ATOMS AND NUCLEI
The atomic nucleus is a tiny massive entity at the center of an atom. Occupying a volume whose radius is 1/100,000 the size of the atom, the nucleus contains most (99.9%) of the mass of the atom. In describing the nucleus, we shall describe its composition, size, density, and the forces that hold it together. After describing the structure of the nucleus, we shall go on to describe some of the limits of nuclear stability.
The nucleus is composed of protons (charge = +1; mass = 1.007 atomic mass units ([μ]) and neutrons. The number of protons in the nucleus is called the atomic number Z and defines which chemical element the nucleus represents. The number of neutrons in the nucleus is called the neutron number N, whereas the total number of neutrons and protons in the nucleus is referred to as the mass number A, where A = N + Z. The neutrons and protons are referred to collectively as nucleons. A nucleus with a given N and Z is referred to as a nuclide. Nuclides with the same atomic number are isotopes , such as 12 C and 14 C, whereas nuclides with the same N, such as 14 C and 16 O, are called isotones. Nuclei such as 14 N and 14 C, which have the same mass number, are isobars. Nuclides are designated by a shorthand notation in which one writes , that is, for a nucleus with 6 protons and 8 neutrons, one writes , or, , or just 14 C. The size of a nucleus is approximately 1 to 10 × 10 −15 m, with the nuclear radius being represented more precisely as 1.2 × A 1/3 × 10 −15 m. We can roughly approximate the nucleus as a sphere and thus we can calculate its density
where 1.66 × 10 −27 kg is the mass of the nucleon. Thus the nuclear density is about 200,000 tonnes/mm 3 and is independent of A. Imagine a cube that is 1 mm on a side. If filled with nuclear matter, it would have a mass of about 200,000 tonnes. This calculation demonstrates the enormous matter/energy density of nuclei and gives some idea as to why nuclear phenomena lead to large energy releases.
Of the 6,000 species of nuclei that can exist in the universe, about 2,700 are known, but only 270 of these are stable. The rest are radioactive, that is, they spontaneously decay. The driving force behind all radioactive decay is the ability to produce products of greater stability than one had initially. In other words, radioactive decay releases energy and because of the high energy density of nuclei, that energy release is substantial. Qualitatively we describe radioactive decay as occurring in three general ways: α -, β -, and γ -decay. Alpha-decay occurs in the heavy elements, and consists of the emission of a 4 He nucleus. Beta-decay occurs in nuclei whose N/Z ratio is different from that of a stable nucleus and consists of a transformation of neutrons into protons or vice versa to make the nucleus more stable. Gamma-decay occurs when excited nuclei get rid of some or all of their excitation energy via the emission of electromagnetic radiation, or via the radiationless transfer of energy to orbital electrons.
The force responsible for holding the neutrons and protons together within the very small nuclear volume must be unusually strong. The nuclear force, or strong interaction, is one of the four fundamental forces of nature (namely, the gravitational, electromagnetic, strong, and weak forces). The nuclear force is charge-independent, meaning that the nuclear force between two protons, or two neutrons, or a neutron and a proton, is the same. The nuclear force is short-ranged, meaning it acts over a distance of 10 −15 to 10 −14 m, that is, the size of nuclei. Of course the nuclear force is attractive, as it binds the nucleons in a nucleus. But some experiments have shown the nuclear force has a "repulsive core," meaning that at very short distances, the force switches from attractive to repulsive, preventing the nucleus from collapsing in on itself. The nuclear force is an "exchange" force, resulting from the virtual exchange of pions (short-lived particles with integra
The atomic nucleus is a tiny massive entity at the center of an atom. Occupying a volume whose radius is 1/100,000 the size of the atom, the nucleus contains most (99.9%) of the mass of the atom. In describing the nucleus, we shall describe its composition, size, density, and the forces that hold it together. After describing the structure of the nucleus, we shall go on to describe some of the limits of nuclear stability.
The nucleus is composed of protons (charge = +1; mass = 1.007 atomic mass units ([μ]) and neutrons. The number of protons in the nucleus is called the atomic number Z and defines which chemical element the nucleus represents. The number of neutrons in the nucleus is called the neutron number N, whereas the total number of neutrons and protons in the nucleus is referred to as the mass number A, where A = N + Z. The neutrons and protons are referred to collectively as nucleons. A nucleus with a given N and Z is referred to as a nuclide. Nuclides with the same atomic number are isotopes , such as 12 C and 14 C, whereas nuclides with the same N, such as 14 C and 16 O, are called isotones. Nuclei such as 14 N and 14 C, which have the same mass number, are isobars. Nuclides are designated by a shorthand notation in which one writes , that is, for a nucleus with 6 protons and 8 neutrons, one writes , or, , or just 14 C. The size of a nucleus is approximately 1 to 10 × 10 −15 m, with the nuclear radius being represented more precisely as 1.2 × A 1/3 × 10 −15 m. We can roughly approximate the nucleus as a sphere and thus we can calculate its density
where 1.66 × 10 −27 kg is the mass of the nucleon. Thus the nuclear density is about 200,000 tonnes/mm 3 and is independent of A. Imagine a cube that is 1 mm on a side. If filled with nuclear matter, it would have a mass of about 200,000 tonnes. This calculation demonstrates the enormous matter/energy density of nuclei and gives some idea as to why nuclear phenomena lead to large energy releases.
Of the 6,000 species of nuclei that can exist in the universe, about 2,700 are known, but only 270 of these are stable. The rest are radioactive, that is, they spontaneously decay. The driving force behind all radioactive decay is the ability to produce products of greater stability than one had initially. In other words, radioactive decay releases energy and because of the high energy density of nuclei, that energy release is substantial. Qualitatively we describe radioactive decay as occurring in three general ways: α -, β -, and γ -decay. Alpha-decay occurs in the heavy elements, and consists of the emission of a 4 He nucleus. Beta-decay occurs in nuclei whose N/Z ratio is different from that of a stable nucleus and consists of a transformation of neutrons into protons or vice versa to make the nucleus more stable. Gamma-decay occurs when excited nuclei get rid of some or all of their excitation energy via the emission of electromagnetic radiation, or via the radiationless transfer of energy to orbital electrons.
The force responsible for holding the neutrons and protons together within the very small nuclear volume must be unusually strong. The nuclear force, or strong interaction, is one of the four fundamental forces of nature (namely, the gravitational, electromagnetic, strong, and weak forces). The nuclear force is charge-independent, meaning that the nuclear force between two protons, or two neutrons, or a neutron and a proton, is the same. The nuclear force is short-ranged, meaning it acts over a distance of 10 −15 to 10 −14 m, that is, the size of nuclei. Of course the nuclear force is attractive, as it binds the nucleons in a nucleus. But some experiments have shown the nuclear force has a "repulsive core," meaning that at very short distances, the force switches from attractive to repulsive, preventing the nucleus from collapsing in on itself. The nuclear force is an "exchange" force, resulting from the virtual exchange of pions (short-lived particles with integra
