Alpha Decay of Uranium Isotopes

Instructor: Sunday Moulton

Sunday earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

This lesson shows how alpha decay affects the atoms of uranium isotopes. It will explain why alpha decay occurs and the concept of alpha decay in relation to the atomic number and atomic mass number.

Unstable Elements

Uranium is one of only a few elements whose isotopes are all unstable, meaning every variation of a uranium atom will undergo some form of nuclear decay. The instability of uranium isotopes makes it a valuable resource for nuclear energy because it takes less input energy to start the nuclear reaction chain that fuels nuclear power plants. In this lesson, we'll look at the three naturally-occurring isotopes of uranium and what happens when they undergo alpha decay, one of the three nuclear decay processes.

Uranium Isotopes

A uranium isotope is a specific composition of an atom that qualifies as uranium because it contains 92 protons in its nucleus. Each isotope will differ only in the number of neutrons its nucleus contains. As a proton and a neutron have nearly the same mass, they contribute almost equally to the mass number, which is the sum of the protons and neutrons found in that atom's nucleus. We label each isotope with the mass number after the element name, such as uranium-238. There are three naturally-occurring isotopes of uranium. These are uranium-234, uranium-235, and uranium-238. Uranium-238 is the most abundant and accounts for 99 percent of all the uranium in the Earth's crust. All three of the isotopes undergo alpha decay as their primary form of nuclear decay.

Alpha Decay

Nuclear decay occurs when the nucleus of an atom is unstable because the forces holding the nucleus together are insufficient to balance the forces pushing the particles in the nucleus apart. Atoms want to reach a stable state and will experience some form of nuclear decay to change the atom into one that is more stable.

Alpha decay occurs because the expulsion of some of the particles in the nucleus can help the atom reach this more stable state. In alpha decay, the nucleus expels an alpha particle. This is a particle made of two protons and two neutrons and is sometimes referred to as a helium-4 nucleus.


Alpha Decay
Alpha Decay


How Alpha Decay Changes Uranium

The loss of an alpha particle changes the original isotope, the parent isotope, in two important ways. First, the loss of two protons changes the atomic number (the number of protons) from 92, in this case, to 90. The atomic number of the atom identifies the element. This means the atom will change from a uranium atom to a thorium atom. We know it becomes a thorium atom because 90 is the atomic number of thorium.

The other way alpha decay changes the uranium atom is by changing its mass number. Losing four nuclear particles reduces the mass number by four. This is helpful to know whenever we try to determine the daughter isotope, the isotope of an atom after it has undergone a type of nuclear decay.

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