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Discovery of Proton

Overview

In the 19th and 20th centuries, three different subatomic particles of an atom, namely neutrons, protons, and electrons, were discovered.

Define proton

Proton is defined as the elementary particle of an atom, denoted by the symbol p+ or p.

  • It has a positive charge of +1 and a smaller mass than a neutron.
  • Positively charged particles are referred to as protons.
  • The letter ‘p’ denotes proton.
  • The atomic number (Z) of an atom is equal to the number of protons in the nucleus.

Mathematically,

  • Atomic Number = Number of Protons
  • The atomic number of a Krypton atom is 36. As a result, the nucleus of the Krypton atom consists of 36 protons.

Discovery

  • E. Goldstein discovered the existence of positively charged particles in an atom in 1886.
  • It was based on the theory that atoms are electrically neutral. It means that they have the same number of negative and positive charges.
  • He carried out a chain of experiments and discovered that when high voltage of electricity was passed through a cathode tube fitted with a perforated plate of cathode, having gas at low pressure, a new type of ray turned out from the positive electrode i.e. anode travelling towards the cathode.
  • He named these new rays as positive rays, canal rays, or anode rays.
  • Rutherford discovered the proton in his popular gold foil experiment in 1909.
  • He bombarded a fine gold foil with particles.
  • Rutherford reasoned that a hydrogen nucleus must be the basic building block of all nuclei and a new radical particle. It is because nothing was known about the lighter nucleus.
  • Rutherford postulated the hydrogen nucleus to be a new particle, which he referred to as proton, in 1920.
  • This was based on the theory of Wilhelm Wien, who discovered the proton in streams of ionised gas in 1898.
  • The word “proton” comes from protos- Greek word meaning "first."

What led to the proton discovery?

Rutherford bombarded alpha particles on an ultrathin gold foil. Then, he detected the scattered particles on a zinc sulphide screen in the gold foil experiment. Rutherford's observation is that:

  • Most of the particles were not deflected and passed right through the foil.
  • A few particles were deflected at an angle.
  • Only a few particles came back (1 in 20,000).

Observations proposed by Rutherford

  • The nucleus is a small core. It contains the majority of positive charge and the atom's mass. The positively charged particle is referred to as a proton.
  • The majority of an atom's volume is empty space.
  • The number of negatively charged electrons dispersed outside the nucleus equals the number of positively charged electrons inside the nucleus.
  • It explains an atom's overall electrical neutrality.

Properties of protons

  • They have a positive charge.
  • The mass of a proton is thought to be 1.672 x 10-24 g.
  • Positive rays are deflected by both magnetic and electric fields.
  • It has a charge of +1.602 x 10-19 coulombs.
  • 4/3 r3 gives the volume of a proton (1.5 x 10-38 cm3)
  • They travel in straight lines.
  • They can cast shadows on anything in their path.
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