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The atom’s existence was first proposed in 400 BC by Indian Maharishi Kamaad and Greek Philosopher Democritus. According to them, on continuing the division of matter, a stage is reached beyond which further division is impossible. All matter comprises extremely small indivisible particles, Indian Manunshi Kamad and Greek Philosopher Democritus called this extremely small indivisible particle as an Atom. The word “Atom” is taken from the Greek word “Atomos”’ which means uncuttable or non-divisible. Thus, all these assumptions related to an atom were based on the philosophers thoughts. Until the 18th century, no specific work was done in the direction of its proof.
After several centuries, in 1808, John Dalton proposed the atomic theory. According to him, all nutters are made from extremely small particles known as atoms. Atoms are indivisible particles. The atoms of an element are identical and differ from other elements.
Subatomic Particles
No attempts were made either by Dalton or his contemporaries to know about the internal structure of matter. However, towards the end of the 19th century and at the beginning of the 20th century. Scientists like JJ. Thomson, Goldstein, Rutherford, Chadwick, Bohr and many more, as a result of their research, established that an atom is made up of subatomic particles; electrons, protons and neutrons. These are known as Fundamental Particles.
Discovery of Electron:-
In 1830, Michael Faraday showed that chemical changes occur when electricity is passed through the electrolyte. Faraday established the relationship between electricity and water and gave the laws of electricity. By the result of Faraday’s experiments, the particle nature of electricity was known. This way, the journey of discovery of electrons started. Sir Julius Plucker, William Crooks, J. Pinien & J.J. Thomson contributed to the study of cathode rays. Thus, the study of cathode rays led to the discovery of electrons. J. J. Thomson performed various experiments for the study of cathode rays and proposed the result that cathode rays are made up of particles. These particles originated from an atom. Thus, the atom was discovered to be divisible. Thomson called these particles as corpuscles. The corpuscular particles discovered by Thomson in 1997 were named ‘Electrons’ by G. Johnstone Stoney. Thus, the credit for the discovery of electrons goes to J.J. Thomson. Thomson was awarded the Nobel Prize for this discovery.
In the nineteenth century, various scientists studied electrical discharge in partially vacuum tubes known as cathode ray tubes. These cathode ray tubes are filled with gas at low pressure and then electric discharge is bypassed at high voltage. The flow of electricity starts from the negative electrode ‘Cathode’ towards the positive electrode ‘Anode’. These are known as Cathode rays.
Properties of Cathode Rays:
On passing the electricity in gas at low pressure in the discharge tube, gas particles collide with cathode by which small particles at high speed are emitted. These small particles are known as the electrons. In this way, cathode rays are the flow of electrons. The main properties of cathode rays are given below:
Cathode rays travel in a straight line: When an opaque object is placed in the path of cathode rays, its shadow is produced at the end opposite to cathode. This shows that cathode rays are travelling in a straight line.
Cathode rays consist of material particles: This was indicated by a light paddlewheel made of mica that starts rotating when placed in the path of cathode rays.
Cathode rays have a negative charge: Cathode rays get deflected towards a positive plate when an electric field is applied. This shows that cathode rays are made up of negatively charged particles.
Cathode rays produce a heating effect: A thin metal foil placed in the path of cathode rays gets heated up due to incandescence.
Cathode rays produce X-rays: They produce X-rays when they strike against hard metals like tungsten, copper, etc.
Discovery of Proton;
Atom is electrically neutral, and electrons are fundamental particles present in it which carry negative charge. So, it is obvious that there must be positively charged particles to neutralise negatively charged electrons. In 1886, Goldstein used a perforated cathode in a discharge tube and observed that rays of positively charged particles move from the anode to the cathode, producing fluorescence on the cathode wall. The rays were called Canal rays, Anode rays or Positive rays. The mass of this particle is equal to the mass of the atom of gas taken in the discharge tube.
The observed anode particle carries a positive charge if hydrogen gas is taken in the discharge tube. The mass of the particles is equal to the mass of hydrogen, i.e., it is a unit. This positive particle is known as Proton. Proton is the fundamental constituent of an atom that carries a positive unit charge of 1.602x 10-¹⁹ coulomb or 48×10-¹⁰ esu charge and the mass of the proton is 1.67 x 10-27 kg or 1 amu, which is the mass of hydrogen. It is represented as ¹H1.
Discovery of proton:-
When fast-moving particles strike Beryllium Metal, then neutral particles are emitted whose mass is equal to the mass of hydrogen; due to their neutral nature, they are known as Neutrons. Chadwick discovered these particles in the year 1932.
Thus, Neutron is the fundamental particle of an atom that has no charge and whose mass is 1.6747 x 10-²⁷ kg (1.008 amu). Normally a Neutron is supposed to be a neutral particle of unit mass. Protons are positively charged particles. It carries a positive unit charge, and its mass is equal to the mass of a hydrogen atom.
Atoms are electrically neutral. Thus, the number of protons present in the atom’s nucleus equals the number of electrons in an atom. This represents the Atomic number which is denoted by Z.
In this way,
Atomic number = Number of protons = Number of electrons
Conclusion
When the electron, Neutron and Proton constitute together, this forms an atom. The Rutherford nuclear model explained that the nucleus has a specific position in the atom. Any atom is identified by the number of protons (positively charged particles) present in the nucleus. In 1913, Moseley calculated the positive charge present in the atomic nucleus, known as the atomic number.
In this way.
An atomic number of any element equals the number of protons in the nucleus of an atom, or
Equal Atomic number = Number of protons to the number of electrons in the neutral atom.
