The lowest unit of matter is an atom, which is made up of three subatomic particles: protons, neutrons, and electrons. The central nucleus is occupied by protons and neutrons, while electrons orbit the nucleus in specific orbits. During the nineteenth and twentieth centuries, these three subatomic particles were identified. Molecules are formed when atoms come together to create molecules, which interact to make matter (solid, liquid or gas).
In this article, We will discuss proton and neutrons and their discovery i.e. how proton/neutron was discovered and properties of protons and neutrons etc.
Discovery of Protons
The presence of positively charged particles 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 the same number of positive and negative charges. He performed a series of experiments and observed that when 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 the positive electrode (anode) which moves towards the cathode. These new rays he termed as canal rays, positive rays, or anode rays.
In 1909, Rutherford discovered protons in his famous gold foil experiment. He bombarded alpha particles on an ultrathin gold foil. Rutherford thought that a hydrogen nucleus must be the 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.”
What is a Proton?
“The fundamental particle of an atom, which is denoted by symbol p or p+. It has a 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 is 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.
Mathematically it can be written as,
Number of Protons = Atomic Number
For example, the atomic number of Krypton (Kr) atoms 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 protons in his gold foil experiment.
How was Proton Discovered?
In a gold foil experiment, Rutherford bombarded alpha particles on an ultrathin gold foil and then detected the scattered alpha particles on a 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 get deflected at a small angle. Very few particles bounced back (1 in 20,000).
Based on these observations, Rutherford proposed the following:
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Most of the atom’s mass and its entire positive charge are confined in a small core, called a nucleus. The positively charged particle is called a proton.
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Most of the volume of an atom is empty space.
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The number of negatively charged electrons dispersed outside the nucleus is the same as the number of positively charged electrons in the nucleus. It explains the overall electrical neutrality of an atom.
Properties of Protons
Protons are also called Positive Rays or Anode Rays. Let us look at the properties of protons here.
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They are positively charged.
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They travel in straight lines and can cast shadows on the object placed in their path.
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These positive rays are deflected by electric as well as magnetic fields.
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Mass of the proton is found to be 1.672 x 10-24 g.
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The charge on the proton is +1.602 x 10-19 coulombs.
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The volume of a proton is given by 4/3 πr3 (1.5 x 10-38 cm3)
What is a Neutron?
Neutron can be Defined as “A subatomic 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 the Nobel Prize in 1935. It is to be noted here that except for hydrogen all atoms contain neutrons. Hydrogen atoms contain only a proton and an electron.
Who Discovered Neutrons?
In his experiment, Chadwick bombarded beryllium atoms with high-energy alpha particles. He observed that some new particles are emitted which carry no charge, and the mass of this particle is the same as that of protons. 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.
Mathematically it can be written as,
Mass Number = (Number of Protons) + (Number of Neutrons)
Or
Number of Neutrons = Mass Number – Atomic Number or number of protons
In the 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
How were Neutrons Discovered?
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James Chadwick used a polonium source to fire alpha radiation at a beryllium sheet. As a result, uncharged, penetrating radiation was produced.
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This radiation was incident on paraffin wax, which is a hydrocarbon with a high hydrogen concentration.
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With the use of an ionisation chamber, the protons ejected from the paraffin wax (when impacted by the uncharged radiation) were seen.
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Chadwick studied the interaction between the uncharged radiation and the atoms of numerous gases and measured the range of the freed protons.
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He came to the conclusion that the unusually penetrating radiation was made up of uncharged particles with the mass of a proton (approximately). Neutrons were later given to these particles.
Properties of Neutrons
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These are neutral particles.
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The mass of neutrons is equal to that of protons (the Mass of the neutron is 1.675 x 10-24 g).
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The specific charge of a neutron is zero.
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The density of the neutron is 1.5 x 1014 g/cc.
What are Electrons?
The electron is defined as a subatomic particle having a negative one elementary electric charge. Electrons are said to be the first generation of the lepton particle family. It is because they have zero known components or substructures, and because of this, they are considered elementary particles.
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 high voltage electricity through a cathode tube containing a gas at low pressure. He observed a new type of ray was produced from a negative electrode (cathode) which moves towards the 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:
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They travel in a straight line.
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They carry mass and possess kinetic energy.
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The mass and charge of the cathode ray particles are independent of the nature of the gas taken in the discharge tube.
An electron is usually represented as “e” and its charge is “-1”. An electron can be defined as:
“The fundamental particle of an atom, which has a negative one elementary charge and it is 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.
Mathematically it can be written as,
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 electrons is maintained when a krypton atom has 36 electrons.
Properties of Electrons
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The specific charge (e/m) of electrons was found by Thomson as 1.76 x 108 coulomb/gram. The specific charge of electrons decreases with an increase in velocity. It is due to an increase in velocity which otherwise increases the mass of electrons.
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The radius of the electron is found to be 10-15 cm.
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The density of electrons was found to be 2.17 x 1017 g/cc.
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Charge on one mole of the electron is 96500 coulombs or 1 faraday.
Discovery of Electrons, Protons and Neutrons
|
Discoverer |
Year of Discovery |
Charge |
Mass |
|
|
Proton |
E. Rutherford |
1909 |
+1.6×10−19 C |
1.67 ×10−27 kg |
|
Neutron |
James Chadwick |
1932 |
0 |
1.67 ×10−27 kg |
|
Electron |
J.J. Thomson |
1897 |
−1.6×10−19 C |
9.11 ×10−31 kg |
The above article is very knowledge full and interesting as it deals with the discovery of protons and neutrons. The properties of neutron and proton are also discussed. Along with this discovery of electrons is also mentioned.