Beyond Science is back to explore another facet of AQA Chemistry Paper 1, this time focusing on Bonding, Structure, and the Properties of Matter.
Bonding, Structure, and the Properties of Matter
States of Matter
The three states of matter are solid, liquid and gas.
The particles in a solid are arranged in a regular pattern. The particles vibrate in a fixed position and are tightly packed together. Particles in a solid have a low amount of kinetic energy.
Solids have a fixed shape and are unable to flow like liquids and gases. The particles cannot be compressed because the particles are very close together.
The particles in a liquid are randomly arranged. The particles are able to move around each other. Particles in a liquid have a greater amount of kinetic energy than particles in a solid.
Liquids are able to flow and can take the shape of the container that they are placed in. As with a solid, liquids cannot be compressed because the particles are close together.
The particles in a gas are randomly arranged. The particles are able to move around very quickly in all directions. Of the three states of matter, gas particles have the highest amount of kinetic energy.
Gases, like liquids, are able to flow and can fill the container that they are placed in. The particles in a gas are far apart from one another which allows the particles to move in any direction.
Gases can be compressed; when squashed, the particles have empty space to move into.
In chemical equations, the three states of matter are represented as symbols:
- solid (s)
- liquid (l)
- gas (g)
- aqueous (aq)
Aqueous solutions are those that are formed when a substance is dissolved in water.
For a substance to change from one state to another, energy must be transferred.
On heating a solid, particles gain energy. This results in some of the particles having enough energy to overcome the attractive forces between particles, causing the solid to melt.
To evaporate or boil a liquid, more energy is needed to overcome the remaining attractive forces between the particles.
Note the difference between boiling and evaporation. When a liquid evaporates, particles leave the surface of the liquid only. When a liquid boils, bubbles of gas form throughout the liquid before rising to the surface and escaping.
The amount of energy needed for a substance to change state is dependent upon the strength of the attractive forces between particles. The stronger the forces of attraction, the more energy it takes to overcome them. Substances that have strong attractive forces between particles generally have higher melting and boiling points.
Identifying the Physical State of a Substance
If the given temperature of a substance is lower than the melting point, the physical state of the substance will be solid.
If the given temperature of the substance is between the melting point and boiling point, the substance will be a liquid.
If the given temperature of the substance is higher than the boiling point, the substance will be a gas.
Formation of Ions
Ions are charged particles. They can be either positively or negatively charged, for example:
When an element loses or gains electrons, it becomes an ion.
Metal atoms lose electrons to become positively charged ions.
Non-metal atoms gain electrons to become negatively charged ions.
Group 1 and 2 elements lose electrons and group 6 and 7 elements gain electrons.
|1||+1||Li → Li+ + e–|
|2||+2||Ca → Ca2+ + 2e–|
|6||-2||O + 2e– → O2-|
|7||-1||Br + e– → Br –|
Metals and Non-metals
Metals are found on the left-hand side of the periodic table.
Metals are generally strong, shiny, malleable and good conductors of heat and electricity.
On the other hand, non-metals are generally brittle, dull, not always solids at room temperature and poor conductors of heat and electricity.
Non-metals are found on the right-hand side of the periodic table.
Metallic bonding occurs between metals only. Positive metal ions are surrounded by a sea of delocalised electrons. The ions are tightly packed and arranged in rows.
There are strong electrostatic forces of attraction between the positive metal ions and negatively charged electrons.
Pure metals are too soft for many uses and are often mixed with other metals to make alloys. The mixture of the metals introduces different sized metal atoms. This distorts the layers and prevents them from sliding over one another. This makes it harder for alloys to be bent and shaped like pure metals.
Ionic bonding occurs between a metal and a non-metal. Metal atoms lose electrons to become positively charged ions. Non-metal atoms gain electrons to become negatively charged ions. Oppositely charged ions are attracted by electrostatic forces, forming an ionic bond.
Ionic compounds form structures called giant lattices. They have strong electrostatic forces of attraction that act in all directions and act between the oppositely charged ions that make up the ionic lattice. E.g. sodium chloride
Properties of Ionic Compounds
- high melting point and high boiling point – because lots of energy is needed to overcome the strong electrostatic forces of attraction between ions
- cannot conduct electricity in a solid state – as the ions are not free to move
- can conduct electricity when molten or in solution – as the ions are free to move and can carry the electrical current
Covalent bonding is the sharing of a pair of electrons between atoms to gain a full outer shell. This occurs between non-metals only, e.g. water and carbon dioxide.
Small molecules containing just a few atoms covalently bonded together are said to have a simple covalent structure.
Substances with a simple covalent structure have low melting and boiling points – this is because there are weak intermolecular forces between the molecules. Only a small amount of energy is needed to overcome these forces. They do not conduct electricity as they do not have any free or delocalised electrons.
Dot and cross diagrams are useful to show the bonding in simple molecules. The outer electron shell of each atom is represented as a circle, the circles from each atom overlap to show where there is a covalent bond, and the electrons from each atom are either drawn as dots or crosses.
The structure of small molecules can be represented in a number of ways.
- 3D model, showing the shape of the molecule
- displayed formula, showing the bonds in the molecule
- dot and cross diagrams, showing the electrons in the molecule
There are pros and cons for each way of representing compounds, depending on what you are trying to show.
Giant Covalent Structures
Each carbon atom in diamond is covalently bonded to four other carbon atoms, making diamond very strong. Diamond has a high melting and boiling point. Large amounts of energy are needed to break the strong covalent bonds between each carbon atom. Diamond does not conduct electricity because it has no free electrons.
This structure contains silicon and oxygen atoms and is similar to that of diamond, in that its atoms are held together by strong covalent bonds. Large amounts of energy are needed to break the strong covalent bonds therefore silicon dioxide, like diamond, has a high melting and boiling point.
Graphite is made up of layers of carbon atoms arranged in hexagons. Each carbon atom is bonded to three other carbon atoms and has one free electron that is able to move within the layers (is delocalised).
The layers are held together by weak intermolecular forces. The layers of carbon can slide over each other easily as there are no strong covalent bonds between the layers.
Graphite has a high melting point because a lot of energy is needed to break the covalent bonds between the carbon atoms. Graphite can conduct electricity because each carbon atom has a free electron.
Graphene is one layer of graphite. It is very strong because of the covalent bonds between the carbon atoms. As with graphite, each carbon atom in graphene is bonded to three others with one free delocalised electron.
Graphene is able to conduct electricity. Graphene, when added to other materials, can make them even stronger. Graphene is useful in electricals and composites.
Polymers are long chain molecules that are made up of many smaller units called monomers. Atoms in a polymer chain are held together by strong covalent bonds.
Between polymer molecules, there are intermolecular forces. Intermolecular forces attract polymer chains towards each other. Longer polymer chains have stronger forces of attraction than shorter ones, therefore, making stronger materials.
Fullerenes and Nanotubes
Molecules of carbon that are shaped like hollow tubes or balls, arranged in pentagons of five or hexagons of six carbon atoms. They can be used to deliver drugs into the body.
Buckminsterfullerene has the formula C₆₀
Carbon Nanotubes are tiny carbon cylinders that are very long compared to their width. Nanotubes can conduct electricity as well as strengthening materials without adding much weight. The properties of carbon nanotubes make them useful in electronics and nanotechnology.
Bonding, Structure, and the Properties of Matter Revision from Beyond
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