9.1:
Radioactivity and Nuclear Equations
The densest region of an atom is the nucleus, containing protons and neutrons—collectively called nucleons.
The type of atom defined by a specific number of protons and neutrons is referred to as a nuclide. The notation for a nuclide comprises an element symbol, atomic number, and mass number.
One of several shorthand notations for nuclides uses the element name, a hyphen, and the mass number.
Nuclides with the same atomic number but different mass numbers are called isotopes of each other. Thus, carbon has three isotopes shown here.
Nuclides are also characterized by their energy state. For example, the single isotope technetium-99 exists in two different states: the lower energy ground state and a long-lived excited state called a metastable state. These two species, even though they have the same numbers of protons and neutrons, are different nuclides.
Interestingly, some elements in the periodic table have stable nuclides, which remain intact indefinitely. In contrast, some elements have only unstable nuclides, called radionuclides. For example, the spontaneous nuclear disintegration of uranium-238 produces thorium-234. The process is termed radioactive decay.
The daughter nuclide produced during the decay may be stable or radioactive. The process is accompanied by the emission of small fragments or electromagnetic radiation.
Alpha particles are equivalent to helium nuclei. Their emission reduces the atomic number by 2 and the mass number by 4.
Beta particles are equivalent to electrons; when emitted, the atomic number of the daughter nuclide increases by 1. As they carry a negative charge, it is called beta-minus radiation.
The emission of a positron, which has the same mass as an electron but opposite charge, decreases the atomic number by 1. It is often termed beta-plus emission.
Gamma rays are high-energy electromagnetic radiation, the emission of which changes neither atomic nor mass number.
Proton emission decreases the mass number and atomic number by 1 each, while neutron emission reduces the mass number by 1.
Nuclear equations map the difference between parent and daughter nuclides and indicate the nature of decay.
The radioactive decay of uranium-238 to thorium-234 is accompanied by the emission of alpha particles.
Nuclear equations are balanced just like chemical equations. The sum of the mass numbers is the same on each side of the equation. As this is alpha decay, so is the sum of the atomic numbers.
9.1:
Radioactivity and Nuclear Equations
Nuclear chemistry is the study of reactions that involve changes in nuclear structure. The nucleus of an atom is composed of protons and, except for hydrogen, neutrons. The number of protons in the nucleus is called the atomic number (Z) of the element, and the sum of the number of protons and the number of neutrons is the mass number (A). Atoms with the same atomic number but different mass numbers are isotopes of the same element.
A nuclide of an element has a specific number of protons and neutrons and is in a specific nuclear energy state. The notation for a nuclide is 
, where X is the symbol for the element, A is the mass number, and Z is the atomic number. There are also several shorthand notations for nuclides, many of which omit the atomic number. For example, 
may be written carbon-14, C-14, or 14C.
If the nuclide is in a temporary excited state, this is typically denoted with an asterisk. If it is in a longer-lived excited state, called a metastable state, this is denoted by adding ‘m’ to the mass number. For example, the isotope technetium-99 exists as ground-state 
and metastable 
. If there is more than one metastable state for a given isotope, they are numbered in increasing energy order. For example, the isotope tantalum-180 has five nuclides: ground-state 
and metastable states 
, 
, 
, and 
.
Nuclear reactions are the transformations of one or more nuclides into another, which occur via changes in the atomic numbers, mass numbers, or nuclear energy states of nuclei. To describe a nuclear reaction, we use an equation that identifies the nuclides and particles involved in the reaction. As with chemical reactions, nuclear reactions obey conservation of mass: the sum of the mass numbers of the reactants equals the sum of the mass numbers of the products.
Many different particles or photons can be involved in nuclear reactions. The most common include alpha particles (α or 
), which are high-energy helium-4 nuclei; beta particles (β), which include electrons (e− or β−) and positrons (e+ or β+); gamma rays (γ); neutrons (
); and protons (p+ or 
).
Some nuclides remain intact indefinitely, or are stable, whereas others spontaneously transform into other nuclides, or are unstable. The spontaneous change of an unstable nuclide into another is radioactive decay. The unstable nuclide is called the parent nuclide, and the nuclide that results from the decay is known as the daughter nuclide. The daughter nuclide may be stable, or it may decay itself.
This text is adapted from Openstax, Chemistry 2e, Section 21.1: Nuclear Structure and Stability and Openstax, Chemistry 2e, Section 21.2: Nuclear Equations.
Additional Sources
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. https://doi.org/10.1351/goldbook. Accessed 2021-01-10
International Atomic Energy Agency, Nuclear Data Section. Live Chart of Nuclides. https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html. Accessed 2021-01-10