Radioactive Decay

Read this text for a brief introduction to nuclear decay. Look at the reaction in the text. Recall from Unit 1 that we can write elements with the mass number (A) as a superscript and the atomic number (Z) as a subscript before the element symbol. The atomic number is the number of protons, while the mass number is the sum of the number of protons and neutrons. In the reaction, the element changes because the atomic number changes. The other product is called an alpha particle, which we will discuss in our next reading.

Once the picture of a nuclear atom was discovered it became clear that the energy released must come from the nucleus. It is for this reason that atomic energy became generally known as nuclear energy rather than atomic energy.(1)

If mass is lost from the nucleus the following questions must be asked:

  • Where does it go?
  • What happens to the remaining mass?
  • What happens to the nucleus?

Photo of Sir William Crookes 1906

William Crookes (1832–1919).

In 1900 William Crookes, working at what is now Imperial College, London, was investigating the chemistry of uranium. He found that if uranium was purified it showed very low (or no) radioactivity. This was counter to previous studies. However, the uranium sample was left for some time its radioactivity increased to the levels previously associated with samples of uranium! Soddy and Rutherford showed that thorium behaved the same as uranium, and they proposed that radioactive disintegration occurred. They proposed that uranium is converted to other more radioactive elements and eventually to lead.

^{238}_{92}\text{U}\rightarrow ^{234}_{90}\text{Th}+^{4}_{2}\text{He}

Based upon the periodic table the difference between uranium and lead would suggest that sequential radioactive disintegration would involve 10 elements. However, by 1914 it was found that there were more than 30 steps between U and Pb. The only way to explain this is the concept of isotopes.

The word isotope is derived from the Greek, meaning same place. Isotopes are any of the different types of atoms (nuclides) of the same chemical element, each having a different atomic mass (mass number), but the same atomic number. Once the concept of isotopes are realized then the reason for all extra elements for the radioactive disintegration.


(1) The United Kingdom Atomic Energy Authority (UKAEA) is the government body, founded in 1954, that oversees nuclear energy in the U.K.

Source: Andrew R. Barron,
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Last modified: Monday, May 17, 2021, 3:35 PM