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(born 600 BC) Acharya Kanada of Ancient India, his real name Kashyap, was a Hindu sage and philosopher. While walking through the town of Prayag, he proposed that paramanu, or atoms, were indestructible particles of matter invisible to the human eye. He also believed that the atom was eternal and has the tendency to bind with other atoms.

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(480-420 BC) Ancient Greek Leucippus is the earliest known atomist. He developed the theory that the universe consists of "the full" and "the void." His student, Democritus, further developed and popularized his concept of atoms.

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(460 - 370 BC) Democritus stated that atoms made up all matter and named the concept, "atomos", meaning indivisible. He believed atoms were the smallest possible particles and were indivisible. He also proposed that there was an infinite amount of atoms of varying size and shape and that they interacted with each other and moved in constant motion throughout the void. Due to a lack of surviving knowledge regarding Leucippus, most work, coming from Democritus's writings, is credited to him.

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(1766 - 1844) John Dalton was an English meteorologist and chemist. Because of his interest in gases, he began to study atomic theory as well. His theory was the first complete attempt to describe all matter in terms of atoms and their properties. He reasserted Democritus's theories and also stated that:

- all atoms of the same element are identical

- atoms are indivisible (later disproven and had to be modified due to discoveries of isotopes and subatomic particles)

- compounds are combinations of at least two types of atoms

- a chemical reaction is a rearrangement of atoms

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The laws Dalton based his theory off of:

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The laws Dalton based his theory off of:

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Dalton based his theory on two laws:

- the conservation of mass - states that matter can not be created or penetrated in a closed space

- constant composition - states that a compound will always have the same proportion of the same element

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His atomic model is commonly referred to as the "Billiard Ball Model." It looked like a mere sphere as subatomic particles had yet to be discovered.

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(1856 - 1940) Joseph John Thomson revolutionized atomic theory by discovering the electron. In 1897, he performed a series of experiments to study the nature of electric discharge in a high-vacuum cathode ray tube. He interpreted the deflection of the rays by electrically charged plates and magnets as proof of the existence of electrons, which he calculated as having a large value for the charge-to-mass ratio. He later estimated the value of the charge itself. He attempted to estimate the amount of electrons in an atom by measuring the scattering of light and gamma rays. His student, Ernest Rutherford, followed this approach in the journey to uncover the nature of negatively charged sub-particles.

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Thompson suggested an atomic model, also known as the "Plum Pudding" model, as a sphere of positive matter with small, distant electrons spaced throughout. Neutrons had yet to be discovered.

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(1871 - 1937) Ernest Rutherford proposed that atoms contained a small, dense nucleus, which took up most of the atom's mass. He believed that only protons, positively charged sub-particles with a mass of one amu each, existed inside the nucleus. While he accurately identified protons, he didn't discover neutrons, which also reside in the nucleus. He correctly suggested that electrons, which are negatively charged and have negligible mass, orbit the nucleus. Because he had a theory that atoms were comprised mostly of empty space, he needed to test it with an experiment, the Gold Foil Experiment of 1909.

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Rutherford's Gold Foil Experiment (1909)

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Rutherford's Gold Foil Experiment (1909)

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The gold foil experiment was conducted by Ernest Rutherford in 1909 to clarify atomic structure. To do this, he placed radium inside a block of lead with a single hole drilled through it, allowing the radium to emit alpha particles, composed of two protons and two neutrons. Alpha particles were charged at an extremely thin sheet of gold foil. Surrounding the foil was a screen made of Zinc Sulfide, which emitted flashes of light when an alpha particle struck it, allowing Rutherford to detect where the particles hit. Most of the particles passed through the foil with minimal deflection, with only a small fraction being deflected. As he could see light flashes, Rutherford knew the alpha particles were hitting the Zinc Sulfide, proving his theory that atoms are mostly empty space. This finding contradicted J.J. Thompson's previously popular "plum pudding" model and allowed the later Niel Bohr's model to gain popularity.

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Rutherford's atomic model, also known as the "Nuclear Model," showcased protons and neutrons in the dense nucleus of the atom, and electrons distributed around the nucleus. The nucleus took up the latter of the atom's mass, and the space occupied by electrons took up the most volume.

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(1885-1962) Niels Bohr largely agreed with Rutherford's atomic model and made a great impact on electron discovery. He proposed the idea that electrons could be found at fixed distances, positions, and energy levels. He also believed that electrons moved in specific, stable orbits around the nucleus without radiating energy, and concluded that electrons will have more energy if located far from the nucleus and will have less energy if located close to the nucleus. Along with this, he stated that when an electron jumps from one orbit to another, it absorbs or emits a specific quantity of energy (photon), which corresponds to the difference in energy between the two orbits.

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Bohr's atomic model depicts the electrons as orbiting in circles around the nucleus. Electrons are placed in orbits depending on their energy levels which correspond to their distance from the nucleus, like rings. In some models, arrows may be illustrated between the orbits to demonstrate how electrons can move from one orbit to another, explaining the release or absorption of light. When an electron moves to a higher orbit, energy is absorbed; when it moves to a lower orbit, energy is emitted as light.

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(1887-1961) Erwin Schrödinger wanted to prove that electrons behaved in an inconsistent wave-like manner. He introduced the concept that electrons behave both as particles and waves, which helped explain the probability aspect behind an electron's location. He formulated an equation that describes how the wavefunction, or probability distribution of the energy state in an electron evolves over time. The solutions to this equation, called orbitals, describe regions surrounding the nucleus where an electron is most likely to be found, or the electron cloud. His discovering paved the way for comprehension of topics like chemical bonding and electron configuration.

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His model of an electron cloud shows a probable distribution of electrons, rather than an often inaccurate, fixed position.

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(1891-1974) James Chadwick discovered the neutron, a neutral particle located in the nucleus with the proton. The existence of neutrons resolved the confusion around how atoms of the same element could have different isotopes with different masses, when at the time, only protons were known, so scientists weren't aware different amounts of neutrons could be in the nucleus. His discovery advanced nuclear and atomic development, as well as medicine and radiology.

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Chadwick's atomic model was the first to present neutrons in the nucleus.