what is an atom ?

 an atom is the smallest particle of a substance which cannot be broken into simpler substance.it may or may not exist freely.

 An atom is a basic unit that makes up all matter. There are many different types of atom each with its own name, mass and size. These are called chemical elements, such as hydrogen or gold. Atoms are very small; the exact size changes depending on the element: they range from 0.1 to 0.5 nanometers.^[1] One nanometer is around a hundred times smaller than the width of a human hair. This makes atoms almost impossible to see. The way they work, and how they act with other atoms has to be seen using equations.

Atoms combine to make molecules or particles: for example, two hydrogen atoms and one oxygen atom combine to make awater molecule, a form of a chemical reaction.

 An atom is a building block of matter.

 An atom is the smallest amount of a substance that we can have that will still retain all the features of that substance. Should it be subdivided any more, the qualities of the substance will be lost. The atom comes from an ancient Greek idea that stated exactly that. Today we know that the "substance" spoken of here is a chemical element, and an atom of an element is the smallest unit into which we can divide that element.  
Atoms, which form chemical molecules, are the basic part of any substance. All matter, including living things and people, is made of atoms. The chemical reactions in the human body are the complex interactions between atoms and molecules in our cells.

 A classic atom has a nucleus (made up of protons and neutrons) and a number of electrons orbiting this nucleus, which in an uncharged atom is equal to the number of protons. Atoms can bond with each other to form molecules, or decay into different atoms, or ionize (lose or gain electrons to become respectively positively or negatively charged ions). The properties of an atom are dictated by the number of protons, neutrons and electrons it contains.


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 All atoms that have been discovered to date are laid out in the periodic table of elements.

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