Why Understanding Neutron Mass Matters in Modern Physics
Neutrons are subatomic particles with no electrical charge. They are found in the nucleus (center) of atoms, together with protons.
The word neutron comes from the Greek word νέος ( neutrós ), meaning “neutral” since they have no charge and hence interact weakly with other particles.
Neutrons have a mass of 940.6 MeV/c² or approximately 1.6749 × 10−27 kg on the atomic mass unit (u) scale. By contrast, protons have a rest mass of 0.93887 u.
Formation
The neutrons in the cosmos are formed by fusion in the core of stars.
When a very high-energy cosmic ray proton collides with an atomic nucleus, it can produce a neutron and a hydrogen nucleus (H) in a reaction that may be written as follows:
proton + atomic nucleus → neutron + H
There is another reaction, the opposite one, where a high-energy neutron collides with an atomic nucleus, producing a proton, an electron (e) and an anti-neutrino (ν):
neutron + atomic nucleus → proton + e + anti-neutrino
These newly created particles are on their own, typically moving at great speed. This makes it very hard to capture these particles due to their high kinetic energy.
How were Neutrons discovered?
Until 1930, it was presumed that two fundamental particles were proton and electron.
In 1932, a physicist named James Chadwick discovered neutrons.
He performed an experiment where he found that on bombarding beryllium with the alpha particles, some neutral radiations were emitted. Such bombardment led to extremely powerful penetrations that could not be deflected by electric or magnetic fields.
According to the application of conservation of energy and momentum, he found that they are not protons because protons are charged particles and can be deflected on a curving path towards the negative plate. It means there is something that has no charge. It is the neutron.
Thus, neutrons are subatomic particles carrying no charge.
Neutron Mass
The mass of the neutron is slightly lesser than the mass of the proton.
The mass of a proton is 1.6726231 x 10⁻²⁷ kg whereas the mass of the neutron, mn = 1.6726231 x 10⁻²⁷ kg
Mass of Neutron in Grams
We know the mass of neutron in kg is 1.6726231 x 10⁻²⁷ Kg.
We also know that 1 kg = 10³ g
So, mass of neutron in grams = 1.6726231 x 10⁻²⁷ x 10³
mn= 1.6726231 x 10⁻²⁴ g
Mass of Neutron in Amu
The rest mass of the neutron that we have calculated above is in the unit of kg.
In amu or atomic mass unit, the mass of neutron is calculated as,
Since 1 kg = 6.0229552894949E+26 amu
So, 1.6749286 x 10⁻²⁷kg = 1.6749286 x 10⁻²⁷ x 6.0229552894949E + 26
Rest Mass of Neutron
The concept of rest mass is very simple. We generally think of mass as being a constant quantity for an object. However, the theory of relativity tells us that energy and mass are interchangeable. It means that the mass of a body increases with the increase in its velocity relative to the observer.
Energy gets affected by increasing an object's mass. So, the minimum mass of an object is when it is stationary. Rest Mass is the mass of a body as measured when it is at rest relative to an observer, given by,
The same value as the mass of a neutron.
\[m=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}\]
Where mo is the rest mass, v = velocity and c = speed of light.
Putting the value of m = 1.6749286 x 10⁻²⁷ kg, v = 2.19 km/s = 2190 m/s, c = 3 x 10⁸ m/s
1.6749286 x 10⁻²⁷ =\[\frac{m_{0}}{\sqrt{1-\frac{2190^{2}}{(3\times 10^{8})^{2}}}}\]
=\[\frac{m_{0}}{\sqrt{(3\times 10)^{2}-\frac{2190^{2}}{(3\times 10^{8})^{2}}}}\]
=\[m_{0}\times \frac{(3\times 10^{8})}{\sqrt{(9\times 10)^{16}}}-(4796100)\]
=\[m_{0}\times \frac{(3\times 10^{8})}{\sqrt{(9\times 10)^{16}}}\]
=\[m_{0}\times \frac{(3\times 10^{8})}{(3\times 10^{8})}\]
Cancelling out the common terms, we get m0 x 1
1.6749286 x 10⁻²⁷ equivalent to m0.
So, we get the value as mo = 1.6749286 x 10⁻²⁷Kg
Mass of One Neutron
The mass of a free neutron is 1.6749286 x 10⁻²⁷ kg or 939,565,346 eV/c².
In common particle physics, units of mass and energy are interchangeable.
Here, eV stands for electron-volt which is equivalent to 1.6 x 10⁻¹⁹ J.
c = speed of light = 3 x 10⁸ m/s.
Since 1 kg = 5.6095883571872E+35 eV
So, 1.6749286 x 10⁻²⁷kg = (5.6095883571872E+35) x (1.6749286 x 10⁻²⁷)
Mass of one neutron, mn = 939,565,413.3 eV
So, regarding mega-electron volt
1 MeV = 10,00,000 eV
Mass of neutron, mn = 939.565346 MeV/c².
Relative Mass of Neutron
An atom contains three subatomic particles, namely proton, electron, and neutron.
The proton and neutron are found inside the nucleus at the centre of an atom.
The nucleus is smaller than the size of an atom as a whole, and the electrons are arranged in shells around them.
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Since protons are about 99.86% as massive as neutrons and electrons are about 0.054% as massive as neutrons.
The relative mass of each particle of an atom is in kilograms.
So, the relative mass of a neutron is 1.
Relative Charge on a Neutron
Neutrons do not hold any charge.
In an experiment conducted by James Chadwick in 1932, he observed that this subatomic particle didn't get deflected by electric and magnetic fields.
That's why the relative charge on a neutron is also 0.
Conclusion
The mass of the neutron is slightly lesser than that of a proton. The rest mass of a neutron is calculated as 1.6749286 x 10⁻²⁷ kg. In common particle physics, units of mass and energy are interchangeable. Here, eV stands for electron-volt, which is equivalent to 1 Neutron that does not hold any charge. The relative mass of a neutron is 1, and the relative charge on a neutron is 0. Students who understand this concept can also go through other related topics like mass of an electron, mass of a proton, mass of an atom, mass of a relative object, mass between two particles and relative charge on two particles. This will give students ample practice to understand the topic better. Vedantu offers online tuition for Science classes. Take Online Tests, participate in Live Discussions and get free assignment help on Vedantu.
FAQs on Neutron Mass: Definition, Value & Significance
1. What is the approximate mass of a neutron in kilograms (kg)?
The mass of a single neutron is extremely small. Its officially accepted value is approximately 1.6749 x 10⁻²⁷ kg. This is a fundamental constant used in many physics and chemistry calculations.
2. How is the mass of a neutron described using atomic mass units (amu)?
In atomic mass units, a neutron has a mass of approximately 1.00866 amu. An atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Using amu makes it much easier to compare the masses of subatomic particles.
3. How does a neutron's mass compare to that of a proton and an electron?
A neutron is very slightly heavier than a proton, but for most school-level calculations, their masses are considered nearly equal (around 1 amu). Both neutrons and protons are significantly more massive than electrons. An electron's mass is about 1/1836th that of a proton, making it almost negligible in comparison.
4. Where are neutrons located inside an atom?
Neutrons are located in the central part of the atom, called the nucleus. They are tightly packed together with protons. The electrons, on the other hand, orbit this nucleus.
5. How can you calculate the number of neutrons in an atom of a specific element?
You can find the number of neutrons by subtracting the atomic number from the mass number. The formula is: Number of Neutrons = Mass Number (A) – Atomic Number (Z). For example, in a Carbon-14 atom (A=14, Z=6), there are 14 - 6 = 8 neutrons.
6. Why is a unit like MeV/c² sometimes used to express a neutron's mass?
The unit MeV/c² (Mega-electron Volts divided by the speed of light squared) comes from Einstein's famous equation, E=mc². This equation shows that mass and energy are interchangeable. In particle physics, it is often more convenient to express the mass of a particle in terms of its equivalent energy, as it simplifies calculations related to nuclear reactions and decays.
7. Can a neutron exist on its own, outside of an atom's nucleus?
A neutron is generally not stable when it is by itself. A free neutron (one outside a nucleus) is unstable and will decay into a proton, an electron, and an antineutrino in about 15 minutes. However, when bound inside a nucleus with protons, neutrons become stable.
8. If a neutron has mass, why is its charge considered to be zero?
Mass and electric charge are two separate and independent properties of a particle. Mass is the measure of an object's inertia, while electric charge determines how it interacts with electromagnetic fields. A neutron has no net electric charge, making it electrically neutral. However, it still possesses mass, just like any other subatomic particle.
9. Why is understanding the mass of a neutron important in science?
The mass of a neutron is crucial for several reasons:
- Isotopes: It helps explain why different isotopes of an element (which have the same number of protons but different numbers of neutrons) have different masses.
- Nuclear Stability: The ratio of neutrons to protons, influenced by their masses, determines whether an atomic nucleus is stable or will undergo radioactive decay.
- Nuclear Reactions: In processes like nuclear fission and fusion, the tiny changes in mass are converted into huge amounts of energy, as described by E=mc².
10. Do all neutrons have the exact same mass?
Yes, the rest mass of a neutron is a fundamental physical constant, meaning every neutron has the same mass when it is at rest. While its relativistic mass can increase if it moves at very high speeds, for all practical purposes in chemistry and introductory physics, you can consider all neutrons to have the same, constant mass.
