Elements, compounds and mixtures Revision Notes

An element is a pure substance composed of only one type of atom. Each element is defined by its atomic number, which is the number of protons in its nucleus. Elements cannot be broken down into simpler substances by chemical means. Examples include oxygen (O), hydrogen (H), and aluminium (Al).

Elements exist as single atoms or molecules made of the same atoms, such as diatomic oxygen (O₂) or nitrogen (N₂). Their chemical properties are determined by their atomic structure.

• Elements contain only one type of atom.
• Cannot be chemically broken down into simpler substances.
• Examples: O₂, H₂, Al.

Compounds are substances formed when two or more different elements chemically combine in fixed proportions. The atoms are joined by chemical bonds, such as covalent or ionic bonds, resulting in a substance with properties different from those of the constituent elements.

For example, water (H₂O) is a compound formed from hydrogen and oxygen atoms chemically bonded in a 2:1 ratio. Sodium chloride (NaCl) is an ionic compound formed from sodium and chlorine ions.

The chemical formula represents the fixed ratio of atoms in a compound. Compounds can only be separated into their elements by chemical reactions, not by physical means.

• Compounds contain atoms of different elements chemically bonded.
• Fixed composition with a definite formula (e.g., H₂O, NaCl).
• Properties differ from constituent elements.
• Separated only by chemical methods.

A mixture consists of two or more substances physically combined without chemical bonding. The components retain their individual properties and can be present in any proportion.

Examples include air (a mixture of gases like nitrogen, oxygen, and others) and salt dissolved in water. Mixtures can be homogeneous (uniform composition) or heterogeneous (non-uniform composition).

Because no chemical bonds form, mixtures can be separated by physical methods such as filtration, distillation, or chromatography.

• Mixtures contain two or more substances physically combined.
• Components retain their own properties.
• No fixed composition; proportions can vary.
• Separated by physical methods.

In compounds, atoms are held together by chemical bonds:

Ionic bonds form by transfer of electrons between metals and non-metals, creating oppositely charged ions attracted electrostatically.

Covalent bonds form by sharing electron pairs between non-metal atoms, creating molecules with specific shapes and properties. These bonds give compounds distinct melting points, boiling points, and chemical behaviours unlike their elements.

• Ionic bonds: electron transfer, electrostatic attraction.
• Covalent bonds: electron sharing between atoms.
• Bonding determines compound properties.

Mixtures can be separated by physical methods depending on the components' properties:

Filtration separates solids from liquids based on particle size.

Distillation separates liquids by boiling point differences, useful for homogeneous mixtures like saltwater or alcohol-water mixtures. Crystallisation can purify solid compounds from solutions by evaporating solvent carefully to form crystals, avoiding overheating to prevent decomposition.

• Mixtures separated by physical methods: filtration, distillation, chromatography.
• Compounds require chemical reactions for separation.
• Crystallisation used to obtain pure solid compounds from solutions.