III.

Neutrons in Atoms

Even before the discovery of neutrons in 1932, physicists realized that atomic nuclei must have an electrically neutral component. The mass of nearly every hydrogen atom is equal to the sum of one proton and one electron. However, for any other atom, the atomic mass is larger than the sum of the electron and proton masses. The neutrons are responsible for the remaining mass of the atom.

Neutrons play an important role in the stability of a nucleus. In the nucleus, two neighboring protons repel each other with an electrical force that is 100 million times stronger than the electrical attraction that binds the electrons around the positively charged nucleus. Protons and neutrons are bound together by the strong nuclear force. Certain combinations of neutrons and protons bind together especially tightly. An example is the helium nucleus, also called an alpha particle, which contains two protons and two neutrons.

Embedded in a nucleus, a neutron is usually stable—that is, it will not decay into a proton and an electron. The nucleus itself is then stable. However, if the nuclear conditions are not optimal—for example, if the nucleus has too many neutrons—one or more of the neutrons may decay. Scientists describe unstable nuclei as being radioactive and describe their changes as nuclear reactions. An example of a radioactive nucleus occurs in the element carbon. Carbon mainly consists of carbon-12 (with six protons and six neutrons) and a small amount of carbon-14 (with six protons and eight neutrons). Carbon-14 is radioactive—its combination of protons and neutrons is unstable and a neutron in its nucleus can decay. When an unstable carbon-14 neutron decays, it splits into a proton and an electron. With seven protons and seven neutrons, the atom is now nitrogen. This natural decay is used in a process called carbon dating to determine the age of anything that was once living matter, such as fossils, wood, and natural fabrics (see Dating Methods: Carbon-14 Method). The carbon content of living matter is continually renewed, so the proportion of carbon-12 to carbon-14 remains the same. Once the organism dies, the carbon is no longer renewed. Because scientists know how much carbon-14 was present in the beginning and how long it takes for the carbon-14 to decay, they can determine the age of a relic by measuring the residual amount of carbon-14 in the object.

In another form of radioactivity, nuclear fission, the unstable nucleus of a large atom splits into two roughly equal smaller nuclei, losing several spare neutrons in the process and releasing energy. The fast-moving free neutrons usually pass through matter, but each one can be captured more easily by another nucleus after the neutron loses some energy and slows down. If a free neutron hits a large nucleus, such as that of uranium, the nucleus can capture it and become unstable. Each new unstable nucleus splits into two roughly equal smaller nuclei and creates more spare neutrons and more energy. Those spare neutrons can then strike more large nuclei to repeat the process in a chain reaction. See also Nuclear Chemistry and See also Nuclear Energy: Nuclear Energy from Fission