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An atom is the most basic building block of matter of a substance.   Any physical substance is made up of atoms that repeat themselves through out the substance.  It is the  smallest indivisible part of the substance, that contributes to physical and chemical properties of a substance.
    Any substance consists of atoms arranged in some structure and some free space between them.

    It consists of subatomic particles called electrons, protons and neutrons.  The size of an atom is pico meters and is invisible to the eyes.

   The chemical elements that we hear like hydrogen, oxygen are composed of their respective atoms.  A good example that we can give is,   a school is like matter of  a substance.  Then rooms in the building are like atoms.  The smallest building block of the building.   Further, in a beehive there are many rooms for the bees, each room is like an atom.

Atom means anu which the elements made; o is oxxygen atom,,, 2 `o combines to form oxygen molecule