Philosopher Democritus first introduced the Atomic Theory. He suggested that there, in nature, exist small, indivisible particles called atoms that make up all we see today. This of course for him was just a theory and he never found a way to prove it. So it was up to John Dalton, who in 1808 published his Atomic Theory, in which he claimed that tiny particles, atoms, make up all matter. He viewed each atom as part of an element, so the atoms of the same element were identical while the atoms of different elements are different. The atom was found to be composed of three basic particles, the electron, neutron and proton. The proton has a positive charge, the electron has a negative charge and the neutron has no charge. The reason why atoms exist is because the positive charge or the nucleus attracts the negative charge of the electrons. J.J. Thompson discovered the electron in 1897. Ernest Rutherford made several advancements. First, in 1902, he showed the electron to be part of the atom, then in 1911 he identified the nucleus of the atom and in 1914 he discovered the proton. James Chadwick discovered the neutron in 1932.

Although the electrons were originally thought to orbit around the atom as a planet orbits the sun, this idea was proven to be impossible because if this were to happen the electron would eventually lose its energy and would be drawn into the nucleus. Niels Bohr challenged the concept of the electron orbits, by introducing his idea of the elliptical paths in which the electron radiates no energy, except when jumping from a higher to lower energy level. The modern understanding of the atom however does not come from Bohr, but from the quantum mechanical theory of the atom. This theory stated that the electron of the atom does not travel in an orbit and that the exact location and velocity at one time could not be found. Electron probability clouds are thus drawn around the atom to indicate where the electron most likely is.

The shapes of the electron probability clouds were found through quantum mechanics. There are several components to the structure of the energy levels. Each energy level has a sublevel designated by the letters s, p, d, f, etc. Each sublevel has a certain number of orbitals in which there can be only two electrons. In the s sublevel, there is only one orbital; the p has three, the d has five and the f has 7. These s, p, d, f, etc. orbitals only signify the probability of an electrons location, not exactly where the electron will be. Electrons also have a property called spin, and they follow Pauli's exclusion principle.

In 1870, Mendeleev formed a periodic table based on the repeating cycle of similar chemical properties between elements. He arranged his periodic table in order of the increasing masses of the elements. The patterns of Mendeleev's periodic table were found to be comparable to the patterns of the electron clouds and to the spectral lines of excited gasses. Many years later, many new elements have been discovered and the Periodic Table has been advanced into its current form. It was thus, through these achievements the atom was advanced into to what we believe it to be today.