http://www.ck12.org Chapter 24. Nuclear Chemistry
through various materials. He did not yet know what these emissions were, and designated them as alpha (α), beta
(β), and gamma (γ) radiation.
We now know that radioactivity involves the spontaneous emission of matter and/or energy from the nucleus of an
atom. The most common radioactive atoms have high atomic numbers and contain a large excess of neutrons. All
isotopes of elements with an atomic number of 84 (polonium) or higher are radioactive, including familiar radioactive
materials such as radon (atomic number 86), uranium (atomic number 92), and plutonium (atomic number 94).
Additionally, two lighter elements (technetium, atomic number 43, and promethium, atomic number 61) have no
stable isotopes. Most elements have both stable and radioactive isotopes. For example, hydrogen-1 and carbon-12
are stable, but hydrogen-3 (tritium) and carbon-14 are both radioactive. These isotopes make up an extremely small
percentage of any naturally occurring sample of their respective elements.
Types of Radioactive Decay
Alpha Emission
FIGURE 24.2
Alpha (α) radiation was eventually found to be made up of particles that consisted of two protons bound to two
neutrons. In other words, analpha particlewas simply the nucleus of a helium-4 atom. Because it does not have
any electrons, the alpha particle carries a charge of +2. In nuclear equations, alpha particles can be represented by
any of the following symbols:
α^42 α^42 HeA typical alpha decay reaction is the conversion of uranium-238 to thorium-234:
238
92 U→
234
90 Th+4
2 α
Many of the largest elements in the periodic table are alpha-emitters. Notice that in this nuclear equation, the total
mass number and the total atomic number are both conserved. The sum of the mass numbers and atomic numbers
of the products are equal to the mass number and atomic number of the parent nucleus. This is a general feature of
nuclear reactions. As a result, knowing that a specific nucleus decays by emitting alpha particles allows us to predict
the product nucleus; we simply need to decrease the mass number by 4 and the atomic number by 2. The change in
atomic number indicates a change in the element’s identity. The new element can be found by looking at the periodic
table.
Beta Emission
Beta (β) emission is a slightly more complicated process. Unlikeα-emission, which simply expels some of the
existing subatomic particles from the nucleus,β-emission also involves the transformation of a neutron into a proton
and an electron. The proton remains bound to the nucleus, while the electron is ejected.