Element vs Compound
Element
An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means. Elements are the entries on the periodic table; some exist as single atoms (helium) and others as molecules of identical atoms (O2).
Example: Oxygen gas, O2: every molecule is made of two oxygen atoms and nothing else — it is an element, not a compound.
Compound
A compound is a pure substance made of two or more different elements chemically bonded together in a fixed, definite ratio. Compounds have properties completely different from their component elements and can only be separated by chemical reactions, not physical methods.
Example: Water, H2O: hydrogen and oxygen bonded in a strict 2:1 ratio — splitting it requires electrolysis, not filtering or boiling.
Key differences
| Aspect | Element | Compound |
|---|---|---|
| Composition | Exactly one type of atom (gold = Au atoms; oxygen gas = O atoms). | Two or more types of atoms chemically bonded in a fixed ratio (water = H2O, always 2:1). |
| Can it be broken down? | Cannot be broken down into simpler substances by any chemical means. | Can be decomposed into its elements by chemical reactions (electrolysis splits water into H2 and O2). |
| How to separate from a mixture | Physical methods work: filtering, distillation, or magnets can separate the sample if it is a mixture of elements. | Physical methods fail; only a chemical reaction can separate the bonded elements. |
| Properties | Each has its own characteristic properties, listed on the periodic table. | Has entirely new properties unlike its elements — hydrogen and oxygen are flammable gases, but water is a liquid that puts out fires. |
| Examples | O2 (oxygen gas), Fe (iron metal), He (helium). | H2O (water), NaCl (table salt), CO2 (carbon dioxide). |
Oxygen gas (O2) is an element, not a compound — many students misclassify diatomic molecules like O2, H2, and N2 because they contain two atoms. The test is atom type, not atom count: a molecule of one element type is still an element. Also, 'mixture' is a third category entirely: air is a mixture of elements and compounds, not a single compound, because its components are not chemically bonded in fixed ratios.