CK-12-Physics-Concepts - Intermediate

22.2. Radioactivity http://www.ck12.org

Each nucleus, therefore, has competing forces. The repulsive force between the protons tends to blow the nucleus
apart and the binding energy tends to hold the nucleus together. If the average binding energy per nucleon overcomes
the repulsion, the nucleus stays together and it referred to as stable. If the repulsion overcomes the average binding
energy per nucleon, the nucleus may blow apart or undergonuclear disintegration. When a nucleus disintegrates,
it throws off pieces of itself and energy in the form ofgamma rays. This disintegration process came to be called
radioactivity.

Early researchers in radioactivity found that the emissions from radioactivity could be classified into three distinct
types according to their penetrating power. One type of radiation could barely penetrate a sheet of paper. The second
type could pass through as much as 3 mm of aluminum. The third type was extremely penetrating and could pass
through several centimeters of lead. They named these three types of radiationalpha(α),beta(β), andgamma
(γ)respectively. Eventually, each type of radiation was further identified. Alpha particles are the nuclei of helium
atoms,^42 He. Beta particles are electrons, and gamma rays are very high energy photons (even higher energy than
x-rays).

When a nucleus emits anαparticle, the remaining particle will contain two less protons and two less neutrons. The
new particle will have an atomic number two less and a mass number four less than the original nuclide.

226

88 Ra−→

222

86 Rn+

4

2 He

The daughter product,^22286 Rn, is different from the parent nucleus,^22688 Ra, by two protons and two neutrons. This
changing of one element into another is calledtransmutation.

Transmutation also occurs when a nucleus undergoesβdecay. An example of beta decay is the emission of an
electron by a carbon-14 nucleus.

14

6 C−→

14

7 N+

0

− 1 e

The symbol^0 − 1 erepresents an electron whose charge corresponds toZ=−1 and since it has no nucleons,A=0. It
must be carefully noted that the electron released during beta decay is NOT an orbital electron but an electron whose
origin was in the nucleus. The process has one neutron becoming one proton and one electron and the electron being
emitted as a beta particle. Since a neutron has been lost AND a proton has been gained, the mass number does not
change. The atomic number, however, has increased one due to the gain of a proton. Therefore, as a result of beta
decay, the daughter product will have the same mass number as the parent and an atomic number one greater than
the parent.

Summary

• The total mass of a stable nucleus is always less than the sum of the masses of its constituent protons and
  neutrons. This is known as mass defect.


• The mass lost in mass defect has gone into energy that is called binding energy.


• Each nucleus has competing forces. The repulsive force between the protons tends to blow the nucleus apart
  and the binding energy tends to hold the nucleus together.


• If the repulsion overcomes the average binding energy per nucleon, the nucleus may blow apart or undergo
  nuclear disintegration.


• Early researchers in radioactivity found that the emissions from radioactivity could be classified into three
  distinct types according to their penetrating power. They named these three types of radiation alpha(α), beta
  (β), and gamma(γ)respectively.


• When a nucleus emits anαparticle, the remaining particle will contain two less protons and two less neutrons.


• As a result ofbetadecay, the daughter product will have the same mass number as the parent and an atomic
  number one greater than the parent.