Chemistry

Discovery of Electron, Proton, Neutron and Nucleus

Discovery of Electron (Study of Cathode Rays)

Electron is the fundamental particle that is discovered by J. J. Thompson. He conducted an experiment in a discharge tube connected to a vacuum pump under low pressure and very high temperature and voltage. He observed some rays traveled from cathode to anode and producing a glow on a fluorescent material.

Properties of Cathode Rays

  • Travels in a straight line.
  • Possess kinetic energy.
  • Ionizes the gases.
  • Deflects in the magnetic field.
  • Deflects towards the positive plate in the electrical field.
  • Made up of negatively charged particles called electrons.

Mass of electron  = 9.10938356 × 10-31 kilograms

Charge of electron = 1.60217662 × 10-19 coulombs

e/m ratio of electron = 1.758820 × 1011 C/kg

Discovery of Proton (Study of Canal Rays)

Proton is discovered by Goldstein. He used a perforated cathode in the discharge tube. Filled a gas at high voltage and very low pressure, a glow is produced behind the cathode.

Properties of Canal Rays

  • Travels in a straight line.
  • Possess kinetic energy.
  • Ionizes the gases.
  • Deflects in the magnetic field.
  • Deflected towards the negative plate in the electrical field.
  • Made up of positively charged particles called protons.

Mass of proton  = 1.6726219 × 10-27 kilograms

Charge of proton = 1.60217662 × 10-19 coulombs

e/m ratio of proton = 9.58 ×  107 C/kg

Discovery of Neutron

Discovered by Chadwick. He bombarded alpha particles on light elements like Be, B, Li and observed a type of penetrating rays called the neutron.

Mass of neutron  = 1.6726219 × 10-27 kilograms

Charge of proton = Neutral

e/m ratio of proton = 0

Discovery of Nucleus

It is based on the Rutherford alpha rays scattering experiment.

Experiment

In this experiment, he bombarded alpha particles from a radioactive source on a gold foil.

Observation

  • Most of the alpha particles went straight which indicates that most of the space in an atom is empty.
  • Few were deflected which indicates that the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume.
  • Very few alpha particles bounced back which indicates that the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.

Postulates

  • An atom is a very small and electrically neutral particle where the positive charge is present at the center.
  • Electron revolves around the nucleus.

Defects

  • He was unable to explain the stability of the atom. A charged particle continuously losses energy in the form of . Thus, the electron continuously losses energy and ultimately should fall into the nucleus and an atom should destroy in a fraction of seconds but an atom is stable.
  • He was unable to explain the line of the atom.

Some Important Terms

Atomic number (Z)

It is equal to the number of protons or electrons in a neutral atom.

Mass Number (A)

It is equal to the sum of protons and neutrons in the nucleus.

Isotopes

Atoms of the same element having the same atomic number but a different mass number.

Isobars

Atoms of the different elements having the same mass number but a different atomic number.

Isotones

Atoms of the different elements having the same mass number of neutrons.

Isoelectronic

Atoms having the same number of electrons.

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