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element oxygen, element hydrogen, Ancient Greek philosophers, atomic nuclei, chemical element

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Atom, tiny basic building block of matter. All the material on Earth is composed of various combinations of atoms. Atoms are the smallest particles of a chemical element that still exhibit all the chemical properties unique to that element. A row of 100 million atoms would be only about a centimeter long. See also Chemical Element.

Understanding atoms is key to understanding the physical world. More than 100 different elements exist in nature, each with its own unique atomic makeup. The atoms of these elements react with one another and combine in different ways to form a virtually unlimited number of chemical compounds. When two or more atoms combine, they form a molecule. For example, two atoms of the element hydrogen (abbreviated H) combine with one atom of the element oxygen (O) to form a molecule of water (H20).

Since all matter—from its formation in the early universe to present-day biological systems—consists of atoms, understanding their structure and properties plays a vital role in physics, chemistry, and medicine. In fact, knowledge of atoms is essential to the modern scientific understanding of the complex systems that govern the physical and biological worlds. Atoms and the compounds they form play a part in almost all processes that occur on Earth and in space. All organisms rely on a set of chemical compounds and chemical reactions to digest food, transport energy, and reproduce. Stars such as the Sun rely on reactions in atomic nuclei to produce energy. Scientists duplicate these reactions in laboratories on Earth and study them to learn about processes that occur throughout the universe.

Throughout history, people have sought to explain the world in terms of its most basic parts. Ancient Greek philosophers conceived of the idea of the atom, which they defined as the smallest possible piece of a substance. The word atom comes from the Greek word meaning “not divisible.” The ancient Greeks also believed this fundamental particle was indestructible. Scientists have since learned that atoms are not indivisible but made of smaller particles, and atoms of different elements contain different numbers of each type of these smaller particles.


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