What Are Elements and Compounds?
An element is a pure substance made of only one type of atom, while a compound is a pure substance made of two or more elements chemically bonded in a fixed ratio. Elements are the periodic table's building blocks; compounds are what you get when those blocks combine chemically. Understanding this distinction is the foundation for everything else in chemistry.
An element cannot be broken down into simpler substances by chemical means (e.g., oxygen, gold, hydrogen), while a compound is two or more elements chemically combined in a fixed ratio and can be broken down into its elements (e.g., water, H₂O, or table salt, NaCl).
- •Made of only one type of atom
- •Cannot be broken down chemically
- •Listed on the periodic table (e.g., O, Au, H)
- •Properties are fixed for that atom type
- •Made of two or more elements chemically bonded
- •Can be broken down into elements by chemical reactions
- •Written as formulas (e.g., H₂O, NaCl, CO₂)
- •Properties differ from the elements that form them
Step-by-step worked examples
Classify oxygen gas (O₂), water (H₂O), and gold (Au) as elements or compounds.
Oxygen gas (O₂): element — made of only one type of atom (oxygen), even though two atoms bond together. Water (H₂O): compound — two different elements (hydrogen and oxygen) chemically bonded in a fixed 2:1 ratio. Gold (Au): element — a single type of atom found directly on the periodic table.
Table salt (NaCl) is heated with electricity in a process called electrolysis and separates into sodium metal and chlorine gas. What does this show about NaCl?
Since NaCl breaks apart into two different pure substances (Na and Cl₂) by a chemical process, NaCl must be a compound, not an element. Sodium and chlorine themselves cannot be broken down further by ordinary chemical means, so they are elements. This confirms the rule: compounds decompose into elements; elements do not decompose into anything simpler.
Carbon dioxide (CO₂) always contains carbon and oxygen in a fixed mass ratio of about 3:8. Why can't you have 'CO₂' with a different ratio of carbon to oxygen?
Compounds obey the law of definite proportions: the elements always combine in the same fixed ratio by mass. If the ratio were different, you would have a different compound entirely (for example, CO instead of CO₂), not variable CO₂. This fixed-ratio rule is what distinguishes a true chemical compound from a simple physical mixture.
Flashcards
Quick quiz
Q1.Which of these is an element?
Q2.What makes a compound different from a mixture of elements?
Q3.How many types of atoms are in a pure element?
Q4.Which process can break a compound down into its elements?
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Common mistakes
Thinking O₂ is a compound because it has two atoms. — Correct: O₂ is an element — both atoms are the same type (oxygen); a compound needs two or more different elements.
Believing mixtures and compounds are the same thing. — Correct: A compound has elements chemically bonded in a fixed ratio; a mixture is just physically combined and can vary in proportion.
Assuming a compound keeps the properties of its original elements. — Correct: Compounds often have totally different properties — sodium (reactive metal) and chlorine (toxic gas) become safe table salt.
Thinking any two elements can combine in any ratio to form the same compound. — Correct: Each compound has one fixed ratio; a different ratio makes a different compound (CO vs. CO₂).
FAQ
What is the difference between elements and compounds?
Elements are made of one type of atom and can't be broken down chemically; compounds are two or more elements chemically bonded in a fixed ratio and can be broken down.
What is an example of elements and compounds?
Oxygen (O₂) and gold (Au) are elements; water (H₂O) and carbon dioxide (CO₂) are compounds.
How do you identify an element vs a compound from its formula?
One chemical symbol means an element (even with a subscript, like O₂); two or more different symbols mean a compound (like H₂O).
What is the elements and compounds formula rule?
Compounds follow the law of definite proportions — the same elements always combine in the exact same fixed mass ratio.