29.THE ATOMIC STRUCTURE – Discovery of neutron.

In the 19th century,the research on the structure of the atom was yielding such incomprehensible results that scientists worldwide were astounded by its revelations.However, this also stoked up the desire to find out more about the atom and thus the nature of the universe.

It is remarkable though that admist this research , a very fundamental particle – the neutron was not discovered till 1932! In 1920, Ernest Rutherford pointed out the disparity between the atomic number and the atomic mass of atoms.For example, why does Helium (Z=2) has mass  four times than that of hydrogen (Z=1) ? Electrons don’t have significant mass.So all the mass of an atom is that of its protons which is concentrated in the nucleus.If Helium has 2 protons and hydrogen has 1 proton in its nucleus , then the mass of helium should be double than that of hydrogen.So, why is it that helium is four times heavier than hydrogen?
Mass of helium = 4×(Mass of hydrogen)…WHY ?

Thus, Rutherford had conceptualised that there was a possibility of existence of some other particle, which is electrically neutral, which is a component of the nucleus. But there was no solid evidence  to prove that.

james_chadwickIt was in  1932, that James Chadwick ,an English Physicist , discovered this sub atomic particle! In his undergraduate days(1911) , James Chadwick was Rutherford’s student in Victoria University of Manchester. In 1913, he was awarded a research scholarship and he went to Berlin,Germany, to study under Hans Geiger.The first world war broke in 1914, and James Chadwick became a prisoner of war in Germany.When the war ended,after 4 years, he  went back to his mentor, Ernest Rutherford in Cambridge University.In 1921 ,Rutherford made him assistant lab director at Cavendish lab in Cambridge University.In 1932, he was awarded the noble prize for the discovery of neutrons!

James Chadwick , who was working with Rutherford in Cavendish lab , was assigned the task of finding out the neutral particle ,which Rutherford had predicted earlier.

In 1931,  Walther Bothe and Herbert Becker, who discovered that when α-particles hit a target of a lighter metal like Beryllium,Lithium or Boron , they emitted strong penetrating radiation.This radiation was first thought to be gamma rays but later it became evident that it was something else.This radiation was far more penetrating than the gamma radiation.
Irene Curie and her husband discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin(an alkane), protons were knocked off from it, which could be detected by a Geiger counter.

 James Chadwick , started his experiments in this direction to find out the nature of this radiation. The experiment he conducted was as follows –



As shown in the above figure , he used  Polonium as a source of α- particles. Polonium (radioactive element) emits α- particles, which hit the  beryllium foil , which in turn lead to emission of some kind of radiation.This emitted radiation was not deflected by electric field.Thus, it was electrically neutral.This radiation was very penetrating.Thus, it knocked off protons(H+) from the paraffin slab.when these protons (H+) enter the gas-filled chamber, they ionised the gas within. The recorder, a Geiger counter, recorded amount of gas ionised ,which was proportional to the amount of protons entering the chamber.

Thus, James Chadwick proposed that it was the neutral particle present in the nucleus of the atom ,whose presence, Rutherford had predicted earlier.Thus, he discovered the neutron!The nuclear reaction that produced neutrons was as follows –


Later he found the mass of this neutron to be nearly as equal to that of the proton(lightly larger than the proton mass).Thus, three fundamental particles of the atom were discovered – THE PROTON,THE ELECTRON and THE NEUTRON. This was a new revelation about the structure of the atom.

It could now be fully understood why the mass of Helium atom was 4 times the mass of hydrogen atom.Helium atom consists of 2 protons and 2 neutrons , which accounted for its mass .The electrons revolved around the nucleus as shown below-


So, was this the final structure of the atom? Was this an inerrant model? Let us find the answers to this question in the next post.Till then,

Be a perpetual student of life and keep learning….

Good Day!


References and further reading –

1) https://en.wikipedia.org/wiki/Discovery_of_the_neutron

Image source –

3)By BruceBlaus – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=33041232

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