An atom is known as a fundamental piece of matter. Everything in the universe is made of matter, so, everything in the universe is made of atoms. An atom itself is made up of three kinds of particles called: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. How do we know all of this? Did we use microscopes to look inside an atom? To answer this there is no “atomic microscope” which would allow one to look inside an atom and say how many protons, electrons and neutrons are inside. The way the structure of the atom was devised was through a long series of experiments. Each one was designed to look at a specific aspect of the atom. At one time the atom was thought to be a solid ball of positive charge with electrons embedded in it. Then in 1909, Ernest Rutherford did an experiment which demonstrated that that idea was wrong and that the positive charge was centered at the center of the atom and occupied a very small volume compared to the whole atom. Before the neutron was discovered in 1932, the nucleus was thought to have both protons and electrons in it. The number of protons was chosen to get the correct atomic weight and the number of electrons was chosen to get the correct nuclear charge. It turned out that this model did not give predictions that agreed with experiment. The discovery of the neutron lead to a revision of the model leading to the current one.In the current model, the number of electrons in the atom is determined by gamma and x-ray spectroscopy. The number of protons in the atom is chosen to balance the charge of the electrons in the atom. The number of neutrons in the atom is chosen to give the correct atomic weight for the element in question. Many additional experiments were performed to confirm the model and the mutual agreement between the experimental results and the predictions based on the model is what is called proof.