Atomic Theory
Greek philosopher Democritus (460 – 370 B. C.) proposed that:
Democritus had ideas – NOT science – ancient philosophers did not perform controlled experiments; he could not verify atoms existed.
Link: Models of the Atom
PDF: Atomic Models
DotPhysics: The Development of the Atom (Includes a video at the end)
John Dalton (1766 – 1844)
Dalton’s Atomic Theory:
Democritus and Dalton had similar theories, but Democritus stated that matter is mostly empty space through which atoms can move, and Dalton made no such claim. Dalton’s theory was based on extensive research and careful measurements, but it wasn’t completely accurate.
Sir William Crookes (1879) conducted experiments on electricity and developed the cathode ray tube.
J.J. Thomson (1897) used the cathode ray tube to discover negatively charged particles in atoms. Proposed the "plum pudding' model. PARTICLE DISCOVERED: Electron
Hantaro Nagaoka (1904) developed the Saturnian model of the atom.
Ernest Rutherford (1911) conducted an experiment in which he shot small, positive particles at some gold foil and noticed that while most went through, some particles bounced back. He inferred that most of the atom is empty space, but there is a very small, dense center to the atom that has a positive charge. Proposed the "nuclear" model. PARTICLE(S) DISCOVERED: Nucleus, and then later, Rutherford isolated the proton.
Robert Millikan (1913) did an oil-drop experiment that determined the charge and mass of an electron.
Niels Bohr (1913) demonstrated that negatively charged particles can occupy certain energy levels around the nucleus and used mathematics to support his findings. Proposed the "planetary" model (solar system).
Werner Heisenberg (1926) Proposed that electrons are particles that behave as waves. Came up with the Uncertainty Principle.
James Chadwick (1932) demonstrated that the atom’s nucleus contains particles that have mass but no charge. PARTICLE DISCOVERED: Neutron
Dmitri Mendeleev (1869) Developed the Periodic Law and re-organized the periodic table to account for missing elements.
The Quantum Mechanical Model (QMM) is a mathematical model based on probability and quantum theory, using Heisenberg's Uncertainty Principle as one of its tenets. Four quantum numbers are used to describe the behavior and location of electrons.
- matter is composed of atoms (called them atomos)
- matter is mostly empty space through which atoms can move
- atoms are indestructible and indivisible
- changes in matter are due to changes in changes in the grouping of atoms
Democritus had ideas – NOT science – ancient philosophers did not perform controlled experiments; he could not verify atoms existed.
Link: Models of the Atom
PDF: Atomic Models
DotPhysics: The Development of the Atom (Includes a video at the end)
John Dalton (1766 – 1844)
Dalton’s Atomic Theory:
- matter is composed of atoms
- atoms are indestructible and indivisible
- changes in matter are due to the changes in the grouping of atoms
- in chemical reactions, atoms separate, combine, or rearrange
- atoms cannot be divided into smaller particles
- elements are made of only one type of atom with the same size, mass, and properties
- different atoms combine in whole number ratios to form compounds (ex: 2 atoms of H to every 1 atom of O)
Democritus and Dalton had similar theories, but Democritus stated that matter is mostly empty space through which atoms can move, and Dalton made no such claim. Dalton’s theory was based on extensive research and careful measurements, but it wasn’t completely accurate.
Sir William Crookes (1879) conducted experiments on electricity and developed the cathode ray tube.
J.J. Thomson (1897) used the cathode ray tube to discover negatively charged particles in atoms. Proposed the "plum pudding' model. PARTICLE DISCOVERED: Electron
Hantaro Nagaoka (1904) developed the Saturnian model of the atom.
Ernest Rutherford (1911) conducted an experiment in which he shot small, positive particles at some gold foil and noticed that while most went through, some particles bounced back. He inferred that most of the atom is empty space, but there is a very small, dense center to the atom that has a positive charge. Proposed the "nuclear" model. PARTICLE(S) DISCOVERED: Nucleus, and then later, Rutherford isolated the proton.
Robert Millikan (1913) did an oil-drop experiment that determined the charge and mass of an electron.
Niels Bohr (1913) demonstrated that negatively charged particles can occupy certain energy levels around the nucleus and used mathematics to support his findings. Proposed the "planetary" model (solar system).
Werner Heisenberg (1926) Proposed that electrons are particles that behave as waves. Came up with the Uncertainty Principle.
James Chadwick (1932) demonstrated that the atom’s nucleus contains particles that have mass but no charge. PARTICLE DISCOVERED: Neutron
Dmitri Mendeleev (1869) Developed the Periodic Law and re-organized the periodic table to account for missing elements.
The Quantum Mechanical Model (QMM) is a mathematical model based on probability and quantum theory, using Heisenberg's Uncertainty Principle as one of its tenets. Four quantum numbers are used to describe the behavior and location of electrons.