by Ron Kurtus (revised 31 December 2008)
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.
These subatomic particles 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. Useful tool: Units Conversion
Classical subatomic particles
Atoms or elements consist of protons and neutrons in their nuclei (except for the case of Hydrogen, which only has a proton in its nucleus). In "obits" or shells around the nucleus are electrons. An electrically neutral atom has the same number of electrons as protons.
Recent experiments have shown that the proton and neutron are not fundamental particles but actually consist of even smaller subatomic particles—quarks. The electron is still a fundamental particle, classified as a lepton.
The Lepton classification of subatomic particles consists of 6 fundamental particles:
- 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 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
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 somewhat silly names to describe these subatomic particles. It is also unfortunate that the scientific community agreed to continue to use these silly names.)
One characteristic of the Quarks is that they have an electrical charge that is either (+2/3) or (−1/3). The reason that it is a fraction of a charge is that when the original definition of electrical charge of was made, no one thought there were actually particles smaller than the electron (having a −1 charge) and a proton (having a +1 charge).
There are also other particles made up of combination of Quarks.
Up and Down Quarks
The Up Quark has a positive (+) electrical charge of (+2/3). The Down Quark has a negative (−) electrical charge of (−1/3).
Note: Think of Up as positive (+) and Down as negative (−).
The Proton is made up of two Up Quarks (+2/3) + (+2/3) and one Down Quark (−1/3). The electrical charge of the proton is then:
(+2/3) + (+2/3) + (−1/3) = (+1).
The Neutron is made up of one Up Quark (+2/3) and two Down Quarks (−1/3) + (−1/3). 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 (+2/3) as the Up Quark but has greater mass. The Top Quark has even greater mass than the Charm Quark.
The Strange Quark has the same electrical charge (−1/3) as the Down Quark but is heavier. The Bottom Quark has even greater mass than the Strange Quark.
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 have been found to be combinations of Quarks.
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