Chemistry : C3 - Structure and Bonding

The particle theory explains:

• In solids, particles are packed together and vibrate in fixed positions.
• In liquids, the particles are close but can flow over eachother and move randomly.
• In gases, the particles are far apart and zoom around randomly.

In melting and boiling, energy is transferred from the surroundings to the substance. In freezing and condensing, energy is transferred from the substance to the surroundings.

The simple particle model is useful in explaining the properties of the three states of matter, but has limitations. Atoms, molecules and ions that make up all substances are not solid spheres with no forces between them.

A compound contains elements that are chemically combined.

Atoms react with eachother to achieve a full outer shell through ionic bonding or covalent bonding. 

Ionic bonding can be represented in a dot and cross diagram.

Ionic bonding is the strong electrostatic forces of attraction between oppositely charged ions that hold them together. Metals form positive ions and non-metals form negative ions. The ionic bonds between the charged particles result in a giant ionic lattice structure. 

In ionic bonding, atoms lose or gain electrons in order to have a full outer shell, forming charged ions. However, the overall charge of an ionic compound is 0, as the charges of the ions it is made of cancel eachother out.

Giant ionic lattices have high melting and boiling points, as it takes a lot of energy to break the strong electrostatic forces of attraction. 

They do not conduct electricity when solid, as the ions are fixed in the lattice so cannot move and carry current. However, when they are molten or in a solution, the ions can move so can carry current.

Most giant ionic structures dissolve in water, as the water molecules attract ions out of the structure.

Covalent bonding happens between non-metals. The share electrons to have a full outer shell. 

Each pair of shared electrons is one covalent bond.

Many substances with covalent bonds consist of small molecules, but some have giant covalent structures, like diamond.

Models are used to help understand bonding, but each has its limitations in representing reality.

Substances made up of simple covalent molecules have low melting and boiling points, because the intermolecular forces are quite weak, so it does not take a lot of energy to break them. 

Intermolecular forces increase with the size of the molecule. For example, polymers are made of small molecules that bond to form long chains of the molecule. They therefore have higher melting and boiling points.

Simple molecules have no overall charge, as they have no delocalised electrons or ions, so they cannot carry a current.

Some covalently-bonded substances have giant structures. Due to their strong covalent bonds, they have high melting and boiling points and are insoluble in water. Excluding graphite, they are also hard and do not conduct electricity.

For example, diamond, made up of carbon atoms, is the hardest known natural substance due to its very strong covalent bonds.

Graphite is also made up of carbon atoms. However, the atoms are only bonded to three other carbon atoms, leaving a sea of delocalised electrons, so it can conduct electricity. Graphite is structured in layers of carbon atoms, which are not covalently bonded but only have weak intermolecular forces. This means the layers slide over eachother easily, making them useful in pencil lead. 

Fullerenes can form large cage-like structures and tubes, based on hexagonal rings of carbon atoms.

Carbon nanotubes have high tensile strength, leading to their use in reinforcing composite materials. They also have high electrical and thermal conductivity, so they are used in the electronics industry. 

Fullerenes can also be used for drug delivery to specific sites within the body.

Graphene is a single layer of graphite, just one layer thick. It is an excellent electrical and thermal conductor, is the most reactive form of carbon, and is very strong for their mass. These properties will help create new developments in the electronics industry in the future, like flexible electronic displays.

The atoms in metals are arranged in layers in a regular pattern, meaning they form crystals. When growing silver crystals on copper wire in silver nitrate solution, the copper displaces the silver in the solution, so silver crystals form on the wore.

In metallic bonding, positively charged metal ions also form giant metallic lattices. The outer electrons from each metal atom can move easily throughout the giant structure, forming a sea of delocalised electrons. Strong electrostatic attraction between the electrons and metal ions bond the ions together.

Metals are made up of giant structures.

Metals are malleable because the layers of ions can slide over eachother. Alloys are usually mixtures of metals. However, most steel is made with iron and carbon, a non-metal. The carbon atoms are a different size to iron atoms, so it is more difficult for the layers to slide over eachother. Therefore, alloys are harder than pure metals.

Metals have high melting and boiling points, as the electrostatic forces of attraction extend in all directions due to the electrons being able to move freely between the metal ions. Therefore, it takes a lot of energy to break these bonds.

Metals are good conductors of thermal and electrical energy due to the delocalised electrons being able to move freely.