Atoms, elements and compounds

• All substances are made of atoms.

• An atom is the smallest part of an element that can exist on its own.

• Atoms have a nucleus containing protons and neutrons, with electrons moving in shells around it.

• Atoms are extremely small, typically about 1 × 10⁻¹⁰ m in diameter.

• An element is a pure substance made up of only one type of atom.

• Each element is represented by a chemical symbol, for example:

  • O – oxygen

  • Na – sodium

  • Cl – chlorine

• There are about 100 known elements, all arranged in the periodic table.

• Elements cannot be broken down into simpler substances by chemical means.

• Some elements exist naturally as molecules made up of two atoms joined together.

• These are called diatomic elements.

• Each molecule contains two atoms of the same element bonded together.

• A compound forms when two or more elements are chemically combined in fixed proportions.

• The compound’s properties are different from the elements it contains.

• Compounds are represented by chemical formulae that show the symbols and ratios of atoms present.

Examples:

• – water (2 hydrogen atoms, 1 oxygen atom)

• – carbon dioxide (1 carbon atom, 2 oxygen atoms)

• – sodium chloride (1 sodium atom, 1 chlorine atom)

• Compounds can only be separated into elements by chemical reactions, not by physical methods.

Compounds are typically named based on the elements they contain:

A molecule includes a few atoms covalently bonded together.

Molecules can contain one type of atom and be an element OR contain more than one type of atom and be a compound.