Alpha Decay: Definition, Equation & Example

Instructor: Anthony Grattini

Tony has a BA in Biology and has taught secondary Life, Earth, and Physical Science, as well as Honors & AP Chemistry.

Learn how unstable elements, like Uranium-238, decay into a more stable state by releasing matter and energy through spontaneous radioactive decay. The type of radioactive decay is alpha decay, or the formation of the helium-4 nuclei.

Alpha Decay Definition

All matter in the universe is trying to reach its most stable state. On a human level, when unstable due to an unbalanced budget, you can either cut back spending or find more work, to become stable again. Or when your're feeling stressed, you might take a vacation. At the elemental level, the most stable elements are found on the far right hand side of the periodic table. These elements even have a name to match this state of bliss and are known as the noble gases. The noble gases all belong to Group 8A and include Helium, Neon, Argon, Krypton, Xenon, and Radon. They are some of the most stable elements on the periodic table. All other unstable elements can reach this state of stability by being combined with other elements or decaying into a more stable form. Alpha decay occurs when unstable elements undergo spontaneous radioactive decay to get into a more stable state.


Radioactivity is when an element with an unstable nucleus, or center of the atom, spontaneously falls apart to form more stable bits of matter. In the process of falling apart they release matter and/or energy. There are three common types of radioactive decay: alpha, beta, and gamma decay. Alpha radiation involves the formation of helium-4 nuclei, beta radiation involves the formation of free electrons, and gamma radiation involves the formation of high-energy photons. In this article we will focus on alpha radiation and the process which forms helium-4 nuclei.

Three Naturally Occurring Uranium Isotopes

Uranium is one unstable element that experiences alpha decay -- or at least, one of its isotopes does. Isotopes are different forms of the same element. The major difference is the number of neutrons within the nucleus of the atom. Uranium is the 92nd element on the periodic table, which means as an uncharged element it will always have 92 protons and 92 electrons. Any bit of matter with 92 protons and 92 electrons will always be known as uranium.

There are unique types of uranium, however, distinguished from one another by the varying number of neutrons they have within the nucleus, or center of the atom. For example, the three naturally occurring isotopes of uranium are uranium-234, uranium-235, and uranium-238, where the numerical suffixes represent the mass numbers. This number can be used to determine the varying number of neutrons by subtracting the number of protons (92) found within uranium. Uranium-234 has 142 neutrons, uranium-235 has 143 neutrons, and uranium-238 has 146 neutrons.

Different isotopes have different natural abundances. For example, 99.3% of naturally occurring uranium is uranium-238, 0.7% is uranium-235, and only a trace amount is uranium-234. Different nuclei also have different stabilities. Indeed, the nuclear properties of an atom depend on the number of protons and neutrons in its nucleus.

Neutron-to-Proton Ratio

The strongest fundamental force in the entire universe is known as the strong nuclear force. It's the force that holds so many protons together in a very tiny space. Neutrons are intimately involved in this attractive force. The more protons packed in the nucleus, the more neutrons are needed to hold the nucleus together. Stable nuclei, up until about the 20th element on the periodic table, have approximately the same number of neutrons and protons. After element 20, the one-to-one ratio is no longer considered the most stable as seen in the graph below. The issue of nuclear stability is very complex. The belt of stability is meant to serve only as a general guide to predicting nuclear stability.

Neutron-to-Proton Ratio

With larger nuclei, more neutrons are necessary to maintain as stable an atom as possible. All nuclei with 84 or more protons are considered radioactive, because the neutron-to-proton ratio gets further away from the happiest 1-to-1 ratio that is seen within the first 20 elements on the periodic table. Uranium-238 is one such highly unstable isotope and spontaneously falls apart, or decays to become a little more stable. It has a neutron-to-proton ratio of 1.59:1.

The Alpha Decay Reaction Equation of Uranium-238

Uranium-238 ---> Thorium-234 + Helium-4

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