Nuclear physics

9.2.1 Nuclear building blocks

We have already seen that the positive charge in atoms is confined to the small nucleus (10—13cm across). The nucleus is composed of protons and neutrons. Because they are the building blocks of the nucleus, we call them nucleons. The proton has charge +e (where — e is the charge on the electron). Since atoms are neutral, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. The chemical properties of an atom depend on the number of orbiting electrons. Therefore, the number of protons in the nucleus ultimately determines the identity of the element. We designate the number of protons by the symbol Z, called the atomic number. The charge on the nucleus is +Ze. The highest naturally occurring value of Z is 92 (uranium), with approximately a dozen "man-made" elements with higher values of Z.

Neutrons are electrically neutral. The mass of the neutron is slightly greater than the mass of the proton. Nuclei with the same numbers of protons can have different numbers of neutrons N. The total number of nucleons in the nucleus is called the mass number, A = Z + N. The mass of a given nucleus is approximately Amp. Two nuclei with the same number of protons but different numbers of neutrons are called isotopes of the same element. We generally designate an element by a letter symbol (e.g. H for hydrogen), preceded by a superscript giving A, followed by a subscript giving Z. (This subscript is redundant since the symbol tells us Z, and is sometimes left out.) For example, the three known isotopes of hydrogen are 1H1, 2H1, 3H1 or simply 1H, 2H, 3H.

For any given number of protons, we do not find an arbitrary number of neutrons. In general, we find that the stable elements have approximately equal numbers of protons and neutrons. For larger values of Z the stable nuclei have a slightly larger number of neutrons than protons.

We now look at the force that holds nuclei together. We generally refer to four forces in nature. In order of decreasing strength, they are strong nuclear, electromagnetic, weak nuclear and gravity. The nuclear forces are shorter ranged, as

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