Nucleon
Protons and neutrons, the components of the atomic nucleus, are both classified as nucleons. Protons carry a positive electric charge, while the neutron is electrically uncharged, but they display the same behavior in their strong force interactions. These particles are classified as nucleons even when they are not inside an atom. Except for hydrogen, whose nucleus is composed of a single proton, all atomic nuclei consist of a combination both types of nucleons. The nucleons are the most stable hadrons, that is, particles affected by the strong nuclear force. Protons are so stable that they have never been observed to decay and they are believed to be stable for the entire lifetime of the universe. Although a free neutron decays to form a proton, an electron and an anti-neutrino in about 15 minutes, neutron bound in an atomic nucleus are stabilized and usually do not decay.
The mass of the proton is 1,836 times that of the electron and the mass of the neutron is a little more than 1,837 times the electron. Unlike the electrons, which act as though they are point particles, nucleons have a measurable size. By observing the scattering effects of particles on a beam of moving electrons, physicists have determined that the positive charge of the proton is distributed in a cloud extending about one femtometer (10-15 m) from its center. The neutron is about the same size as the proton.
Neutrons are electrically neutral overall, but experiments demonstrate an internal structure with a slight positive charge in the core surrounded by a shell of negative charge. Nucleons cannot penetrate one another and act like hard balls on contact. The distance between nucleons inside a nucleus is always the same. So the density of all nuclei is the same.
Nucleons are affected by the three forces of nature that act on all particles, including the electrons--gravitation, electromagnetism, and the weak force. However, nucleons also exert a very powerful force on each other--the strong, or nuclear, force. At very small distances between nucleons, less than one femtometer (10-15 m), the force is repulsive, pushing the nucleons apart. At distances of about one to two femtometers, roughly the diameter of a nucleon, the force is strongly attractive and binds nucleons together by energy millions of times stronger than the electromagnetic repulsion of charged protons. Beyond a few femtometers, the attraction of the strong force drops off rapidly with distance, having a much smaller effect than the nuclear particles than does the electromagnetic force. Because of the short range of the strong force effects, it is not felt outside an atom. Within the nucleus, the strong force overwhelms the electromagnetic repulsion of the charged protons, binding the nucleons together. The strong force interactions between nucleons are the same whether the two particles are protons, neutrons, or a proton and neutron. Because of their electromagnetic repulsion, two protons cannot be bound together without a neutron, so there is no helium isotope consisting only of two protons.
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