Discovery of Protons and Neutrons

Discovery of Protons

The discovery of protons dates back to the year 1815 when the English chemist William Prout suggested that all atoms are made up of hydrogen atoms (which he referred to as protyles). When canal rays (positively charged ions formed by gases) were discovered by the German physicist Eugen Goldstein in the year 1886, it was observed that the charge-to-mass ratio of the hydrogen ion was the highest among all gases. It was also observed that the hydrogen ion had the smallest size among all ionized gases.

The nucleus of the atom was discovered by Ernest Rutherford in the year 1911 in his famous gold foil experiment. He concluded that all the positively charged particles in an atom were concentrated in a singular core and that most of the atom’s volume was empty. He also stated that the total number of positively charged particles in the nucleus is equal to the total number of negatively charged electrons present around it.

Who Discovered Protons?

The discovery of the proton is credited to Ernest Rutherford, who proved that the nucleus of the hydrogen atom (i.e. a proton) is present in the nuclei of all other atoms in the year 1917.

The Proton was Discovered by Ernest Rutherford

Based on the conclusions drawn from the gold-foil experiment, Rutherford is also credited with the discovery of the atomic nucleus.

How was the Proton Discovered?

  • Ernest Rutherford observed that his scintillation detectors detected hydrogen nuclei when a beam of alpha particles was shot into the air.
  • After investigating further, Rutherford found that these hydrogen nuclei were produced from the nitrogen atoms present in the atmosphere.
  • He then proceeded to fire beams of alpha particles into pure nitrogen gas and observed that a greater number of hydrogen nuclei were produced.
  • He concluded that the hydrogen nuclei originated from the nitrogen atom, proving that the hydrogen nucleus was a part of all other atoms.
  • This experiment was the first to report a nuclear reaction, given by the equation: 14N + α → 17O + p [Where α is an alpha particle which contains two protons and two neutrons, and ‘p’ is a proton]
  • The hydrogen nucleus was later named ‘proton’ and recognized as one of the building blocks of the atomic nucleus.
    Discovery of Protons

Discovery of Neutrons

The discovery of neutrons can be traced back to the year 1930 when the German nuclear physicists Herbert Becker and Walther Bothe observed that a penetrating form of radiation was produced when the alpha particles emitted by polonium was incident on relatively light elements such as lithium, beryllium, and boron. This penetrating radiation was unaffected by electric fields and was, therefore, assumed to be gamma radiation.

In the year 1932, the French scientists Frederic Joliot-Curie and Irene Joliot-Curie observed that this unusually penetrating radiation, when incident on paraffin wax (or other compounds rich in hydrogen), caused the ejection of high energy protons (~5 MeV). The Italian physicist Ettore Majorana suggested the existence of a neutral particle in the nucleus of the atom which was responsible for the manner in which the radiation interacted with protons.

The presence of neutral particles in the nuclei of atoms was also suggested by Ernest Rutherford in the year 1920. He suggested that a neutrally charged particle, consisting of a proton and an electron bound to each other, also resided in the nuclei of atoms. He coined the term ‘neutron’ to refer to these neutrally charged particles.

Who Discovered Neutrons?

The British physicist Sir James Chadwick discovered neutrons in the year 1932. He was awarded the Nobel Prize in Physics in the year 1935 for this discovery.

The Neutron was Discovered by James Chadwick

It is important to note that the neutron was first theorized by Ernest Rutherford in the year 1920.

How were Neutrons Discovered?

  • James Chadwick fired alpha radiation at beryllium sheet from a polonium source. This led to the production of an uncharged, penetrating radiation.
  • This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high hydrogen content.
  • The protons ejected from the paraffin wax (when struck by the uncharged radiation) were observed with the help of an ionization chamber.
  • The range of the liberated protons was measured and the interaction between the uncharged radiation and the atoms of several gases was studied by Chadwick.
  • He concluded that the unusually penetrating radiation consisted of uncharged particles having (approximately) the same mass as a proton. These particles were later termed ‘neutrons’.

Discovery of Neutrons

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Practise This Question

When beryllium is bombarded with α - particles, extremely penetrating radiations which cannot be deflected by electrical or magnetic field are given out. These are

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