A nuclear reaction is a response to disequilibrium in the nucleus of an atom. The responders to the disequilibrium are the nucleons (the neutrons and the protons) in the nucleus of the atom. The three primary nuclear reactions are: Radioactivity, fission and fusion. These nuclear reactions are expressions of Pj problems.
Radioactivity is the emission of particles or energy (radiation) from the the nucleus of an atom undergoing spontaneous disintegration (also called spontaneous nuclear decay or radioactive decay). An atom that undergoes radioactivity is said to be radioactive.
Figure NR1 indicates the atomic mass (A), the number of protons (Z) and the symbol (X) of an arbitrary atom. The difference A - Z is the number of neutrons in the atom. Atoms with the same atomic number but different atomic mass are called isotopes. The nucleus of a specific isotope is called a nuclide. The following are four types of radioactivity:
(1) Alpha emission: this type emits an alpha particle (24He) from the decaying nucleus. For example, Uranium-238 emits an alpha particle as it decays to Thorium-234. Nuclides with atomic numbers greater than 83 often emit alpha particles. The nuclear equation is as follows:
92238He ---> 90234Th + 24He
(2) Beta emission: this type emits a beta particle (high-speed electron, -10e) from the decaying nucleus. In other words, a beta emission is a conversion of a neutron (01n) into a proton (11H). The product nuclide gains a proton. The electron has mass, however, its mass is approximated to zero because it is very small relative to the mass of the nuclide. The following equation illustrates this conversion:
01n ---> 11H + -10e
For example, a carbon-14 nucleus decays into a nitrogen-14 nucleus by emitting a beta particle.
614C ---> 714N + -10e
(3) Positron emission: this type emits a positron ( a particle with the same mass as an electron but with a positive charge). Positron emission is a conversion of a proton to a neutron
11H ---> 10n + 10e
For example, a sodium-22 decays to a neon -22 nucleus by emitting a positron.
1122Na ---> 1022Ne + 10e
(4) Gamma emission: this type emits pure energy (a photon) with a single wavelength from an excited nucleus said to be in a metastable state that is in many instances a consequence of an alpha emission, or beta emission, or positron emission. This excited nucleus calms down by emitting a massless and chargeless gamma ray. For example, a metastable Technetium-99 (4399mTc) calms down to Technetium-99 by emitting a gamma ray (00γ)
4399mTc ---> 4399Tc + 00γ
Radioactivity is essentially a nucleus' search for equilibrium. This stability is sometimes not realized after a single decay step. For example, the product nuclide of a decay step may itself be unstable and so it may decay into another product nuclide which itself is unstable and so must decay. The sequence of decay steps necessary to realize a stable nucleus is called a decay series. In general, most naturally occurring radioactive nuclides fall into one of these decay series: (a) Uranium-238 --> Lead-206 (b) Uranium-235 --> Lead-207 (c) Thorium-232 --> Lead-208. Radioactive carbon-14 and potassium-40 occur naturally in small quantities but they are not in these series.
The stability of a nuclide determines its spontaneous nucleic behaviour. The atomic mass of most stable nuclides is about 50. Nuclides with atomic mass greater than 50 might have the tendency to split into two lighter and more stable nuclei. This splitting is called fission. Non-spontaneous fission can be induced by bombarding heavy nuclei with neutrons. For example, uranium-235 will split in many different ways when it is bombarded with neutrons and if a supercritical mass of uranium-235 is used, the fission can be a chain reaction which is a self-sustaining fission that is due to sufficient availabitity of uranium-235 and sufficient product neutrons. A rapid chain reaction in uranium-235 releases an immense amount of energy. This the reason it is a primary component of atomic bombs. Uranium-235 is only about 0.7% of naturally occurring uranium. So uranium-238 which is naturally abundant is enriched to 90% uranium-235 for its use in atomic bombs.
Nuclides with atomic mass less than 50 combine to form heavier more stable nuclei. This process is called fusion. Fusion is the principal nuclear reaction in celestial stars. For example, the Sun which is about 75% hydrogen and 25% helium, produces its helium from the fusion of hydrogen nuclei. A lot of energy is released in both fission and fusion.
Containership (nucleons in the nucleus of an atom); identity (neutrons and protons); force (weak force in spantaneous nuclear decay, neutron bombardment in fission); motion (high-speed electrons in beta emission, high-speed neutron in nuclide bombardment); change (formation of new nuclides); grouping/interaction (combination of nuclides in fusion, combination of neutrons and nuclides in chain reactions) and equilbrium (realization of stable nuclei) are Pj Problems expressed in nuclear reactions.