September 30, 2020
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Atomic Model Timeline
⟶ Updated 26 Oct 2018 ⟶
List of edits
JOHN DALTON (1766-1844 CE) date of discovery (1803 CE) All matter is made up tiny indivisible particles called atoms (solid, hard, impenetrable, movable particles) where different atoms of elements are spheres of different sizes and colors. His atomic model provides much of the framework that is used in modern chemistry efforts.
DMITRI MENDELEEV (1834-1907 CE) date of discovery (1869 CE) Created the first periodic table that arranged the known elements according to their of atomic mass and organized them into groups possessing similar properties. His periodic table predicted the existence of three new elements (gallium, scandium, and germanium) and when these elements were discovered, they fit correctly into the three missing elements in his table.
HENRY MOSELEY (1887-1915 CE) date of discovery (1913 CE) His work was used to reorganize the periodic table based upon atomic number instead of atomic mass. He used x-ray tubes to determine the charges on the nuclei of most atoms. Created the world’s first atomic battery (beta cell) and he discovered that an element’s atomic number is identical to how many protons it has.
J.J. THOMSON (1856-1940 CE) date of discovery (1897 CE) Discovered negatively charged particles (electrons) using cathode ray tubes and proposed the "plum pudding" model of the atom which consisted of a uniform sphere of positive charge with negative electrons embedded in the sphere. Even though Thomson’s atomic model was inaccurate and had a few drawbacks, Thomson's model is important because it introduced the notion that an atom consisted of charged particles and it provided the base for sev
MARIE CURIE (1867-1934 CE) date of discovery (1898 CE) Atom is composed of particles and the nuclei of heavy, unstable elements lose mass (decay) and give off energy (radioactivity). Discovered unknown elements polonium and radium and deduced that radioactivity does not depend on how atoms are arranged into molecules, but rather that it originates within the atoms themselves.
ERNEST RUTHERFORD (1871-1937 CE) date of discovery (1907 CE) Fired alpha particles at gold foil and deduced that the atom is composed mostly of empty space with electrons orbiting in a set, predictable paths around fixed, positively charged nucleus. Although his model was partially inaccurate, it helped form the basis of the quantum mechanics and helped the future development of quantum mechanics.
HANTARO NAGAOKA (1865-1950 CE) date of discovery (1904 CE) Atom is a large, massive, positively charged sphere, encircled by many light-weight, negatively charged electrons bound by electrostatic forces analogous to Saturn's rings (Saturn model). His model predicted a massive atomic center (nucleus) with electrons revolving around the nucleus and bound by electrostatic forces. Both predictions were successfully confirmed by Ernest Rutherford in 1911.
NIELS BOHR (1885-1962 CE) date of discovery (1913 CE) Atom is composed of negatively charged electrons traveling in separate orbits around a positively charged nucleus. Atoms absorb or emit radiation when electrons on subatomic level jump between the allowed and stationary states. Discovered that the number of electrons in the outer orbit determines the properties of an element. Bohr’s work contributed significantly to scientists' understanding of nuclear fission.
JAMES CHADWICK (1892-1974 CE) date of discovery (1932 CE) Atom is composed of a nucleus containing neutrons (no electric charge) and protons (positively charged) with electrons (negatively charged) orbiting the nucleus at separate levels. Discovered the existence of the neutron by bombarding beryllium with alpha particles. His findings were pivotal to the discovery of nuclear fission and ultimately the development of the atomic bomb.
ERWIN SCHRODINGER (1887-1961 CE) date of discovery (1924 CE) Atom is composed of a nucleus containing protons and neutrons with electrons moving in a set of waves around the nucleus (electron cloud of probability). Established the wave mechanics formulation of quantum mechanics, which portrayed electrons as waves, spread out rather than in any given location.
LOUIS DE BROGLIE (1892-1987 CE) date of discovery (1924 CE) Atom is portrayed as a nucleus surrounded by an electron cloud. This model introduced the concept of sub-energy levels. Discovered the wave nature of electrons and suggested that all matter have wave properties Inspired the birth of wave mechanics.
DEMOCRITUS (460-370 BCE) Everything is composed of “atoms”, which are physically indivisible. Between atoms, there lies empty space. The solidity of a material corresponded to the shape of the atoms involved. Influenced others like the Roman philosopher Lucretius in the 1st century CE and other scientists during the Scientific Revolution.
ARISTOTLE (384-322 BCE) Atoms do not exist and everything is made up of 4 elements: earth (cool), air (light), fire (hot), water (wet). All matter is made up of varying levels of all four elements to take on their unique composition. His refusal to accept atoms as a scientific reality may have set the model of the atom back for centuries.
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