[Physics Class Notes] on Neutron Mass Pdf for Exam

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}}{(3times 10^{8})^{2}}}}]

=[frac{m_{0}}{sqrt{(3times 10)^{2}-frac{2190^{2}}{(3times 10^{8})^{2}}}}]

=[m_{0}times frac{(3times 10^{8})}{sqrt{(9times 10)^{16}}}-(4796100)]   

     

=[m_{0}times frac{(3times 10^{8})}{sqrt{(9times 10)^{16}}}]

=[m_{0}times frac{(3times 10^{8})}{(3times 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 m_o   = 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. offers online tuition for Science classes. Take Online Tests, participate in Live Discussions and get free assignment help on .