Discovery of proton neutron and electron
Dalton provides a basic idea about the structure of an atom. But after the discovery of the electron, proton, and neutron by Thomson, Golstine, and Chanweak it does not grant its position. Today’s atomic structure based on the charge mass and arrangement of these elementary particles like proton, neutron, and electron.
Discovery of the electron by Thomson
Thomson Cathode rays experiment shows that all the atom contain negatively charged elementary particles like electron.
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 rays impinge on the metal targets placed on their path x-ray is produced.
Charge and mass of an 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. Metallic silver reducing at cathode by gaining one electron.
Ag⁺ + e → Ag
When the Avogadro number of electrons produced 1 mole of silver at the electrode. 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 electricity required for the production of 1 gm equivalent of a substance 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 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
The nature of these positive rays protons investigated by Thomson.
- 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 and 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 a 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 the neutron 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 number of protons of the atom = Z.
∴ (A – Z) shortfall of mass number due to other particles like a neuron.
Neutrons and protons in an 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.
- Protium, deuterium, and tritium are three isotopes of hydrogen with zero, one and two protons in the nucleus of the hydrogen atom.
- Oxygen-16, oxygen-17, and oxygen-18 are three isotopes of oxygen with 8, 9, 10 number of protons on the nucleus.