An atom is the smallest unit of matter that can not be broken down by any chemical means and exhibits all the properties of that matter. The term Atom comes from the Greek word Atomos which means Indivisible. There is no existence of an atom independently, they combine together to form a molecule. Each molecule is made up of two or more atoms. We can not see an atom with naked eyes.
Lets understand the concept of atom with an example- Suppose we have a piece of copper wire and if we keep on cutting this piece of copper wire into smaller pieces, there will be a point beyond which it can not be cut any further. The smallest piece of copper is a copper atom.
Examples- O(oxygen), N(Nitrogen), H(hydrogen), etc are examples of atoms. Two or more atoms combine together to form molecules. H2 contains two atoms of Hydrogen.