element: The Atom

The smallest unit of a chemical element that has the properties of that element is called an atom. Many elements (e.g., helium) occur as single atoms. Other elements occur as molecules made up of more than one atom. Elements that ordinarily occur as diatomic molecules include hydrogen, nitrogen, oxygen, and the halogens, but oxygen also occurs as a triatomic form called ozone. Phosphorus usually occurs as a tetratomic molecule, and crystalline sulfur occurs as molecules containing eight atoms.

Sections in this article:

• Introduction
• Discovery of the Elements
• Evolution of Modern Concepts
• Greek Concept of the Elements
• The Elements through the Ages
• Official Symbols and Names for the Elements
• Properties of the Elements
• Atomic Mass and Atomic Weight
• Isotopes
• Atomic Number and Mass Number
• The Atom
• Bibliography

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.

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