How Were Neutrons Discovered?

Till 1930, it was presumed that two fundamental particles are 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.

Where the mass of a proton is 1.6726231 x 10⁻²⁷ Kg of mass of  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

Neutron amu

The rest mass of a neutron we 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 


mn  (amu) = 1.008664904(14)  amu

Rest Mass of Neutron

The concept of rest mass is very simple.

We generally think 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 or its rest mass.

Rest mass is the mass of a body as measured when it is at rest relative to an observer, given by, 

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 = \[\frac{m_{0}}{\sqrt{1 - \frac{(2190)^{2}}{(3 \times 10^{8})^{2}}}}\]

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

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

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

                              = \[m_{0}\] x \[\frac{(3 \times 10^{8})}{(3 \times 10^{8})}\]

Canceling out the common terms, we get \[m_{0}\] x 1

1.6749286  x 10⁻²⁷ equivalent to \[m_{0}\].             

So, we get the value as


m = 1.6749286 x 10⁻²⁷Kg

The same value as the mass of a neutron.

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 center 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.

FAQs (Frequently Asked Questions)

Q1: What is the Approximate Mass of a Neutron?

Ans: A neutron is a subatomic particle that forms a part of the nucleus. The mass of a neutron is equivalent to that of the mass of a proton. It weighs 1 amu.

Q2: Do All Neutrons have the Same Mass?

Ans: The rest mass of a neutron is a constant value. Therefore, all the neutrons have the same masses.

Q3: What is MeV/c2?

Ans: MeV/c2 stands for million electron volts upon charge square. The rest energy of a particle can be calculated in units of MeV by multiplying its rest mass in units of MeV/c2 by c2.

Q4: How Do You Find a Neutron?

Ans: The mass number of an atom is the sum of the number of protons and neutrons in the nucleus, and the atomic number is equal to the number of protons.

For example, Carbon,  6C14 , here no of protons = 6, mass number = 14

So,  mass number (A) = no of neutrons + no of protons

                                 14 =  X + 6

We get number of neutrons (X) = 8

Q5: Do Neutrons Exist Alone?

Ans: No, neutrons cannot exist alone because the half-life of neutrons is about 10 minutes, after that they decay into protons, electrons, and anti-electron neutrinos.

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