Element, Compound or Mixture

An element is a substance made of only ONE type of atom. There are about 118 known elements, and every one of them has its own box on the periodic table. An element cannot be broken down into anything simpler by chemical means.

Examples: copper (Cu), oxygen (O₂), sulfur (S₈), helium (He), iron (Fe), carbon (C).

Important idea — elements can be made of molecules. An element does NOT have to be single atoms. Many elements exist as molecules made of the same type of atom joined together:

• Oxygen exists as O₂ — two oxygen atoms bonded together.
• Sulfur exists as S₈ — eight sulfur atoms in a ring.
• Hydrogen, nitrogen and chlorine exist as H₂, N₂ and Cl₂.

Because every atom in O₂ or S₈ is the same kind of atom, it is still an element — not a compound. The test is the number of types of atom, not the number of atoms.

A compound is a substance made of two or more different elements chemically bonded together in a fixed ratio. The fixed ratio is set by the chemical formula:

• Water (H₂O) — always 2 hydrogen atoms to 1 oxygen atom.
• Carbon dioxide (CO₂) — always 1 carbon to 2 oxygen.
• Sodium chloride (NaCl) — always 1 sodium ion to 1 chloride ion.

Two facts examiners love:

1. A compound's properties are DIFFERENT from the elements it contains. Once elements bond, the new substance behaves nothing like the originals:

• Sodium is a soft, silvery, violently reactive metal.
• Chlorine is a yellow-green poisonous gas.
• Sodium chloride (the compound) is a safe, white, edible solid — completely different from either element.

2. A compound can only be separated by a CHEMICAL reaction. The bonds must be broken chemically (e.g. electrolysis of water gives back H₂ and O₂; heating calcium carbonate gives CaO and CO₂). You cannot separate a compound by filtering, evaporating or using a magnet — those are physical methods that only work on mixtures.

A mixture is two or more substances that are physically together but NOT chemically bonded. Key features of a mixture:

• The components are present in any proportion (no fixed ratio).
• Each substance keeps its own properties.
• It can be separated by physical methods (filtering, distillation, a magnet, dissolving) — no chemical reaction needed.

Common examples:

• Air = a mixture of N₂ (about 78%), O₂ (about 21%), Ar (about 0.9%) and CO₂ (about 0.04%).
• Sea water = water + dissolved salts (mainly NaCl).
• Brass = a mixture (alloy) of copper and zinc atoms.

The classic iron + sulfur demonstration. If you mix iron filings and sulfur powder you get a mixture: the iron is still magnetic (you can pull it out with a magnet) and the sulfur is still yellow — each keeps its own properties, and they are in no fixed ratio. But if you heat them strongly, they react chemically to form iron(II) sulfide (FeS) — now a compound with completely new properties (it is not magnetic and cannot be separated with a magnet).

Exam questions often show boxes of circles and ask you to classify each as an element, compound or mixture. Use two checks: how many colours (types of atom), and whether the atoms are joined (bonded).

How to decide:

What you see in the box	Classification
All circles the same colour, single	Element
All molecules made of the same colour atoms (e.g. paired)	Element (made of molecules)
Different colours bonded together, always the same pattern	Compound
Different particles side by side, not all bonded, mixed up	Mixture

The two traps:

1. Seeing molecules (like O₂) and calling it a compound — wrong, both atoms are the same, so it is an element.
2. Seeing a box with two different molecules that are each compounds, all jumbled — that is a mixture (of two compounds), because they are not bonded to each other.