11.2. Radioactive Decay http://www.ck12.org
4
2 He. The superscript 4 is the mass number (2 protons + 2 neutrons). The subscript 2 is the charge of the particle as
well as the number of protons.
FIGURE 11.6
Alpha decay results in the loss of two
protons and two neutrons from a nucleus.An example of alpha decay is the decay of uranium-238 to thorium-234. In this reaction, uranium loses two protons
and two neutrons to become the element thorium. The reaction can be represented by this equation:
238
92 U→234
90 Th+4
2 He+EnergyIf you count the number of protons and neutrons on each side of this equation, you’ll see that the numbers are the
same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction
is unstable, so it will undergo radioactive decay as well. The alpha particle(^42 He)produced in the reaction can pick
up two electrons to form the element helium. This is how most of Earth’s helium formed.
Problem Solving
Problem:Fill in the missing subscript and superscript to balance this nuclear equation:^20884 Po→??Pb+^42 He+Energy
Solution:The subscript is 82, and the superscript is 204.
You Try It!
Problem:Fill in the missing subscript and superscript to balance this nuclear equation:^222 ?Ra→ 86 ?Rn+^42 He+Energy
Beta Decay
Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is an electron. It has
a charge of -1. In nuclear equations, a beta particle is represented by the symbol−^01 e. The subscript -1 represents
the particle’s charge, and the superscript 0 shows that the particle has virtually no mass. Nuclei contain only protons
and neutrons, so how can a nucleus emit an electron? A neutron first breaks down into a proton and an electron (see
Figure11.7). Then the electron is emitted from the nucleus, while the proton stays inside the nucleus. The proton
increases the atomic number by one, thus changing one element into another.
An example of beta decay is the decay of thorium-234 to protactinium-234. In this reaction, thorium loses a neutron
and gains a proton to become protactinium. The reaction can be represented by this equation: