Elementary particles like Electron, proton, neutron are discovered by Thomson, Golstine, and Chanweak which constitutes the atoms of all chemical elements in the periodic table. The idea about these elementary particles resulted from faradays famous experiment of electrolysis in 1837. Faraday made an intensive study of the decomposition of salt, acid, and base by the passage of electric current. He establishes the quantitative relations between the amount of electrolysis and the quantity of electricity. These relationships are known as Faraday’s law of electrolysis for learning chemistry. He suggested that current was carry in the solution by charged elementary particles or ions. Therefore, the idea of the atomicity of electricity was finely developed from the ionization of gases. Our present-day, for understanding the chemical bonding of the atom is based on the orbital structure and electronic configuration.
Discovery of Electron Particles
Thomson Cathode rays experiment shows that all the atoms possess negatively charged elementary particles like the photon. He suggested when gases at low pressures subjected to high potential form various luminous effects. When the pressure quite low (0.01 mm), the tube remains dark (Crooks dark space) but a streak of rays, named cathode rays, traveled from cathode to anode.
Cathode Rays and Electrons
- Production of fluorescence on the opposite wall where the rays impinge.
- The rays travel in straight lines confirmed by the shadows of an object placed on their path.
- The cathode rays defected from the path they travel by electric or magnetic field. The direction of deflection suggested that cathode rays are a negative charge electron.
- When these electromagnetic rays impinge on the crystalline solid metal targets placed on their path x-ray is produced.
Charge and Mass of Electron
An electron carrying negatively charged with the value of
= – 4.8 × 10-10 esu
= – 1.60 × 10-19 coulombs
Let the mass of an electron = m and charge = e
∴ e/m = 1.76 × 108 coulomb/gram.
Mass of an electron = (1.60 × 10-19)/(1.76 × 108) gram
= 9.11 × 10-28 gram
Determination of Charge of an Electron
Faraday’s law of electrolysis of silver nitrate used for the determination of the charge of an electron in chemical science. Metallic silver uses as reducing agents at the cathode by decreasing the oxidation number or gaining one electron.
Therefore, the Avogadro number of electrons produced 1 mole of silver at the electrode from this redox process. At the same time, 1 mole of electrons removed from the anode and 1 mole of nitrate ions discharged. But according to Faraday’s law, 96500 coulombs of electrical energy required for the production of 1 gm equivalent of the molecule at the electrode.
∴ The charge carried by each electron
e = (96500-coulomb mol-1)/(6.023 × 1023 mol-1)
= 1.60 × 10-19 coulombs
Discovery of Protons Particles by Goldstein
Electrons contribute negligibly to the total mass of an atom but an atom is electrically neutral. Thus nucleus of an atom must carry elementary particles protons which account both for the mass and positive charge. Therefore, Goldstein added a new feature to the discharge tubes by using holes in the cathode. With this modification, he observed that there appeared not only cathode rays but also a beam of positively charged ions traveling from anode to cathode. Thus some of the positively charged particles passed through the hole in the cathode and produce a spot on the far end of the discharge tube.
Positive Rays Protons Particles
The nature of these positive rays protons investigated by Thomson in physics or chemistry.
- On deflection by a magnetic and electric field, the positive ray beam produced a large diffuse spot on the tube.
- The e/m ratio and velocity of these elementary particles are not the same as electrons.
- Thomson further demonstrated that each different gas placed in the apparatus gave a different assortment of e/m.
Electrons Protons Neutrons in Hydrogen
The hydrogen is the simplest discovery with one electron and proton without any neutron. Therefore the nucleus of the hydrogen atom carry unit positive charge. When this electron of the hydrogen has removed the nucleus contains unit charge and mass. Therefore the particle represented by hydrogen ion called a proton considered as an elementary particle which accounts entire positive charge of the nucleus.
Charge and Mass of Proton
The proton carrying positively charged and the charge of the proton
= +4.8 × 10-10 esu
= +1.60 × 10-19 coulomb
Let the mass of proton = m and charge = +e
∴ e/m = 9.3 × 104 coulomb/gram
∴ Mass of proton = 1.6725 × 10-24 gm
Discovery of Neutron Particles by Chadwick
The entire mass of an atom concentrated in the nucleus and the weight of electrons being negligible. Atomic number and mass number of hydrogen = 1. Therefore protons alone account for the total mass of hydrogen atom.
But except hydrogen, the proton alone cannot account for the total mass of the nucleus. Helium atom 4 times heavy as an atom of hydrogen, hence helium nucleus must be 4 times heavier than a proton. Therefore, to resolve this anomaly Chanwick discovery a new elementary particle called the neutron. Let the mass number of an atom = A, nuclear charge, or the number of protons of the atom = Z. Therefore, (A – Z) shortfall of mass number due to other particles like a neuron.
Neutrons and Protons Particles in Atom
Rutherford suggested this shortfall must be made up by another elementary particle. These elementary particles have electrically neutral, and mass equal to that of the proton. Rutherford named this particle in advance as a neutron. But this glory of discovery neutron went to Chadwick, one of Rutherford students. Mass number and the atomic number of oxygen 16 and 8 respectively suggested that the atomic nucleus of oxygen composed of 8 protons and 8 neutrons and 8 electrons.
But some of the species with the same number of protons varying numbers of neutrons inside the nucleus are called isotopes. Such species must belong to the same element and must vary only in their mass numbers.