated over and over again.
Ions are arranged in such a way that the electrostatic attraction between the oppositely charged ions is greater than the electrostatic repulsion between ions with the same charge.
State the coordination number of sodium chloride
6
State the coordination number of caesium chloride
8
Define coordination number
the coordination number of an ion gives the number of its nearest neighbours.
List and explain the physical properties of ionic solids.
High melting temperatures
- the giant lattices are held together by strong electrostatic forces between the oppositely charged ions. It takes a large amount of energy to overcome these forces of attraction.
Often soluble in water
- water molecules are polar. In solution, the oxygen ends of the water molecules are attracted to the positive ions and the hydrogen ends of the water molecules are attracted to the negative ions.
Hard but brittle
- when force applied, layers of ions slide over each other causing ions of the same charge to be next to each other. Ions repel each other and the crystal shatters
Poor electrical conductivity when solid, but good when molten/dissolved
- In solid state the ions are fixed in position by the strong ionic bonds. When molten or dissolved, the ions are free to move and will move to the electrode of opposite sign, so will carry current
What are allotropes?
Different forms of the same element in the same state.
List the two allotropes of carbon.
Diamond
Graphite
Describe the structure of diamond.
Each carbon atom is covalently bonded to four others. The atoms arrange themselves in a tetrahedral shape. The bonding forces are uniform throughout the structure.
List the physical properties of diamond.
- Very high melting temperature
- the energy needed to break the strong covalent bonds is very high. - Extremely high.
- due to strength of the covalent bonds and the geometrical rigidity of the structure. Many cutting tools are tipped with diamonds. - Insoluble in water.
- there are no ions to attract the polar water molecules - Poor conductor of electricity
- no free electrons or ions present.
Describe the structure of graphite.
Consists of layers of hexagonal rings.
Each carbon is joined to three others by strong covalent bonds. The fourth electron from each carbon atom is delocalised within the layer.
Hexagonal layers are held together by weak Van der Waals forces.
List the physical properties of graphite.
- Very high melting temperature.
- strong covalent bonds in hexagon layers - Soft
- weak forces between the layers are easily broken, so the layers can slide over each other. Used as a lubricant - Insoluble in water.
- no ions to attract the polar water molecules. - Good conductors of electricity.
- delocalised electrons are free to move along the layers so an electric current can flow. delocalised electrons are unable to move from one layer to the next so graphite can only conduct electricity parallel to its layers - Low density.
- relatively large amount of space between layers because the length of the covalent bonds in the layers is much shorter than the length of the Van der Waals forces between the layers. It is used to make strong, lightweight sports equipment
Describe structure of simple molecular solids.
Have covalent bonds within molecules held together by weak intermolecular forces.
List the physical properties of simple molecular solids.
- Low melting and boiling temp.
- although covalent bonds within the molecule are strong, intermolecular forces holding the molecules together are weal and don’t require much energy to break. - Soft.
- weak intermolecular forces between the molecules are easily broken - Insoluble in water (normally)
- no ions to attract polar water molecules. however, compounds that can form hydrogen bonds with water will be soluble - Poor conductors of electricity
- no delocalised electrons or ions.
Describe the structure of iodine.
Atoms are covalently bonded in pairs to form diatomic I2 molecules. These molecules are held together by weak Van der Waals forces and are arranged in a regular pattern.
Describe the structure of ice.
Molecules of water are arranged in rings of six held together by hydrogen bonds.
Water molecules are further apart than they are in the liquid state. The structure creates large areas of open space inside the rings and as a result at 0 degrees Celsius. Ice is less dense than liquid water
Describe the structure of metals.
Metals consist of a regular arrangement of metal cations surrounded by a ‘sea’ of delocalised electrons.
There are electrostatic forces of attraction between the nucleus of the cations and the delocalised electrons. (metallic bonding)
List the physical properties of metals
- High melting temperatures
- large energy needed to overcome the strong forces of attraction between the nuclei of the metal cations and the delocalised electrons. The melting temperature is affected by the number of delocalised electrons per cation and the size of the cation - Hard.
- metallic bond is very strong - Insoluble in water
- no ions to attract the polar water molecules. - Good conductors of electricity in solid and molten
- delocalised electrons can carry a current (when potential difference applied, electrons will be attracted to and move towards the positive terminal of the cell).
- They are also good thermal conductors because the delocalised electrons can pass kinetic energy to each other. - Malleable and ductile
- force applied to a metal the layers of cations can slide over each other. however, the delocalised electrons move with the cations and prevent forces of repulsion forming between layers.
