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What is an atom - definition of atom?

Atom - small basic building block of matter. 

All materials on Earth are composed of different combinations of atoms.

Atoms are the smallest particles of a chemical element that still have all the unique chemical properties of this element.

A row of 100 million atoms would be about a centimeter long.

Understanding the atoms is the key to understanding the physical world.
More than 100 different elements exist in nature, each with its own unique atomic structure. The atoms of these elements react with each other and make different combinations, forming virtually unlimited number of chemical compounds.
When two or more atoms combine, they form a molecule. 
For example, the two hydrogen atoms (abbreviated to H) combined with an atom of the element oxygen (O),  form a water molecule (H20).
Since all matter, since its creation in the early universe to current biological systems is made up of atoms, understanding their structure and properties play an important 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 compounds they form play a role in almost all processes occurring on Earth and in space.
All living organisms rely on a set of chemical compounds and chemicals to digest food, energy transport reactions, and to reproduce.
Stars like the Sun rely on reactions in the atomic nucleus to produce energy. Scientists duplicate these reactions in their laboratories on Earth and study them to learn more about the processes that occur in the universe.
Atom Diagram - What is an atom - definition of atom?
Throughout history, people have sought to explain the world in terms of its most basic parts.
Ancient Greek philosophers conceived the idea of ​​the atom, which they define as the smallest part of a possible substance.
The word atom comes from the Greek word meaning "not divisible."
The ancient Greeks also believed that fundamental particle was indestructible.
Scientists have learned that atoms are not indivisible, but small particles and atoms of different elements contain different numbers of each type of these smaller particles.
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