21.1 Nuclear Structure and Stability

21.1 Nuclear Structure and Stability

• The basic idea of nuclear structure was introduced in the chapter on atoms, molecule, and ion.
• The number of protons in the nucleus is called the atomic number, and the mass number is called the mass number.

• There are different mass numbers for the same element.
• A nuclide is referred to by the name of the element followed by a hyphen and a mass number.

• The nucleus is small compared to the entire atom, which is 10 to 10 meters.
• Nuclei are denser than bulk matter, with an average density of 1.8 x 1014 grams.
• Water has a density of 1 gram per cm3 and iridium has a density of 22.6 g/ cm3.
• If the average nuclear density was equal to the earth's, the earth's radius would be about 200 meters.

• When the core of a very massive star collapses, the star's outer layers explode in a supernova.
• They are the densest known stars in the universe, with densities comparable to the average density of an atomic nucleus.
• A star with a mass equal to the solar mass of the sun has a diameter of 26 km.

• The U-235 nucleus can be treated as a sphere.

• The values are similar but the nucleus is more dense than the star.

• Find the density of a neutron star with a mass of 1.97 solar mass and a diameter of 13 km and compare it to the density of a hydrogen nucleus, which has a diameter of 1.75 fm.

• The star has a density of 3.4 x 1018 kg/m3.
• 6.0 x 1017 is the density of a hydrogen nucleus.
• The hydrogen nucleus is less dense than the neutron star.

• Strong attractive forces are needed to hold positively charged protons together in a small nucleus.
• There is a force between protons and neutrons.
• The attraction between opposite charges is different from the electrostatic force that holds negatively charged electrons around a positively charged nucleus.
• The strong nuclear force is much stronger than the electrostatic repulsions between protons, and it is notexistent over larger distances and outside the nucleus.

• There are four fundamental forces.

• The energy associated with the strong nuclear force can be seen in the helium atom, composed of two protons, two neutrons, and two electrons.

• The mass of an 42He atom is less than the combined mass of its six particles.
• In the case of helium, the mass defect indicates a loss in mass.
• The conversion of mass into energy that is evolved as the atom forms is what leads to the loss in mass.
• Nuclear reactions have greater energy changes than chemical reactions.

• When matter is converted into energy, this equation can be used.

• The 42He nucleus has a mass defect of 0.0305 amu.
• The mass-energy equivalence equation can be used to determine the binding energy.
• To accommodate the requested energy units, the mass defect must be expressed in kilograms.

• The mass defect is 0.0305 g/mol.
• To accommodate the units of the other terms in the mass-energy equation, the mass must be expressed in kilograms.
• The mass defect is 3.05 x 10-5 kg/mol.

• This tremendous amount of energy is associated with the conversion of a very small amount of matter.

• Remember that 1 eV is 1.602 x 10-19 J.

• The energy changes for breaking and forming bonds are small compared to the energy changes for breaking or forming nuclei, so the mass changes during chemical reactions are not visible.

• If a nucleus can't be transformed into another configuration without adding energy from the outside, it's stable.

• About 250 nuclides are stable.
• The stable isotopes fall into a narrow band according to the plot of the number of neutrons versus the number of protons.
• The lighter stable nuclei have the same number of protons and neutrons.
• Nitrogen 14 has seven protons and seven neutrons.
• The heavier the nucleus, the more neutrons it has.
• The stable nuclide lead- 207 has 125 neutrons and 82 protons, an n:p ratio of 1.52, while iron 56 has 30 neutrons and 26 protons, an n:p ratio of 1.15.

• Larger nuclei have more repulsions of protons and need larger numbers of neutrons to hold the nucleus together.

• The plot shows the nuclides that are stable.
• The stable nuclides are shown in blue and the unstable nuclides are shown in green.
• All elements with atomic numbers greater than 83 are unstable.
• The line is solid

• They change spontaneously into other nuclei that are close to the band of stability.
• The nature and products of this radioactive decay will be discussed in subsequent sections of this chapter.

• There is a relationship between the stability of a nucleus and its structure.

• It is more likely that the nucleus is stable if it has even numbers of protons, neutrons, or both.
• The complete shells of the nucleus are made by the numbers of protons or neutrons.
• The stable electron shells observed for noble gases are similar to these.

• Double magic is when he, 8 O, 20 Ca, and 82 Pb are all stable.
• The quantum mechanical model of nuclear energy states may be used to rationalize the trends in nuclear stability.
• The model is beyond the scope of this chapter.

• The nucleus is 28.4 MeV.

• The binding energy per nucleon is the largest for nuclides.

• The binding energy curve shows that the iron nuclide 56 26 Fe is one of the most stable nuclide.