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442 BCE
Democritus
Greek philosopher who developed the atomic theory of the universe. He theorized that atoms were specific to the material they composed. Democritus also believed atoms differed in size and shape, were in constant motion in a void, and collided with each other. -
400 BCE
The Alchemists
The Alchemists created most early technology and chemistry, which we still use today. They developed theory of the atom. They also thought all metals are made or mercury or sulfur and that base metals can be turned into gold. -
400 BCE
Plato
Plato was another Greek philosopher. He also strongly disagreed with Democritus' theories. Plato theorized that a fifth atomic type must exist, which Aristotle later calls 'ether'. The heavens, and objects in the heavens (stars, planets, Sun) are composed of atoms of ether. -
300 BCE
Aristotle
Aristotle was a Greek philosopher. He did not agree with Democritus' theories. He believed not all things on Earth were made of atoms, but of the four elements. He greatly slowed down the evolution of atomic theory by saying it does not exist. -
Robert Boyle
Robert Boyle was an Anglo-Irish natural philosopher, chemist, physicist, and inventor. Boyle is largely regarded today as the first modern chemist. He helped develop a definition of an element ( any substance that can be broken into 2 or more substances is not an element) and helped with " the death" of the four elements. -
Lavoisier
Antoine Lavoisier was a prominent French chemist. Through a series of experiments, he found that the total mass of products and reactants in a chemical reactions is always the same. He established the law of conservation of mass -
John Dalton
John Dalton was an English meteorologist and chemist. He invented the theory of chemical combination. Dalton’s atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. Dalton’s atomic theory also stated that all compounds were composed of combinations of these atoms in defined ratios -
Solid Sphere of "Billiard Board" Model
The Solid Sphere Model was the first atomic model and was developed by John Dalton in the early 19th century. Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Dalton added these so the model atoms could be joined together with hooks and used to model compounds. -
Dmitri Mendeleev
Dmitri was a Russian chemist and teacher. He found that, when all known chemical elements were arranged in order of increasing atomic weight, the table displayed a recurring pattern. This eventually led to Mendeleev creating the Periodic table of elements. -
J.J. Thomson
Joseph John Thomson was a British physicist who proposed the "plum pudding" model of the atom. He showed that all atoms contain tiny negatively charged subatomic particles or electrons. -
The Curies
Marie Curie was a polish and and naturalized-French physicist and chemist. She conducted experiments on uranium rays. From this, she theorized that the rays came from the element's atomic structure. This idea created the field of atomic physics. Marie and her husband Becquerel discovered radioactivity. -
Planck's Quantum Theory of Light
In 1900, the German physicist Max Planck explained the ultraviolet catastrophe by proposing that the energy of electromagnetic waves is quantized rather than continuous. In physics, Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature, when there is no net flow of matter or energy between the body and its environment. -
"Plum Pudding" Model
Thomson proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus. The plum pudding model (also known as Thomson’s plum pudding model) is a historical scientific model of the atom. The plum pudding model has electrons surrounded by a volume of positive charge, like negatively charged "plums" embedded in a positively charged "pudding". -
Albert Einstein
Albert Einstein was a German mathematician and physicist who developed the general theories of relativity. He mathematically proved the existence of atoms. Atomic theory says that liquid is made up of molecules and that these molecules are always in random, ceaseless motion. -
Photoelectric effect
The photoelectric effect is a phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. -
Ernest Rutherford
Ernest Rutherford was a New-Zealand-born British physicist. He is known for his gold-foil experiment. He conducted this experiment in college as a research project. The gold-foil experiment showed that the atom consists of a small, massive, positively charged nucleus with the negatively charged electrons being at a great distance from the center. -
Robert Millikan
Robert Millikan was an American physicist honored the Nobel Prize for his study for the elementary of the electric charge and photoelectric effect. He showed that electrons' charge always was a multiple of precisely determined charge. -
Neils Bohr
Neils Bohr was a Danish physicist who was first to apply the quantum concept. He won the Nobel Prize for the idea that an atom is a small, positively charged nucleus surrounded by orbiting electrons. Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. -
Henry G.J. Mosely
Henry Gwyn Jeffreys Mosley was an English physicist who experimentally demonstrated that the major properties of an element are determined by the atomic number, not by the atomic weight. He established the relationship between atomic number and the charge of the atomic nucleus. -
Solar System Model
In 1913, Niels Bohr proposed the "Solar System" model of the atom, also known as the Rutherford-Bohr model, because it was a modification of Rutherford's model. This modification was based off of quantum physics. The "solar system" model describes an atom as a central massive positive entity (the nucleus/sun) and, orbiting around it, the negative entities (the electrons/planets). -
Electron Cloud Model
The electron cloud model says that we cannot know exactly where an electron is at any given time, but the electrons are more likely to be in specific areas. These areas are specified by orbitals. The electron cloud model was developed by Erwin Schrödinger and Werner Heisenberg. -
Schrodinger Equation
Schrodinger's equation shows all of the wave like properties of matter and was one of greatest achievements of 20th century science. The Schrödinger equation describes the form of the probability waves (or wave functions) that govern the motion of small particles, and it specifies how these waves are altered by external influences. -
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist and one of the key pioneers of quantum mechanics. He is best known for his uncertainty principle of quantum mechanics. In Niels Bohr's theory of the atom, electrons absorb and emit radiation of fixed wavelengths when jumping between fixed orbits around a nucleus. -
James Chadwick
James Chadwick was a British physicist who was awarded the Nobel Prize for his discovery of the neutron. The Chadwick atomic theory arose from James Chadwick's discovery of the neutron lead to the nucleus. His theory also made it possible to understand radioactivity. As part of the study of nuclei and radioactivity, scientists have further dissected protons and neutrons to find that they are themselves composed of smaller particles called quarks.