Atom Timeline

By: Lizzy Edwards

John Dalton's Model of the Atom... (1803)

Five Parts to his Theory -

1. Elements are made of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

J.J. Thomson's Model of the Atom... (1897)

The electron, a negatively charged part of every atom. Though several alternative models were advanced in the 1900's by Lord Kelvin and others, Thomson held that atoms are uniform spheres of positively charged matter in which electrons are embedded. Popularly known as the plum-pudding model, it had to be abandoned (1911) on both theoretical and experimental grounds in favor of the Rutherford atomic model.

Hantaro Nagaoka's Model of the Atom... (1904)

Nagaoka rejected Thomson's model on the grounds that opposite charges are impenetrable. In 1904, Nagaoka proposed an alternative planetary model of the atom in which a positively charged center is surrounded by a number of revolving electrons, in the manner of Saturn and its rings.

- Nagaoka's model made two predictions:

1. A very massive atomic center (in analogy to a very massive planet)

2. Electrons revolving around the nucleus, bound by electrostatic forces (in analogy to the rings revolving around Saturn, bound by gravitational forces).

Ernest Rutherford's Model of the Atom... (1911)

By 1911 the components of the atom had been discovered. The atom consisted of subatomic particles called protons and electrons. However, it was not clear how these protons and electrons were arranged within the atom. J.J. Thomson suggested the "plum pudding" model. In this model the electrons and protons are uniformly mixed throughout the atom.

Louis de Broglie's Model of the Atom... (1924)

In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.

Erwin Schrodinger's Model of the Atom... (1926)

Four Parts to his Theory -

1. Electrons occupy only certain orbits around the nucleus. Those orbits are stable and called "stationary" orbits.

2. Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.

3. Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.

4. The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.

James Chadwick's Model of the Atom... (1932)

The theory at the time was that there were "nuclear electrons" in the atomic nucleus, along with additional protons. The extra protons were thought to provide the extra atomic mass, while the additional electrons would cancel out their positive charge, leaving the atom electrically neutral. Eventually, however, calculations using Heisenberg's uncertainty principle showed it was not possible for electrons to be contained in the nucleus.
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