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Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. http://viewpure.com/IdTxGJjA4Jw

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Cathode Rays (electrons)

Observations: Cathode Ray Tube produces rays with constant charge to mass ratio.

Conclusions and Hypotheses:

Cathode rays (electrons) were found in all substances tested.

Cathode rays (electrons) were attracted to the positive plate every time.

Electrons are negatively charged.

Neutral atoms are made up of equal amounts of (+) and (-) particles.

All atoms contain tiny particles called electrons.

The electron has a specific size and charge.

Thomson’s Atomic Model (1897)

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Thomson-

Discovered electrons: cathode rays were

deflected by magnets and electric plates

Found charge-to-mass ratio for electrons

Proved electrons are a part of all atoms

Goldstein (1886) - Started with the hypothesis that when a neutral

hydrogen atom loses an electron, a positive particle should remain.

- Using cathode ray tube. He found rays traveling in the opposite direction.

- Concluded that these were made a positive subatomic particles, which would later be called protons.

Rutherford’s “Gold Foil Experiment”

Alpha particles are helium nuclei, He2+ Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Radioactive source

(+) http://viewpure.com/wzALbzTdnc8

Rutherford’s Findings (1911)

The atom is mostly empty space. The nucleus is dense. The nucleus is positively charged Electrons, e-, are moving large distances outside the nucleus.

Observations: Most of the alpha particles passed right through Some alpha particles were deflected slightly VERY FEW were greatly deflected

Conclusions:

Rutherford’s Conclusion (1911)…

Small, dense, positive nucleus.

Equal amounts of (-) electrons at large distances outside the nucleus.

Millikan http://viewpure.com/XMfYHag7Liw

In 1916, he discovered the charge of the electrons!

Chadwick (1932) discovered the neutron.

http://viewpure.com/_7DAlvRI1M4 fast forward to 5:00 min..

Neils Bohr’s Atomic model (1913)

Small, dense, positive nucleus. Equal amounts of (-) electrons at specific orbits around the nucleus.

This incorrect version of the atom is often used to represented atoms because it shows energy levels for electrons.

Bohr’s Model of the Atom • Bohr’s Model

- an early conceptual model of the atom

- classic planetary model in which electrons whirl around the small but dense nucleus: like planets orbiting the Sun

- developed by the Danish physicist Niels Bohr in 1911

- each electron has a certain energy that is determined by it’s path around the nucleus

Erwin Schrodinger (1926 ) Quantum Mechanical Model Like the Bohr model, the quantum mechanical model of the atom restricts the energy of electrons to certain values. Unlike the Bohr model, however, the quantum mechanical model does not involve an exact path the electron takes around the nucleus.

The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus.

If you need more review of the history of atomic theory: https://www.youtube.com/watch?v=NSAgLvKOPLQ