In this explainer, we will learn how to describe the properties of quarks and the composition of protons and neutrons.
Lots of advanced scientific experiments were conducted during the first half of the 20th century to determine the structure of the atom. Scientists used relatively sophisticated scientific experiments to show that there was a positively charged nucleus at the center of each atom. The scientists demonstrated that the nucleus was made up of protons and neutrons, and they also proved that the atomic nucleus was surrounded by negatively charged electrons.
Scientists continued to conduct even more complex scientific experiments, and they eventually ended up proving that the atom was even more complex than they first thought. Scientists used incredibly long particle accelerators to prove that subatomic neutrons and protons were themselves composed of even more fundamental particles.
The subatomic particles that make up protons and neutrons are known as quarks.
Definition: Quarks
Quarks are subatomic particles and are fundamental constituents of matter.
Murray Gell-Mann and George Zweig independently proposed the quark model in 1964. The quark model states that many large particles are made up of different types of quarks. Quarks can be considered to be fundamental building blocks of matter, because they make up larger particles such as protons and neutrons.
The quark model was later used as part of the standard model of physics, which is shown below.
The image of the standard model shows that there are six different types of quarks. These are known as flavors of quarks.
The six flavors of quarks are as follows:
- Up
- Down
- Charm
- Strange
- Top
- Bottom
Example 1: Identifying the Incorrect Flavor of Quark
Which of the following is not a type of quark?
- Bottom
- Strange
- Left
- Up
- Down
Answer
Quarks are fundamental subatomic particles and fundamental building blocks of matter. Quarks combine to make particles such as protons and neutrons. There are six different types of quarks (also known as flavors of quarks). The flavors of quarks are up, down, strange, charm, top, and bottom.
From the list of options given in the question, we can see that all are flavors of quarks except for C, “left.” Therefore, the correct answer is C.
Like protons and electrons, quarks contain an electric charge. However, unlike protons and electrons, these are fractional charges. Quarks either have a charge of e or e, where e is the elementary charge: the electrical charge carried by a single proton. The table below shows the electrical charge for each flavor of quark.
| Flavor of Quark | Symbol | Electric Charge |
|---|---|---|
| Up | u | e |
| Down | d | e |
| Charm | c | e |
| Strange | s | e |
| Top | t | e |
| Bottom | b | e |
Example 2: Identifying the Electric Charges of Quarks
Which of the following denotes the two values of electrical charge a quark can have?
- e and e
- 0 and e
- e and e
- e and e
- e and e
Answer
Quarks are fundamental subatomic particles and building blocks of particles such as protons and neutrons. Protons have a charge of + e and electrons have a charge of e, where e is the elementary charge. Quarks can have two different fractional charges of e. The electric charge of a quark depends upon its type, otherwise known as its flavor.
Up, charm, and top quarks have an electric charge of e, while down, strange, and bottom quarks have an electric charge of e. These two charges correspond to the ones in answer D, and therefore, this is the correct answer.
Quarks cannot be observed individually. They are only found in particles where they are combined with other quarks. Protons and neutrons are made up of combinations of up and down quarks.
Protons are composed of two up quarks and one down quark. The structure of a proton is shown in the diagram below.
Summing the charges of the three quarks gives the charge of the proton:
Hence, the charge of a proton is, as expected, e.
Neutrons are composed of one up quark and two down quarks. The structure of a neutron is shown in the diagram below.
Summing the charges of the three quarks gives the charge of the neutron:
Hence, the charge of a neutron is, as expected, 0.
Example 3: Identifying a Particle from Its Composition of Quarks
Which of the following is composed of 3 quarks in the form udd?
- An electron
- An alpha particle
- A proton
- A beta particle
- A neutron
Answer
Quarks are fundamental subatomic particles and can combine in different ways to form composite particles.
Neutrons and protons are both composed of three quarks. Neutrons and protons are exclusively formed of up and down quarks. So, option C or E could be the correct answer for this question.
Electrons are fundamental particles, and they are not composed of any quarks. Similarly, a beta particle can be a high-energy electron or positron and is therefore not composed of quarks.
An alpha particle is formed of two protons and two neutrons, and it is equivalent to the atomic nucleus of a helium atom. Each proton or neutron is composed of three quarks, and this means that an alpha particle must contain a total of 12 quarks. Option B cannot be the correct answer to this question.
The question tells us the particle is composed of the udd quark combination. A proton is composed of two up quarks and one down quark. A neutron is composed of one up quark and two down quarks. Therefore, the correct answer is E: a neutron.
Example 4: Determining the Electric Charge of a Particle from Its Composition of Quarks
A composite particle is composed of 2 up quarks and 1 down quark. What is the overall electric charge of this particle?
Answer
Quarks are fundamental subatomic particles and building blocks of particles such as protons and neutrons. There are six different types of quarks (also known as flavors of quarks). The flavors of quarks are up, down, strange, charm, top, and bottom.
Up, charm, and top quarks have an electric charge of e, while down, strange, and bottom quarks have an electric charge of e.
The overall charge of a particle composed of 2 up quarks and 1 down quark can be determined by summing up the charges of each quark:
Therefore, the overall electric charge of the particle is . It is highly likely that this particle is a proton.
Example 5: Calculating the Number of Up Quarks in a Nucleus
Calculate the total number of down quarks in the nucleus of an element that has atomic number 9 knowing that its nucleus contains 28 up quarks.
Answer
Quarks are fundamental subatomic particles and building blocks of particles such as protons and neutrons. A proton is composed of two up quarks and one down quark. A neutron is composed of one up quark and two down quarks.
To solve this question, the total number of protons and neutrons in the nucleus needs to be determined.
The question states that the element has an atomic number of 9, meaning it has 9 protons. As protons are made from two up quarks and one down quark, the 9 protons account for up quarks, which equals 18 up quarks.
The question states the nucleus contains 28 up quarks. 18 of these are from the protons, which means there are 10 up quarks left to account for. As the nucleus of an atom only contains protons and neutrons, then the remaining 10 up quarks must come from the neutrons.
Neutrons are composed of one up quark and two down quarks. Therefore, the remaining 10 up quarks correspond to 10 neutrons.
We have now determined that the atomic nucleus for this element contains 9 protons and 10 neutrons.
As protons contain one down quark, then 9 protons contain 9 down quarks.
As neutrons contain two down quarks, then 10 neutrons contain 20 down quarks.
Adding the number of down quarks together gives us . Therefore, the total number of down quarks in the nucleus of the atom is 29.
Other particles can be formed from the combination of two, three, or even five quarks. However, the most stable and important are the protons and neutrons that form atomic nuclei.
Key Points
- Quarks are fundamental, subatomic particles found inside protons and neutrons.
- There are six types (flavors) of quarks: up, down, charm, strange, top, and bottom.
- Quarks have fractional charges of either e or e.
- Up, charm, and top quarks have charges of e, while down, strange, and bottom quarks have charges of e.
- Protons are composed of two up quarks and one down quark.
- Neutrons are composed of one up quark and two down quarks.
