The three different sub-atomic particles of an atom namely, protons, neutrons and electrons were discovered in the nineteenth and twentieth century.
Discovery of Protons
The presence of positively charged particle in an atom had been first observed in 1886 by E. Goldstein based on the concept that atoms are electrically neutral i.e., it has same number of positive and negative charge. He performed a series of experiments and observed that when a high voltage electricity passed through a cathode tube fitted with a perforated cathode (pierced disk) containing gas at low pressure a new type of ray was produced from positive electrode (anode) which moves towards cathode. These new rays he termed as canal rays, positive rays, or anode rays.
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In 1909, Rutherford discovered proton in his famous gold foil experiment. He bombarded alpha particles on an ultrathin gold foil. Rutherford thought that a hydrogen nucleus must be fundamental building block of all nuclei, and also possibly a new fundamental particle as well, since nothing was known from the nucleus that was lighter. Based on Wilhelm Wien’s theory, who in 1898 discovered the proton in streams of ionized gas, Rutherford postulated the hydrogen nucleus to be a new particle in 1920, which he called proton. Rutherford named it the proton, from the Greek word "protos," meaning "first."
Proton can be Defined as:
“The fundamental particle of an atom, which is denoted by symbol p or p+. It has
positive electric charge of +1e elementary charge and a mass slightly less than that
of a neutron”
The particles that are positively charged are called protons.
A proton usually represented as p its charge is “+1”
The number of protons in the nucleus of an atom is equal to the atomic number (Z)
of the atom.
Number of Protons = Atomic Number
For example, the atomic number of Krypton (Kr) atom is 36. Hence, the nucleus of
the Krypton atom contains 36 protons.
Who Discovered Protons?
Goldstein observed positive rays in the anode ray experiment in 1886.
In 1909, Rutherford discovered proton in his gold foil experiment.
How was Proton Discovered?
In gold foil experiment, Rutherford bombarded alpha particles on an ultrathin gold foil and then detected the scattered alpha particles on zinc sulphide (ZnS) screen. According to Rutherford’s observation,
Most of the alpha particles were not deflected; they passed through the foil.
Some alpha particles got deflected at a small angle.
Very few particles bounced back (1 in 20,000).
Based on these observations, Rutherford proposed the following:
Most of the atom’s mass and its entire positive charge are confined in a small core, called nucleus. The positively charged particle is called proton.
Most of the volume of an atom is empty space.
The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.
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Properties of Protons/Positive Rays/Anode Rays:
They are positively charged.
They travel in straight lines and can cast shadow of the object placed in their path.
These positive rays are deflected by electric as well as magnetic field.
Mass of proton is found to be 1.672 x 10-24 g.
Charge on the proton is +1.602 x 10-19 coulombs.
The volume of a proton is given by 4/3 πr3 (1.5 x 10-38 cm3)
Discovery of Neutrons
By 1920, scientists knew that most of the mass of the atom is in the core of nucleus
and this central core contains proton. However, in 1932, James Chadwick discovered
that the core also contains a new uncharged particle; he called it neutron.
Neutron can be Defined as:
“A sub-atomic particle of an atom denoted by n or n0. It has no net electric charge
and a mass slightly greater than that of a proton.”
For his novel observation, Chadwick was awarded with Nobel Prize in 1935. It is to be noted here that except hydrogen all atoms contain neutrons. Hydrogen atom contains only a proton and an electron.
Who Discovered Neutron?
In his experiment, Chadwick bombarded beryllium atom with high-energy alpha particles.
He observed that some new particles are emitted which carry no charge, and the mass of this particle is same as that of proton. A neutron is usually represented as “n” and its charge is zero.
The total number of protons and neutrons present in an atom indicates the mass number of that atom.
Mass Number = (Number of Protons) + (Number of Neutrons)
Number of Neutrons = Mass Number - Atomic Number or number of protons
In case of krypton,
Mass number = 83.80
Protons = 36
83.80 = 36 + (Number of Neutrons)
Number of Neutrons = 83.80 – 36 = 47.8 or 48
Properties of Neutrons:
These are neutral particles.
The mass of neutrons is equal to that of protons (Mass of neutron is 1.675 x 10-24g).
The specific charge of neutron is zero.
Density of neutron is 1.5 x 1014 g/cc.
Discovery of Electrons
In 1897, J.J. Thomson discovered electrons by working on a cathode ray tube. Thomson demonstrated that cathode rays were negatively charged by passing a high voltage electricity through a cathode tube containing a gas at low pressure. He observed a new type of ray was produced from negative electrode (cathode) which moves towards anode. These new rays of particles were called cathode rays (as they come out of cathode). The key characteristics of cathode rays are as follows:
They travel in straight line.
They carry mass and possess kinetic energy.
The mass and charge of the cathode ray particles are independent of the nature of gas taken in the discharge tube.
An electron is usually represented as “e” and its charge is “-1”. Electron can be defined as:
“The fundamental particle of an atom, which has negative one elementary charge and its denoted by e−. It has mass approximately 1/1836 that of the proton.”
Atoms do not carry any specific electrical charge. Therefore, a balance between the protons and the electrons is necessary for which atoms contain equal numbers of protons and electrons.
Number of Electrons = Number of Protons = Atomic Number
For example, the nucleus of an atom of krypton has 36 protons in it. The balance between protons and the electrons is maintained when a krypton atom has 36 electrons.
Properties of Electrons:
The specific charge (e/m) of electrons was found by Thomson as 1.76 x 108 coulomb/gram. The specific charge of electron decreases with increase in velocity. It is due to increase in velocity which otherwise increases the mass of electron.
Radius of electron is found to be 10-15 cm.
Density of electron found to be 2.17 x 1017 g/cc.
Charge on one mole of electron is 96500 coulombs or 1 faraday.