The History of the

ATOMATOM

Who are these men?In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called…

ATOMAATOMA

(greek for indivisible or cannot

be divided)

This theory was ignored and forgotten for more than 2000 years!

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808 John Dalton

• Suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them…

ATOMSATOMS

Dalton’s Theory

1. All elements are composed of atoms.

2. Atoms of the same element are exactly alike.

3. Atoms of different elements are different.

4. Compounds are formed by the joining of atoms of two or more elements.

Dalton’s Atom

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

• Found that atoms could sometimes eject a far smaller negative particle which he called an…

ELECTRONELECTRON

• To learn about J.J.Thomson’s cathode ray tube experiment, click here

HISTORY OF THE ATOMHISTORY OF THE ATOM

• Thomson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere. • The electrons are surrounded by a soup of positive charge to balance the electron's negative charge like plums surrounded by pudding.

1904

PLUM PUDDINGPLUM PUDDING

MODELMODEL

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest Rutherford

• Oversaw Geiger and Marsden carrying out his famous experiment.

• Fired Helium nuclei at a piece of gold foil which was only a few atoms thick.

• Found that although most of them passed through. About 1 in 10,000 hit

HISTORY OF THE ATOMHISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back.

helium nuclei

HISTORY OF THE ATOMHISTORY OF THE ATOM

• Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleuscentral nucleus.

• He suggested that the positive chargepositive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction.

However, this was not the end of the story.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

• Studied under Rutherford at the Victoria University in Manchester.

• Refined Rutherford's idea by adding that the negatively charged electrons were in orbitsorbits, like planets orbiting the sun.

• Each orbit is only able to contain a set number of electrons.

• Calculated the exact energy levels for the hydrogen atom.

Bohr’s Atomic Theory

• The Bohr model was a modification of the Rutherford model.

• Bohr placed electrons in certain circular orbitsorbits around the nucleus called shellsshells.

+

• Proposed the electron cloud.

• Stated that the electron do not travel in exact orbits but travel in clouds around the nucleus.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1926 Erwin Schrodinger

Electron cloud model

* Although this is an important model to recognize, we will be focusing on the Bohr Model in class.

• Scientists could not figure out what made atoms so heavy.

• Chadwick proposed the idea of neutrons—neutral particles that are located in the nucleus with the protons.

• The new Bohr model includes these particles.

0

Neutrons

HISTORY OF THE ATOMHISTORY OF THE ATOM

1932 James Chadwick

New Bohr Model of a HELIUM ATOM

+N

N

+-

-

proton

electron neutron

Shell

Today’s Understanding of the Atom

With the help of new technology, such as particle accelerators, scientists now think protons and neutrons are made of smaller particles.

Evolution of the Atomic

Model