James Chadwick was born in Cheshire, England on October 20, 1891, the son of John Joseph Chadwick and Anne Mary Knowles. He went to Manchester High School, and studied at the Universities of Manchester and Cambridge. In 1913 Chadwick went to work with Hans Geiger at the Technical University of Berlin. He also worked with Ernest Rutherford. He acted as an Allied spy in World War I at the Zivilgefangenenlager concentration camp in Ruhleben, Germany.
After the war, Chadwick returned to Cambridge where he worked with Ernest Rutherford in investigating the emission of gamma rays from radioactive materials. They also studied the transmutation of elements by bombarding them with alpha particles, and investigated the nature of the atomic nucleus.
In 1932 Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge. In contrast with the helium nuclei (alpha particles) which are positively charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. In this way, Chadwick prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. For this important discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935. Later, he found out that a German scientist had discovered the neutron at the same time. But Hans Falkenhagen (Rostock) was afraid of publishing his results. When Chadwick learned of Falkenhagen's discovery, he offered to share the Nobel Prize with him. Falkenhagen, however, modestly refused the honor. He was also nicknamed "Jimmy Neutron" because of his discovery of the neutron.
Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory. His discovery particularly inspired Enrico Fermi, Italian physicist and Nobel laureate, to discover nuclear reactions brought by slowed neutrons, and led Otto Hahn and Fritz Strassmann, German radiochemists in Berlin, to the revolutionary discovery of “nuclear fission”, which triggered the development of the atomic bomb.
Chadwick became professor of physics at Liverpool University in 1935. As a result of the Frisch-Peierls memorandum in 1940 on the feasibility of an atomic bomb, he was appointed to the MAUD Committee that investigated the matter further. He visited North America as part of the Tizard Mission in 1940 to collaborate with the Americans and Canadians on nuclear research. Returning to England in November 1940, he concluded that nothing would emerge from this research until after the war. In December 1940 Franz Simon, who had been commissioned by MAUD, reported that it was possible to separate the isotope uranium-235. Simon's report included cost estimates and technical specifications for a large uranium enrichment plant. James Chadwick later wrote that it was at that time that he "realised that a nuclear bomb was not only possible, it was inevitable. I had then to start taking sleeping pills. It was the only remedy."
He shortly afterward joined the Manhattan Project in the United States, which developed the atomic bombs dropped on Hiroshima and Nagasaki. Chadwick was knighted in 1945.
After the war, Chadwick moved to Cambridge University as master of Gonville and Caius College.
He died in Cambridge on 24 July 1974.