Definition
A chemical element is a pure substance made up of only one type of atom. Elements cannot be broken down into simpler substances by ordinary chemical means. Every element is defined by its unique atomic number — the number of protons in each of its atoms — and is represented by a one- or two-letter chemical symbol.
Key Characteristics
- There are 118 known elements, all listed in the Periodic Table of Elements.
- Each element has a unique atomic number (e.g., hydrogen = 1, carbon = 6, gold = 79).
- Elements are represented by symbols: H (hydrogen), O (oxygen), Fe (iron), Au (gold).
- Elements exist as pure substances and cannot be separated into simpler materials by chemical reactions.
- Elements can combine with other elements to form compounds (e.g., H and O combine to form H₂O).
- Groups of elements share similar properties — for example, the noble gases (Group 18) are all largely unreactive.
Real-World Example
Gold (Au) is a pure element. Every atom in a gold ring has exactly 79 protons. Gold cannot be chemically broken down into a simpler substance, and its properties — metallic lustre, electrical conductivity, resistance to corrosion — are entirely determined by those gold atoms. This is why gold has been used as jewellery and currency for thousands of years.
An element is a pure substance made of one type of atom only, identified by its atomic number (number of protons). All 118 elements are organised in the Periodic Table, where elements in the same column (group) share similar chemical properties. Unlike compounds, elements cannot be broken down by chemical reactions.
Elements vs Compounds
| Feature | Element | Compound |
|---|---|---|
| Composition | One type of atom | Two or more different atoms bonded |
| Example | O₂ (oxygen gas) | H₂O (water) |
| Can be chemically separated? | No | Yes |
Practice Problems
Identify which of the following are elements and which are compounds: oxygen (O₂), water (H₂O), iron (Fe), carbon dioxide (CO₂).
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An element has atomic number 17. How many protons does one atom of this element have, and what element is it?
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Why can't you separate an element like copper into simpler substances using a chemical reaction?
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A student mixes iron filings and sulfur powder. Is the result a new element, a compound, or a mixture? How could you tell, and how could you separate it?