Nuclear Chemistry

Nuclear chemistry involves the alteration of the nuclei of atoms. Here are some general terms to know:

  • Atomic number is the number of protons in an atom’s nucleus.
  • Mass number is the sum of the number of protons and the number of neutrons in an atom’s nucleus.
  • Where Z is atomic number, A is mass number, and X is an element, a nucleus can be represented as follows: For instance, deuterium, a hydrogen atom with one neutron, is represented as
  • Isotopes are different nuclei of atoms with the same atomic number but with different numbers of neutrons. They are written as “Element-A.” For instance, hydrogen has three isotopes: hydrogen-1, hydrogen-2 (deuterium), and hydrogen-3 (tritium).
  • Nucleons are protons and neutrons – elementary particles that lie in the nucleus. Electrons are not.
  • A nuclide is a nucleus with a specific number of protons and neutrons.

Mass Defect

The mass of a nucleus is less than the actual combined mass of its constituent protons and neutrons. This mass defect is due to the fact that some of the mass has been converted to energy in accordance to Albert Einstein’s famous equation E=mc2, where E is energy, m is mass, and c is the speed of light (about 299,792,458 m/s). This mass defect corresponds to the nuclear binding energy, which is derived from the strong nuclear force and is the energy required to disassemble a nucleus into the same number of free unbound neutrons and protons.

Fusion and Fission

Fusion is a nuclear reaction in which nuclei combine to form more massive nuclei. Fusion accounts for the energy of the sun, where hydrogen fuses to form helium, releasing energy. Fission is a nuclear reaction in which a massive nucleus splits into smaller nuclei. Both fusion and fission are very exothermic (though fusion is typically more exothermic). Atoms with mass numbers smaller than that of iron tend to undergo fusion, while atoms with mass numbers greater than that of iron tend to undergo fission.

Types of Nuclear Decay

Radioactive decay is the spontaneous decay of nuclei. Radioactive decay is a first-order process and thus obeys first-order rate calculations.

Alpha Decay

In alpha decay, a nucleus releases an alpha (α) particle – a helium nucleus. Thus a nuclei that undergoes alpha decay experiences a decrease in mass number of 4 and a decrease in atomic number of 2. For instance, uranium-238 alpha decays:

Alpha decay is a mode of radioactive decay seen only in heavier nuclides.

Beta Decay

Beta decay is a type of radioactive decay in which a beta (β) particle – an electron – is emitted. In the process, a neutron is converted to a proton. Atoms that undergo beta decay experience an increase in atomic number by 1, but no change in mass number. For instance,  cesium-137 beta decays:

Positron Emission

In positron emission, energy is used to convert a proton into a neutron and a positron (β+) - an antiparticle with the same mass but opposite charge as an electron) is released. Thus, atomic number decreases by 1 and mass number is unchanged. For instance, sodium-22 emits positrons:

Electron Capture

In electron capture, an electron is captured by a nucleus. A proton is converted into a neutron, so atomic number decreases by 1, while mass number remains the same. For instance, aluminum-26 can undergo electron capture:

Half-Life

The half-life of an isotope can be calculated as follows, where lambda (λ) represents the decay constant.

Thus, where N0 is the original amount of radioactive substance and t represents elapsed time, we can use exponential principles to solve half-life problems involving nuclear decay:

e is a mathematical constant that is approximately 2.718.

 

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