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Structure of Atom


Do you know what type of charge a neutron has? Positive, negative or is it neutral? Well, neutrons play a crucial role in nuclear power production. Atom, the smallest unit of matter, is composed of a centre – the nucleus which is surrounded by the subatomic particles called electrons. A nucleus, where all the mass of an atom is concentrated, in turn, is composed of protons and neutrons. Let’s find out more about these neutral subatomic particles below.

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 Who Discovered Neutron

It was observed after the discovery of electrons and protons that all mass of an atom cannot be accounted on the basis of just electrons and protons. Rutherford was unable to explain any other atoms of different elements except for hydrogen on the basis of protons and electrons.

Scientists realized that the model given by Rutherford was incomplete. For example, the atomic mass of helium should be double of the hydrogen atom. However, on calculation, the mass of helium atom was found to be four times more than that of hydrogen. Therefore, Rutherford postulated the presence of neutral particles in an atom. He believed that the neutral particle had the same mass as that of a proton. However, experimental evidence was lacking.

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Experiments Conducted and  their Observations

Later on, various experiments were conducted and several theories and observations resulted in the discovery of neutrons. Few noteworthy experiments and their observations are listed below.

  • Bothe and H. Becker while bombarding beryllium with alpha particles observed electrically neutral radiation in the year 1930. The scientists believed it to be photons with high energies, also known as gamma rays.
  • In the year 1932, Irène and Frédéric Joliot-Curie demonstrated that this ray has the potential to eject protons when it strikes paraffin or any H-containing compounds.
  • This however raised many questions such as how a photon that does not have mass can release a particle 1836 times heavier than an electron (protons). Therefore, it was concluded that the ejected rays cannot be photons.

In the year 1932, James Chadwick conducted the same experiment but he used different bombardment targets other than paraffin. After research and analysis of the various targets, he discovered the presence of a new particle. This new particle has the mass similar to that of the proton but no charge. He named these particles neutrons.

Furthermore, he derived the nearly accurate mass of this new particle by applying the conservation of energy and momentum. He measured the mass of a neutron to be nearly same as that of a proton. M= 1.00866 u = 1.6749 X 10-27 kg


Chadwick Experiment (Source Credit: Wikimedia)

Function of Neutrons

  • James Chadwick concluded the nucleus consists of proton and neutron. Together these two subatomic particles are termed as nucleon.
  • The discovery helped to understand the atomic mass and the atomic number more clearly. It also helped in understanding the concept of isotopes which forms the basis of radioactivity.

The beryllium reaction when it is bombarded with the alpha particle is:


neutrons, azx atomic symbol

X= Chemical Symbol of a particular element
N= Neutron number
Z= Atomic Number= Number of protons
A= Atomic Mass number= Z+N

After the discovery of the neutron, every chemical element present in the periodic table was modified and written accordingly. For instance, uranium was written as 23592 U. This means that a single atom of uranium contains 235 nucleons (protons + neutrons). Out of 235 nucleons, 92 are protons and 143 neutrons.

You can download Structure of Atom Cheat Sheet by clicking on the download button below

Structure of Atom

Properties of Neutrons

  • Neutrons reside inside the nucleus with protons.
  • Hydrogen atom lack nucleus.
  • Mass of a neutron is equivalent to the mass of a proton. Therefore, the mass of neutron= 1.676 × 10-27 kg = 1.676 × 10-24 g. However, the mass of an electron is negligible and was found to be 1/1840 times that of the proton.
  • Neutrons do not have charge and thus they are electrically neutral.


Subatomic Particle Representation Mass Charge
Electron e 9.1×10-31  kg -1.60×10-19 C
Proton p+  (H+) 1.672×10-27 kg 1.60×10-19 C
Neutron n0 1.674×10-27 kg 0.0 C

Solved Questions for You

Question: Neutron was emitted by bombardment of beryllium and _______  ?

  1. alpha particles
  2. gamma particles
  3. beta particles
  4. none

Solution: The answer is 1 (alpha particles).

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