The Atomic Story So Far… The model of the atom had all of the positive charged particles, the proton, in a central nucleus. Electrons were moving in the region outside of the nucleus. The question was, “How do all the positive protons stay together when they should repel each other?” Rutherford predicted that a neutral particle would be found in the nucleus.

Rutherford’s gold foil experiment lead scientists to try different materials. Bothe and Becker tried using Polonium to emit alpha particles, hitting Beryllium foil and creating neutral emissions that scientists then tried to explain.

The Analysis: Curie-Jolie’s Interpretation They said gamma radiation was coming from the collision of alpha particles with the Beryllium foil. This “gamma radiation” was causing the protons to be ejected from the paraffin. Chadwick said they were wrong!

Animation of Chadwick’s Experiment Equipment used Process inside link

The Data from the Experiment The particle coming from the beryllium foil had no charge. There was not a direct way to measure it. The paraffin target when hit by the neutral particles emitted a positive particle. The positive particles coming from the paraffin could be detected by an ionization chamber and were identified as protons, already discovered particles.

Chadwick’s Analysis However, as Chadwick knew from the Compton effect, high energy photons could only eject small particles (electrons). Gamma radiation would be unable to eject a proton that is 2000 times bigger than an electron.

Chadwick’s Inference Therefore, in order to eject a particle as large as a proton, the particle hitting the paraffin, needed to have greater momentum than the proton. Thus the mass of the neutron needed to close to the size of a proton. Chadwick tested his idea with Helium, Nitrogen and other elements as targets of his proposed neutron emissions to confirm his conclusion. He named this particle, NEUTRON, as it had a neutral or no charge. He received the Nobel Prize in 1935 for this work.

HE CALLS NEUTRON 'DIFFICULT CATCH‘! CAMBRIDGE, England, Feb. 28, 1932 -- Dr. James Chadwick, 41-year-old Cambridge physicist, last week discovered a new particle in the nucleus: The NUETRON! HE CALLS NEUTRON 'DIFFICULT CATCH‘!

Take Home Message What we now know about the neutron: Chadwick is known for the discovery of the neutron Neutrons seem to be buffers between the positive protons in the nucleus As Neutrons are without an +/- electric charge they were harder to discover

Sources High School reading http://www.light-science.com/chadwick.html College level reading http://www.chemcases.com/nuclear/nc-01.htm Sites with animation: Neutron emissions: http://images.google.com/imgres?imgurl=http://www-outreach.phy.cam.ac.uk/camphy/neutron/side1_neutron.jpg&imgrefurl=http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron1_1.htm&usg=__O9XFEfmyQldpXyAES9rQr-UTmX4=&h=180&w=225&sz=10&hl=en&start=11&um=1&tbn Compton effect: http://www.youtube.com/watch?v=zYZNTzviBxs Sites with images: http://images.google.com/imgres?imgurl=http://www.kutl.kyushu-u.ac.jp/seminar/MicroWorld3_E/3Part1_E/3P13_E/Chadwick_apparatus_E.jpg&imgrefurl=http://www.kutl.kyushu-u.ac.jp/seminar/MicroWorld3_E/3Part1_E/3P13_E/DiscoverNeutron_E.htm&usg=__J5CvRl9twV1uwes8oCnOSfHIjCw=&h=278&w=393&sz=14&hl=en&start=2&sig2=skioJSJgAUt7dRvft1JIgA&um=1&tbnid=-ACfR5ilzHkymM:&tbnh=88&tbnw=124&prev=/images%3Fq%3DChadwick%2Bneutron%26hl%3Den%26rls%3Dcom.microsoft:en-us%26sa%3DN%26um%3D1&ei=B8o2SqLiD5GktAOFoP3HBg http://images.google.com/imgres?imgurl=http://library.thinkquest.org/27954/Image24.gif&imgrefurl=http://library.thinkquest.org/27954/neutron.html&usg=__sY1svk3fUD7WUf23On9Ew9ZM9qE=&h=319&w=594&sz=7&hl=en&start=4&sig2=ovtFGJcDVSfI8NsIOxIHlw&um=1&tbnid=EuRho_xoSRTHOM:&tbnh=73&tbnw=135&prev=/images%3Fq%3DChadwick%2Bneutron%26hl%3Den%26rls%3Dcom.microsoft:en-us%26sa%3DN%26um%3D1&ei=B8o2SqLiD5GktAOFoP3HBg http://images.google.com/imgres?imgurl=http://resources.edb.gov.hk/physics/articlePic/AtomicPhysics/matterstr_pic02e.gif&imgrefurl=http://resources.edb.gov.hk/physics/articleIE/matterstructure/matterstructure_e.htm&usg=__GnKhxNQmLQ0fCHGseVz7bBmIpZo=&h=224&w=454&sz=15&hl=en&start=9&sig2=QFi7WFlEK4qai4i8NdYlmA&um=1&tbnid=afqKYNXS32XoLM:&tbnh=63&tbnw=128&prev=/images%3Fq%3DChadwick%2Bneutron%26hl%3Den%26rls%3Dcom.microsoft:en-us%26sa%3DN%26um%3D1&ei=B8o2SqLiD5GktAOFoP3HBg