http://www.ck12.org Chapter 3. Chemical Interactions
Q:Nuclei contain only protons and neutrons, so how can a nucleus emit an electron in beta-minus decay or a positron
in beta-plus decay?
A:Beta decay begins with a proton or neutron. You can see how in theFigure3.111.
FIGURE 3.111
Q:How does beta decay change an atom to a different element?
A:In beta-minus decay an atom gains a proton, and it beta-plus decay it loses a proton. In each case, the atom
becomes a different element because it has a different number of protons. You can see an animation showing how
this happens at the following URL. http://www.furryelephant.com/player.php?subject=physics&jumpTo=re/10Ms44
Equations for Beta Decay
Radioactive nuclei and particles are represented by nuclear symbols.. For example, a beta-minus particle (electron) is
represented by the symbol^0 − 1 e. The subscript -1 represents the particle’s charge, and the superscript 0 shows that the
particle has virtually no mass (no protons or neutrons). Another example is the radioactive nucleus of thorium-234.
It is represented by the symbol^23490 Th, where the subscript 90 stands for the number of protons and the superscript
234 for the number of protons plus neutrons.
Nuclear symbols are used to write nuclear equations for radioactive decay. Let’s consider the example of the beta-
minus decay of thorium-234 to protactinium-234. This reaction is represented by the equation:
234
90 Th→
234
91 Pa +
0
− 1 e + energyThe equation shows that thorium-234 becomes protactinium-234 and loses a beta particle and energy. The protactinium-
234 produced in the reaction is also radioactive, so it will decay as well.
A nuclear equation is balanced if the total numbers of protons and neutrons are the same on both sides of the arrow.
If you compare the subscripts and superscripts on both sides of the equation above, you’ll see that they are the same.
Q:What happens to the electron produced in the reaction above?
A:Along with another electron, it can combine with an alpha particle to form a helium atom. An alpha particle,
which is emitted during alpha decay, consists of two protons and two neutrons.