3.60. Alpha Decay http://www.ck12.org
FIGURE 3.110
chemical symbol for helium, the subscript 2 is the number of protons, and the superscript 4 is the mass number (2
protons + 2 neutrons).
Nuclear symbols are used to write nuclear equations for radioactive decay. Let’s consider an example. Uranium-238
undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number
of protons plus neutrons.) In this reaction, uranium-238 loses two protons and two neutrons to become the element
thorium-234. The reaction can be represented by this nuclear equation:
238
92 U→234
90 Th +4
2 He + EnergyIf you count the number of protons (subscripts) as well as the number of protons plus neutrons (superscripts), you’ll
see that the total 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 also unstable, so it will undergo radioactive decay as well. The alpha
particle (^42 He) produced in the reaction can join with two free electrons to form the element helium. This is how
most of Earth’s helium formed.
Q:Fill in the missing subscript and superscript to balance the following nuclear equation for alpha decay of
Polonium-210.
210
84 Po→
?
?Pb +
4
2 He + EnergyA:The subscript of Pb is 82, and the superscript is 206. This means that the new element produced in the reaction
has 82 protons. You can find the element with this number of protons in the periodic table. It is the element lead
(Pb). The new element also has 124 neutrons (206 –82 protons = 124 neutrons).
If you’re still not sure how alpha decay occurs or how to write a balanced equation for it, watch the animation at this
URL: http://www.furryelephant.com/player.php?subject=physics&jumpTo=re/10Ms24
How Dangerous Is Alpha Decay?
All types of radioactive decay pose risks to living things, but alpha decay is the least dangerous. That’s because
alpha particles are relatively heavy, so they can travel only a few centimeters through the air. They also are not very
penetrating. For example, they can’t pass through a sheet of paper or thin layer of clothing. They may burn the skin,
but they can’t penetrate to the tissues underneath the skin. However, if alpha particles are emitted inside the body,
they can do more damage. One way this can happen is by inhaling cigarette smoke. People who smoke actually