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550 BCE
Asharya Kanada
Acharya Kanad is believed to be the first to have realized the idea of an indestructible particle of matter. The theory occurred to Kanad while he was walking with food in his hand and he was breaking it into small pieces. He did so until he was no longer able to break it down into smaller pieces. It was then that he realized that he could not divide the food into further parts and thought of the idea of a particle that could not be divided any further. He called the particle “Anu,” or atom. -
450 BCE
Empedocles
Empedocles lived during the dawn of scientific thought in Ancient, and arguably his greatest contribution to the atomic theory is his theory of natural selection, in which he proposed that everything is made up of the four elements (Earth, Wind, Fire, Water). Now although his idea is currently disproven, he was one of the first people to try and classify existence into different elements. -
420 BCE
Democritus
Democritus is credited to one of the first people to propose the notion that all things are made up of small geometrically indivisible particles that he called "atoms". Democritus alongside his teacher Leucippus and Epicurus proposed the idea about how all things are made up of atoms and the different properties each item has, is related to the type of atom and how they are bonded. -
John Dalton
John Dalton's contribution to the atomic theory is that he theorized that all matter is composed of atoms. Now the idea originally belongs to Democritus, but Dalton is the first to create a modern description of the atomic theory. In which he states that atoms are fundamental building blocks of Chemical structures and that each chemical element is composed of atoms, and these atoms can be combined to form a more complex structure in which he came to the realization through experimentation. -
J.J. Thomson
J.J Thomson's contributions started with his experiments with cathode ray tubes. Using his experiment Thomson came up with the idea of the electron which he called cathode ray particles. Through his experimentation, he concluded that the particles must be negatively charged and their mass is very small. Thomson knew that atoms were neutral and as such he expanded of Dalton theory by coming up with the plum pudding model where the atoms were plum pudding with both negative and positive charges. -
Marie Sklodowska Curie
Curie contributed to the atomic theory through her research in radioactivity. Curie was studying uranium rays when she made the claim the rays were not dependent on the uranium's form, but on its atomic structure. Her theory created a new field of study, atomic physics, and Marie herself coined the phrase "radioactivity." She defined radioactivity at the time to be this activity of rays to be dependent on uranium's atomic structure, the number of atoms of uranium. -
Nagaoka Hantaro
Hantaro, in particular, developed the “Saturnian” system in 1904. The atom, as postulated in this model, was inherently unstable because, by radiating continuously, the electron would gradually lose energy and spiral into the nucleus. He predicted two things with his model, one that the atom has a very massive atomic center, and electron revolve around the center by electrostatic forces. -
Max Planck
Even though Max Planck did not exactly affect the atomic theory, his discoveries helped other scientists such as Neils Bohr to improve the atomic theory. His main discoveries were that energy is radiated in discrete amounts which he called quanta, which is a theoretical explanation of the spectrum radiation emitted by an object that glows when heated. -
Albert Einstien
Albert einstein's contribution to the atomic theory was that he mathematically proved the existence of atoms. That same year he published his paper of the Special Theory of Relativity, which later led to the General Theory of Relativity. -
Lise Meitner
In 1909, Meitner and Hahn discovered radioactive recoil, finding that when an atomic nucleus emits an alpha particle, the nucleus will recoil like a gun that has fired a bullet. The recoiling positively charged nucleus can be attracted to a negatively charged electrode. Meitner and Hahn demonstrated that radioactive recoil can be used to produce elements with very high purity, collecting them on the negative electrode. -
Ernest Rutherford
Rutherford's great contribution to the atomic theory was nuclear model of the atom. In which he describes the positive charge is in the middle of the atom with the negative charges around it and most of the atom is nothing but empty space. He came to this discovery through his gold foil experiment where he shot alpha particle at a thin sheet of gold and he observed how not all the alpha particles pass-through the gold sheet. -
Niels Bohr
Niels Bohr's contribution was that he proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well-defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths. -
Louis de Broglie
In the early 20th century experiments had indicated that the electron must move around a nucleus and that, and that there are restrictions on its motion. De Broglie’s idea of an electron with the properties of a wave offered an explanation of the restricted motion. A wave confined within boundaries imposed by the nuclear charge would be restricted in shape and, thus, in motion, because any wave shape that did not fit within the atomic boundaries would interfere with itself and be canceled out. -
Wolfgang Pauli
Wolfgang Pauli contributed to the atomic theory when he introduced two new numbers and formulated the Pauli principle, which proposed that no two electrons in an atom could have identical sets of quantum numbers. It was later discovered that protons and neutrons in nuclei could also be assigned quantum numbers and that Pauli's principle applied here too. -
Erwin Schrodinger
Schrödinger proposed the electron cloud model, which no longer depicts electrons moving around the nucleus in an orbit. Instead, Schrodinger proposed a model whereby scientists could only make educated guesses as to the positions of electrons. Hence, their locations could only be described as being part of a ‘cloud’ around the nucleus where the electrons are likely to be found. -
Werner Heisenberg
Heisenberg contributed to the atomic theory through formulating quantum mechanics in terms of matrices and also discovering his uncertainty principle. In which he states that we can know a particle position but not it's momentum, or we can know it's momentum but not it's position. -
James Chadwick
James Chadwick's main role in the atomic theory is that he discovered the Neutron in atoms and that neutrons are located in the center of an atom, in the nucleus along with the protons. Also, neutrons have neither a positive nor a negative charge but contribute to atomic weight with the same effect as a proton. Chadwick discovered this subatomic particle by using a neutron chamber in his experiments. -
Peter Higgs
Higgs contributed to the atomic theory by theorizing that all mass is attained by how particles interact with something he called the Higgs field created by the Higgs boson. At the time he wasn't taken seriously but with the upcoming but was recently given light as a cause of the particle accelerators. -
Murray Gell-Mann
When new accelerators and apparatuses helped identify many new elementary particles. In theoretical works from the same period, Murray Gell-Mann classified particles and their interactions. He proposed that observed particles are in fact composite, that is, comprised of smaller building blocks called quarks. According to this theory, as-yet-undiscovered particles should exist. He first explained how protons, neutrons, are made up of quarks. -
Ronald J. Gillespie
Gillespie contributed to the atomic theory by creating the widely used Valence Shell Electron Pair Repulsion (VSEPR) theory. Which is used to predict the shape based off of the number of electrons pairs in the outer shell -
Richard F.W. Bader
Richard Bader discovered that electron density is very important in explaining the behavior of atoms in the molecules. According to his theory, there are no atomic orbitals in the molecules. This was a new idea and went against accepted theories. He fought hard for his revolutionary ideas and found it difficult to publish. In the end, the theories became more accepted and published a book Atoms in Molecules, a Quantum Theory in 1991.
