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Atom is the basic building block of all matter. Everything, including things we see and things that we can't see, is made up of atoms. They are incredibly tiny; so, scientists have done a lot of hard works and experiments, they are too brilliant. A normal atomic structure consists of three smaller particles: proton, neutron, electrons. The atom has protons and neutrons at the center of it, which together makes the nucleus. Revolving the nucleus are the electrons. The electrons move in a very complicated pattern and so it is very difficult to trace the exact location of electrons, as they are continuously moving in different directions. Electrons weigh hardly anything; so, the atomic mass is just the nucleus - the sum of protons and nucleus.

A little bit of extra information - all these particles have something called charge. Protons have a one plus charge(1+) meaning they are positively charged. Electrons are vice-versa - they have a one minus charge(1-) meaning they are negatively charged. 
I believe if we have not visualized a thing well enough, the learning stays incomplete. So, here are some pictures of the atom: