11.S: Nuclear Chemistry (Summary)
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In most atoms, a nucleus containing an “excess” of neutrons (more neutrons than protons) is unstable and the nucleus will decompose by
radioactive decay
, in which particles are emitted until a stable nucleus is achieved. Common particles emitted during radioactive decay include:
- Alpha particles , consist of two protons and two neutrons. This is equivalent to a helium nucleus and an alpha particle has a charge of 2+ . Because it is positive, it will be attracted towards a negative charge in an electric field. The atomic symbol for an alpha particle is , or sometimes . Alpha particles are slow-moving and are easily absorbed by air or a thin sheet of paper. When an element ejects an alpha particle, the identity of the element changes to the element with an atomic number that is two less than the original element. The mass number of the element decreases by four units.
- Beta particles are electrons, are considered to have negligible mass and have a single negative charge. They will be attracted towards a positive charge in an electric field. The atomic symbol for a beta particle is , or sometimes . Beta particles have “intermediate” energy and typically require thin sheets of metal for shielding. A beta particle is formed in the nucleus when a neutron “ejects” its negative charge (the beta particle) leaving a proton behind. When an element ejects a beta particle, the identity of the element changes to the next higher atomic number, but the mass number does not change.
- Gamma particles (gamma rays) are high-energy photons. They have no mass and can be quite energetic, requiring thick shielding.
- Positrons are anti-electrons, are considered to have negligible mass and have a single positive charge. They will be attracted towards a negative charge in an electric field. The atomic symbol for a positron is symbol . Positrons have “intermediate” energy and typically require thin sheets of metal for shielding. A positron is formed in the nucleus when a proton “ejects” its positive charge (the positron) leaving a neutron behind. When an element ejects a positron, the identity of the element changes to the next lower atomic number , but the mass number does not change.
- In a nuclear equation , elements and sub-atomic particles are shown linked by a reaction arrow. When you balance a nuclear equation, the sums of the mass numbers and the atomic numbers on each side must be the same.
- Radioactive elements decay at rates that are constant and unique for each element. The rate at which an radioactive element decays is measured by its half-life ; the time it takes for one half of the radioactive atoms to decay, emitting a particle and forming a new element. The amount of an original element remaining after n half-lives can be calculated using the equation: \[R=I\left ( \frac{1}{2} \right )^{n} \nonumber \] where I represents the initial mass of the element and R represents the mass remaining.
- In nuclear fission , a nucleus captures a neutron to form an unstable intermediate nucleus, which then splits (undergoes fission) to give nuclei corresponding to lighter elements. Typically, neutrons are also ejected in the process. For heavy isotopes, the process of fission also releases a significant amount of energy. A nuclear equation for a classical fission reaction is shown below: \[_{92}^{235}U+_{0}^{1}n\rightarrow _{56}^{141}Ba+_{36}^{92}Kr+3_{0}^{1}n \nonumber \]
- In nuclear fusion , nuclei combine to form a new element. For light isotopes, the process of fusion also releases a significant amount of energy. A nuclear equation for the fusion cascade that typically occurs in stars the size of our sun is shown below:
\[2_{1}^{1}\rho \rightarrow _{+1}^{0}\beta +_{2}^{3}He \nonumber \]
\[2_{2}^{3}He \rightarrow 2_{1}^{1}\rho +_{2}^{4}He \nonumber \]