By: Hillary Nguyen, Sydney Lenz, Colin Blythe James Chadwick By: Hillary Nguyen, Sydney Lenz, Colin Blythe

The Problem With The Previous Model When Ernest Rutherford discovered the proton, scientists thought that the atomic structure was complete. He believed that negatively charged electrons orbited a tiny nucleus made up of positively charged protons. This model explained why the atom was neutral. Rutherford’s model failed to explain how the atom’s electrons are arranged in the space around the nucleus. It didn't explain why the negative charged electrons are not pulled into the positive nucleus.

Experimental Design Chadwick used a system called the neutron ionization detection chamber. A disk of polonium metal (the alpha source) was placed behind beryllium. The polonium gave a source of radiation, capable of kicking neutrons out of the beryllium nuclei.

Experimental Design (cont.) The radiation would hit the paraffin wax, which is high in protons. Some protons were knocked out of the paraffin wax. They used a Geiger counter, which clicks when radiation is detected, and he concluded that there were neutrons and it has the same mass as protons.

Reasoning Behind Developing a New Model The reasoning behind developing a new model was that the atomic number was less than the atomic mass. In 1932 the Joliot-Curies believed they had managed to remove protons from the wax using gamma rays. Chadwick thought that gamma rays were not strong enough to do this it was actually the radioactive element polonium. Chadwick came to the conclusion that the protons had been hit by neutrons. The existence of neutrons explained why atoms weighed more than their total mass of the protons and electrons.

Chadwick's New Atomic Model The nucleus of the atom contains protons and neutrons, and the nucleus is surrounded by electrons.

Interesting facts Chadwick was a prisoner for four years in Germany after he was given a scholarship to study there in the beginning of WWI. In 1943-1946, he was in the United States as Head of British Mission to work on the Manhattan Project.

Citations http://www.famousscientists.org/james-chadwick/ https://suite.io/isaac-m-mcphee/mn02y0 http://www.thocp.net/biographies/chadwick_james.htm http://atomic-molecular-optical-physics.suite101.com/article.cfm/the_discovery_of_the_neutron http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html http://www.vias.org/physics/bk2_05_05.html http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron1_1.htm