James Chadwick

Born on October 20, 1891, James Chadwick won the Nobel Prize in Physics in 1935 for his discovery of the neutron.

In 1932 Chadwick discovered a nuclear particle that did not have any charge (one of his letters announcing the discovery can be found here). These chargeless particles, called neutrons, differ from the previously discovered protons in that they do not have any charge. Because they do not have any charge they can be combined with other nuclei without having to overcome electrostatic repulsion.

Atomic nuclei are composed of two different types of particles, protons and neutrons. They both have about the same mass (neutrons are slightly more massive than protons) but differ in the amount of electrostatic charge they carry. Protons carry a positive charge and neutrons have no charge. Because they are all positively charged protons repel each other and therefore it is difficult to add alpha particles (alpha particles have two protons) to a nucleus. A neutron has no charge and can be used to bombard a nucleus without having to overcome the electrostatic charge.

Chadwick also discovered that atomic number is determined by the number of protons that are found in a nucleus. This is now the definition of atomic number. What element a particular atom is is determined by the number of protons present in its nucleus. For example, 1 proton is hydrogen, two protons is helium, three protons is lithium, etc. A full list of atom:atomic number corespondences can be found on a periodic table.

Chadwick later work with particle accelerators contributed to the making of the atomic fission bomb.

For discovering the neutron James Chadwick is the Dead Scientist of the Week for the week of October 18-24, 2009.

References:

James Chadwick Wikipedia entry

James Chadwick Nobel biography

James Chadwick Answers. com biography