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Alpha Particle: Definition, Symbol & Properties

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  • 0:00 What Is an Alpha Particle?
  • 0:40 Creating Alpha Particles
  • 2:05 Symbol
  • 2:50 Properties
  • 3:50 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what an alpha particle is and what symbols are used to represent it, and will also describe the properties of alpha particles. A short quiz will follow.

What Is an Alpha Particle?

Alpha particles are considered to be a form of dangerous radiation -- a result of radioactive decay -- and can cause significant damage to human tissue. But what, exactly, are they?

Alpha particles are helium nuclei and are produced by some radioactive substances known as 'alpha sources.' An alpha particle, just like a helium nuclei, contains two protons and two neutrons. You might, therefore, be inclined to just use the term 'helium nuclei,' and a lot of scientists would agree with you -- the name usually depends on context.

Creating Alpha Particles

There are a number of sources of alpha particles: alpha decay, particle accelerators, fusion reactions, and cosmic rays. Let's review these sources in more detail.

Alpha decay is the most common way that alpha particles are created. Alpha decay is a kind of radioactive decay, where larger radioactive atoms like uranium-235 decay into smaller atoms. The laws of physics tell us that mass in the universe is conserved, and therefore if a uranium-235 atom turns into a lighter element, that mass has to go somewhere. Where does it go? It is released in the form of an alpha particle.

Alternatively, particle accelerators like synchrotrons and cyclotrons can be used to create alpha particles. Helium fusion in stars, where helium atoms are fused together to form carbon, produce extra alpha particles as a byproduct. And cosmic rays from space also contain alpha particles, though currently scientists are not certain as to how these are created.

The term alpha particle is usually used when the helium nucleus was created by alpha decay, through fusion reactions in a star, or when they reach us as cosmic rays. In most other contexts, they are simply called helium nuclei. The distinction has little logic behind it and is largely historical, like many names in science.

Symbol

Since an alpha particle can be referred to as a helium nucleus, it has multiple chemical symbols to represent it.

Strictly speaking, an alpha particle is represented by the first letter of the Greek alphabet, alpha:

But since it is also a helium nuclei, it can equally be represented by the chemical symbol for helium, He. However, since it is specifically a helium nuclei and does not contain any electrons, it is important to include the 2+ (or +2) charge when using the He symbol. So in summary, any of the following symbols are acceptable for any alpha particle:

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