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Subatomic Particles

by Ron Kurtus (revised 12 July 2007)

An atom is made up of subatomic particles. Although the Proton, Neutron and Electron have been considered the fundamental particles of an atom, recent discoveries from experiments with atomic accelerators have shown that there are actually 12 subatomic particles. They are divided into two classes, consisting of Leptons and Quarks. The proton and neutron are no longer considered fundamental particles in this subatomic classification but are actually made up of smaller subatomic particles.

Questions you may have include:

• What are the Leptons?
• What are the Quarks?
• How does this all fit with atomic physics?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Useful tools: Metric-English Conversion | Scientific Calculator.

Leptons

The Lepton classification of subatomic particles consists of 6 fundamental particles:

• Electron
• Muon
• Tau
• Electron Neutrino
• Muon Neutrino
• Tau Neutrino

Electron, Muon and Tau Leptons

The Electron remains a fundamental particle, as it was in the original Atomic Theory. It has an electrical charge of (-1) and plays an active role in chemical reactions.

The Muon is similar to an Electron, only heavier. It is primarily seen as a result of a high-energy collision in an atomic accelerator. The Tau particle is similar to a Muon, only heavier yet. Muon and Tau particles are unstable and exist in nature for a very short time.

Neutrinos

Neutrinos are extremely small and have no electrical charge. This makes them extremely difficult to detect. They can possess a large amount of energy and the very rare times they do collide with another particle, that energy can be released.

The types of neutrinos are:

• Electron Neutrino, which has no charge and is extremely difficult to detect
• Muon Neutrino, which is created when some atomic particles decay
• Tau Neutrino, which is heavier than the Muon Neutrino.

Quarks

Another group of subatomic particles are the Quarks. Just like their name, they exhibit unusual characteristics. The fundamental particles among the Quarks are:

• Up and Down Quarks
• Charm, Strange, Top and Bottom Quarks

(Note: It is unfortunate that scientists choose these somewhat silly names to describe these subatomic particles. It is also unfortunate that the scientific community agreed to continue to use these names.)

An usual characteristic of the Quarks is that they have an electrical charge that is either (+2/3) or (-1/3). The reason is that when the original definition of electrical charge of (+1) and (-1) was made, the fact that there were actually smaller particles than the electron (having a -1 charge) and a proton (having a +1 charge) was known.

Other particles are made up of combination of Quarks.

Up and Down Quarks

The Up Quark has an electrical charge of (+2/3). The Down Quark has an electrical charge of (-1/3).

The Proton is made up of  two Up Quarks and one Down Quark. The electrical charge of the proton is then: (+2/3) + (+2/3) + (-1/3) = (+1).

The Neutron is made up of one Up Quark and two Down Quarks. The resulting electrical charge of the Neutron is: (+2/3) + (-1/3) + (-1/3) = (0).

Charm, Strange, Top and Bottom Quarks

The Charm Quark has the same electrical charge as the Up Quark but is heavier. The Top Quark is then heavier than the Charm.

The Strange Quark has the same electrical charge as the Down Quark but is heavier. The Bottom Quark is then heavier than the Strange.

Summary

According to subatomic Theory, the fundamental particles now consist of 6 Lepton particles and 6 Quark particles. Other particles are made up of combination of Quarks. An Electron is still a fundamental particle, but Protons and Neutrons are combinations of Quarks.

Answers to Readers' Questions

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Resources

The following resources provide information on this subject:

Websites

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Books

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Miscellaneous

Mini-quiz to check your understanding

1. How were subatomic particles discovered?

2. Why is a neutrino so difficult to detect?

3. How can a Quark have a fraction of a charge?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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