What Is an Element?
An element is a pure substance made of only one type of atom. Elements cannot be broken down into simpler substances by chemical reactions.
There are 118 known elements. Some familiar ones are:
- Iron (Fe) — used in construction and tools
- Oxygen (O) — needed for breathing and combustion
- Carbon (C) — found in all living things
- Gold (Au) — used in jewellery and electronics
All elements are listed in the periodic table, arranged by their atomic number (number of protons).
What Is a Compound?
A compound is a pure substance made from two or more different elements that are chemically bonded together. The atoms are joined by chemical bonds and cannot be separated by physical means (like filtering or evaporation).
Key properties of compounds:
- The properties are completely different from the elements they came from.
- They have a fixed ratio of atoms (always the same formula).
- They can only be separated back into elements by chemical reactions.
Table salt (sodium chloride, NaCl) is a compound made from sodium and chlorine. Sodium is a silvery metal that reacts violently with water. Chlorine is a poisonous yellowish gas. Yet together, in a fixed 1:1 ratio, they form harmless white crystals that you put on your food. This shows how the properties of a compound differ from its elements.
Key Differences
| Feature | Element | Compound |
|---|---|---|
| Types of atom | Only one | Two or more different types |
| Formula | Just a symbol (e.g. Fe) | Shows ratio of atoms (e.g. H₂O) |
| Can be broken down chemically? | No | Yes, by chemical reactions |
| Properties | Properties of that element | Different from the constituent elements |
| Examples | O₂, Cu, S | CO₂, NaCl, H₂O |
Elements vs Compounds vs Mixtures
It is important not to confuse a compound with a mixture:
- In a compound, elements are chemically bonded in a fixed ratio. The compound has one set of properties and can only be separated chemically.
- In a mixture, substances are just physically mixed together. They are not chemically bonded, their properties remain their own, and they can be separated by physical methods.
For example, air is a mixture of nitrogen, oxygen, and other gases — they are not chemically joined. Water (H₂O) is a compound of hydrogen and oxygen — they are chemically bonded.
A quick test: if you can separate two substances by a physical method (filtering, evaporating, using a magnet), they were in a mixture. If you need a chemical reaction to separate them, they were in a compound.
Naming Compounds
Simple compounds with two elements are usually named using both element names, with the second element having an "-ide" ending:
- Sodium + Chlorine → Sodium chloride (NaCl)
- Hydrogen + Oxygen → Hydrogen oxide (water, H₂O) — note water has its common name
- Iron + Sulfur → Iron sulfide (FeS)
Real world: Salt — from reactive elements to safe food
Sodium (Na) is a soft, silvery metal that reacts explosively with water. Chlorine (Cl₂) is a poisonous green gas used as a chemical weapon in WWI. Yet when they combine chemically in a 1:1 ratio, they form sodium chloride (NaCl) — ordinary table salt, completely safe and essential for cooking. This demonstrates how a compound has completely different properties from its elements.
Real world: Water and its elements
Hydrogen (H₂) is a highly flammable gas — it famously caused the Hindenburg airship disaster. Oxygen (O₂) supports combustion and is needed for fires to burn. Yet their compound, water (H₂O), is used to put out fires. Water's properties bear no resemblance to those of the hydrogen and oxygen it contains.
Real world: Copper wire vs. copper compounds
Copper (Cu) as an element is a shiny, ductile metal used in electrical wires because it conducts electricity well. However, copper sulfate (CuSO₄) — a compound of copper, sulfur, and oxygen — is a blue crystalline solid used as a fungicide. The compound behaves entirely differently from pure copper metal.
Where do you see this in real life? Look at ingredient labels — "sodium chloride" is salt (a compound). "Iron" supplements in food labels are the element iron. Recognising whether something is an element or compound helps you understand what it does and how it behaves.
Practice Problems
Water (H₂O) and a hydrogen/oxygen gas mixture are both made of hydrogen and oxygen. What is the key difference between them?
Show answer
Is carbon dioxide (CO₂) an element, a compound, or a mixture? Explain.