Beginner Topic 4 of 1102 Atomic Structure & Periodicity
Compound
A pure substance of two or more elements chemically combined in a fixed ratio.
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In-depth explanation of Compound
A compound is a pure substance in which two or more elements are chemically bonded in a fixed, whole-number ratio of atoms. Its properties are usually very different from those of its constituent elements, because the new bonds create a different electronic structure. Its composition is written as a chemical formula, such as H2O or NaCl, which fixes the ratio of atoms. Compounds can be separated into their elements only by chemical reactions, for example electrolysis or reduction. Ionic and molecular compounds are the two main classes, and they differ in how their atoms are held together.
Going deeper: Compound
Key ideas: A compound has a fixed ratio of elements by mass and by atoms, written as a formula such as H2O or CaCO3. Subscripts count atoms, coefficients count formula units or molecules. Its properties differ sharply from its elements (Na is a reactive metal, Cl2 a toxic gas, NaCl table salt). Compounds can be split only by chemical means such as electrolysis or heating.
Common mistakes and connections: Students mix up compounds with mixtures, expecting properties of the parts to persist. Another error is changing subscripts to balance an equation, which changes the compound itself. This idea underlies the laws of definite proportions and molar mass calculations.
Visualization of Compound
Water is a compound: hydrogen and oxygen bonded in a fixed 2 to 1 ratio, with properties unlike either gas on its own. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Sodium is a violently reactive metal and chlorine is a poisonous gas, yet the compound NaCl is harmless table salt.
Frequently asked questions about Compound
What is Compound?
A pure substance of two or more elements chemically combined in a fixed ratio.
Is Compound a beginner, intermediate or advanced topic?
Compound is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Compound?
Study these earlier topics in Atomic Structure & Periodicity first: Matter, Atom, Element.
What comes after Compound?
The next topics in Atomic Structure & Periodicity are Mixture and Pure Substance.
Can I practise Compound with quizzes?
Yes. The interactive lesson includes a self-check question on Compound, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Compound fits in Atomic Structure & Periodicity
Compound is topic 4 of 115 in Atomic Structure & Periodicity.