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Thales decided that fundamentally, everything must be made up of the same thing. He concluded that all matter is made up of water.

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Empedocles argued that all matter was made up of four basic elements: fire, water, earth, and air. He said that the ratio of these elements affected the properties of matter.

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Leucippus is believed to be the first to theorize the atom. The basis for his atomic model was that the universe was made up of two fundamental elements: the atom, and the void. He argued that the atom was the smallest division of matter, making them indivisible

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Democritus, the student of Leucippus, theorized that all matter is made of atoms. He argued that all materials were made up of small indivisible structures called "atoms." He believed that atoms were specific to the material they composed.

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Aristotle famously disagreed with Democritus's early atomic theory. He argued for Empedocles's idea that all matter was made up of four elements (fire, water, earth, and air), and that these elements in miniscule amounts made up all substances.

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John Dalton is credited with the first modern atomic theory. His theory was that all elements were composed of tiny indivisible particles called atoms, and that atoms of the same element were identical and atoms of different elements were different. He believed atoms could mix and join together chemically.

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Johann Dobereiner discovered that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements. This gave other scientists a clue that relative atomic masses were important when arranging the elements.

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Dmitri Mendeleev devised the periodic classification of the chemical elements, in which the elements were arranged in order of increasing atomic weight. Essentially he created the framework for the modern periodic table.

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He was the first person to recognize the periodic trends in the properties of elements, and developed Periodic Law. This states that the elements, when listed in order of their atomic numbers, fall into recurring groups, so that elements with similar properties occur at regular intervals.

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JJ Thompson experimented with cathode ray tubes and showed that all atoms had tiny negatively charged subatomic particles. He was the first to discover and name electrons. He proposed the plum pudding model of the atom, which showed electrons scattered in a positively charged "soup." He was the first to prove atoms were not indivisible.

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Maxwell Planck developed the theory that different atoms and molecules can emit or absorb energy in discrete quantities only. He named the smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation as quantum. He was able to determine that the energy of each quantum is equal to the frequency of the radiation multiplied by a universal constant that he derived, now known as Planck's constant.

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Nagaoka proposed an alternative planetary model of the atom in which a positively charged center is surrounded by a number of revolving electrons, in the manner of Saturn and its rings. This model was the first to contain a nucleus.

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Albert Einstein mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. He also demonstrated mathematically that Brownian motion has to arise from a force, proving the atomic theory of matter.

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Robert Millikan determined the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity.

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Ernest Rutherford contributed to the atomic theory through his discovery of the nucleus of the atom and through the discovery of the proton. During his famous gold foil experiment, Rutherford discovered that atoms contained a dense, positively charged core which he named the nucleus. He also discovered that the majority of the atom is different space.

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Niels Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another. This was the first model of the atom to incorporate quantum theory.

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Louis de Broglie introduced the idea that particles, such as electrons, could be described not only as particles but also as waves. This was substantiated by the way streams of electrons were reflected against crystals and spread through thin metal foils in the electron diffraction experiment.

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Henry Moseley collected the x-ray spectra of a variety of elements and found that the frequency of x-ray radiation has a precise mathematical relationship to an element's atomic number. (Concluded the Atomic Number is equal to the number of protons). This relationship is now called Moseley's Law and allowed scientists a new, more accurate way to organize elements.

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Wolfgang Pauli introduced two new quantum numbers and formulated the Pauli principle, which proposed that no two electrons in an atom could have identical sets of quantum numbers.

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Assuming that matter (e.g., electrons) could be regarded as both particles and waves, Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms. He developed a mathematical treatment into which both wave and particle nature of matter could be incorporated, known as quantum mechanics.

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Werner Heisenberg formulated quantum mechanics in terms of matrices and discovered the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly

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James Chadwick proved the existence of neutrons – elementary particles devoid of any electrical charge. The discovered it through an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. The resulting radiation showed high penetration through a lead shield, which could not be explained via the particles known at that time.