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Structure and properties of materials

A chemical element is a substance made up of only one type of atom. This includes isotopes of that particular atom, which only differ in the number of neutrons present.

Elements are the simplest pure substances in nature and cannot be broken down further into simpler pure substances.

An element can occur in nature as a single atom or as a molecule containing more than one atom.

Neon gas exists as single atoms ($$\ce{Ne}$$) while hydrogen gas exists as a molecule consisting of two hydrogen atoms ($$\ce{H2}$$).

As of 2011, there are 118 known chemical elements. Many of these elements do not exist naturally but have been created in a laboratory.

All matter is made of these elements or various combinations of these elements. The following table shows images of some elements.

Neon gas: $$\ce{Ne}$$
Neon gas: $$\ce{Ne}$$
Bromine liquid: $$\ce{Br}$$
Bromine liquid: $$\ce{Br}$$
Iron: $$\ce{Fe}$$
Iron: $$\ce{Fe}$$
Gold: $$\ce{Au}$$
Gold: $$\ce{Au}$$

A compound is a substance made up of two or more types of atoms. Atoms can bond together to form a large number of different compounds.

Unlike elements, compounds can be broken down into simpler substances.

Compounds often exist as molecules or as ionic lattice structures.

Water (formula: $$\ce{H2O}$$) is a molecule made of one oxygen ($$\ce{O}$$) atom and two hydrogen ($$\ce{H}$$) atoms.

The bonds between atoms in a compound can be broken if energy is added (for example: in the form of heat or electricity).

An input of energy (usually in the form of electricity) can separate atoms in a water molecule, converting it into oxygen and hydrogen gas.

$$$\ce{2H2O -> O2 + 2H2}$$$

Molecular diagram of water
Molecular diagram of water

The different types of atoms in a compound are always present in a fixed ratio.

Each water molecule always consists of one oxygen atom and two hydrogen atoms. Any volume of water will contain hydrogen and oxygen atoms in a 2:1 ratio.

A mixture is made of two or more chemical substances that are combined but not chemically bonded to each another.

Separating substances in a mixture does not require breaking chemical bonds.

Salt water is just a mixture of salt ($$\ce{NaCl}$$) and water ($$\ce{H2O}$$). The salt can be retrieved by boiling off the water.

The substances in a mixture can be added together in any proportion. This differs from compounds, where the components are always present in a fixed proportion.

Mixing water and salt in any proportion will still give salt water.

Combining two hydrogen atoms to two oxygen atoms will not give water ($$\ce{H2O}$$). Another compound will be formed instead ($$\ce{H2O2}$$ in this case).

Mixture Compound
Can contain different elements or compounds Consists of atoms of two or more elements, chemically combined together
Components can be present in any proportion Components are present in a fixed ratio
Components are separated by physical methods (e.g. filtration, distillation) Components are separated by chemical methods (e.g. electrolysis, decomposition)