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General Chemistry Principles, Patterns, and Applications, 2011

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The atomic numbers of the parent <strong>and</strong> daughter nuclides differ in Equation 20.11, although the mass<br />

numbers are the same. To write a balanced nuclear equation for this reaction, we must explicitly include<br />

the captured electron in the equation:<br />

Equation 20.12<br />

F2655e + e-10 ® M2555n + x - ray<br />

Both positron emission <strong>and</strong> electron capture are usually observed for nuclides with low neutron-to-proton<br />

ratios, but the decay rates for the two processes can be very different.<br />

Gamma Emission<br />

Many nuclear decay reactions produce daughter nuclei that are in a nuclear excited state, which is similar<br />

to an atom in which an electron has been excited to a higher-energy orbital to give an electronic excited<br />

state. Just as an electron in an electronic excited state emits energy in the form of a photon when it<br />

returns to the ground state, a nucleus in an excited state releases energy in the form of a photon when it<br />

returns to the ground state. (For more information on electron ground states, see Chapter 6 "The<br />

Structure of Atoms".) These high-energy photons are γ rays. Gamma (γ) emissioncan occur virtually<br />

instantaneously, as it does in the alpha decay of uranium-238 to thorium-234, where the asterisk denotes<br />

an excited state:<br />

Equation 20.13<br />

U92238 ® T 90234h *excitednuclearstate +a24- ®<br />

- - - - relaxationT 90234h +g 00<br />

If we disregard the decay event that created the excited nucleus, then<br />

Equation 20.14<br />

T 88234h*® T 88234h +g 00<br />

or more generally,<br />

Equation 20.15<br />

XZA*® XZA +g 00<br />

Gamma emission can also occur after a significant delay. For example, technetium-99m has a half-life of<br />

about 6 hours before emitting a γ ray to form technetium-99. (Them is for metastable, which is explained<br />

in Chapter 14 "Chemical Kinetics", Section 14.5 "Half-Lives <strong>and</strong> Radioactive Decay Kinetics".)<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1844

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