Classification of elementary particles
Dalton’s atomic model should as the primary basis of chemistry and it grows up in the last century. The chemical reaction in chemistry clearly explained by this model.
- All the matter made of atoms is indivisible and indestructible.
- All the atoms of a given element are identical mass and properties.
- Compounds are formed by the combination of two or more same or different kinds of atoms.
- A chemical reaction is a rearrangement of atoms.
But Dalton’s model of indivisibility of atom discarded by the discovery of various list elementary particles like proton, neutrons, and electrons.
The modern structure of an atom
Rutherford has remarked that it is not in the nature of things for any one man to make the sudden violet discovery. Science goes step by step and every man depends on the work of his predecessor.
The journey from Dalton’s model to the modern structure of an atom was a long and arduous one. This century much valuable information was being compiled.
This study clearly indicates that Dalton’s atomic model no longer enjoyed the exalted position to grant it. Today an atom considered to made up of a tiny nucleus carrying neutrons and protons.
The nucleus of an atom has around itself a certain number of the negatively charged particles carrying negligible mass, called electrons, arranged in a definite order.
JJ Thomson cathode ray experiment
Gases at low pressures, when subjected to high potential, becomes conducting and the various luminous effect was observed. When the pressure quite low (0.01 mm), the tube remains dark (Crooks dark space) but a streak of rays, named cathode rays, emanating from the cathode.
Characteristics of cathode rays
- 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 particle.
- When these rays impinge on the metal targets placed on their path x-ray is produced.
Charge and mass of an electron
The electron carrying negatively charged and the charge of an electron
= – 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
The electrodeposition of silver atom from an aqueous solution of silver salt. This is a suitable experiment for the measurement of the charge of an electron.
Faraday’s Law readily interpreted by reference to the electrolysis of silver nitrate. The change at the cathode requires one electron for every silver ion reduced.
Ag⁺ + e → Ag
If the electrons consumed at this electrode equal to Avogadro number, 1 mole of a silver atom produced. At the same time, 1 mole of electrons removed from the anode and 1 mole of nitrate ions discharged.
96500 coulomb of electricity necessary to produce 1 equivalent mass of a substance at the electrode will be total charge carried by 1 mole of electrons.
∴ The charge carried by each electron
e = (96500-coulomb mol⁻¹)/(6.023 × 10²³ mol⁻¹)
= 1.60 × 10⁻¹⁹ coulombs.
Discovery of protons experiment
Electrons contribute negligibly to the total mass of an atom and an atom is electrically neutral. Thus nucleus of an atom must carry atomic particles which will account both for the mass and positive charge of the atom.
The production of cathode rays in discharge tubes inspired physicists to look for the oppositely charged ions, namely positive ions.
Goldstein added a new feature to the discharge tubes by using holes in the cathode. With this modification, it is observed that on operating such discharge tubes there appeared not only cathode rays traveling from the cathode to anode but also a beam of positively charged ions traveling from around anode to cathode.
Some of the positively charged atomic particles passed through the hole in the cathode and produce a spot on the far end of the discharge tube.
The nature of these positive rays extensively investigated by Thomson. It proved much more difficult to analyze the beam of the positive rays than to analyze a beam of electrons.
On deflection by a magnetic and electric field, the positive ray beam produced a large defuse spot indicating that the e/m ratio of the constituents of the beam was not the same and that the atomic particles moved with different velocities.
Thomson further demonstrated that each different gas placed in the apparatus gave a different assortment of e/m.
The hydrogen atom is the simplest atom with one electron revolving around the nucleus. The nucleus of the hydrogen atom carry unit positive charge. When the only electron of the hydrogen atom has removed the nucleus of hydrogen atom contains unit charge and mass.
The particle represented by hydrogen ion called a proton considered as a list elementary particle that accounts for the positive charge of the nucleus.
Relative charge and mass of a proton
The proton carrying positively charged and the charge of a proton
= +4.8 × 10⁻¹⁰ esu
= +1.60 × 10⁻¹⁹ coulomb.
Let the mass of an proton = m and charge = +e.
∴ e/m = 9.3 × 10⁴ coulomb/gram.
∴ Mass of a proton = 1.6725 × 10⁻²⁴ gm.
Discovery of the neutron by Chadwick
We have study the entire mass of an atom is concentrated in the nucleus and the weight of electrons being negligible.
Atomic number and mass number of hydrogen = 1 and 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.
But helium atom contains 2 protons, which account for two units of mass. The other two units must have been due to other particles.
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.
Atom nucleus on the basis of neutrons and protons
Rutherford suggested this shortfall must be made up by another elementary particle. This elementary particle has electrically neutral, and mass equal to that of the proton. Rutherford named this particle in advance as a neutron. The glory of discovering the neutron went to Chadwick, one of Rutherford students.
The study of the atomic nucleus on the basis of neutrons and protons now a simple affair. 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.
Since neutrons contribute only to the mass of the element do nothing towards the charge.
Some of the species with the same number of protons varying numbers of neutrons inside the nucleus.
Such species must belong to the same element and must vary only in their mass numbers. They are called isotopes.
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.