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Neutrons, Isotopes, Isotones, and Isobars

Neutrons, Isotopes, Isotones, and Isobars

Neutrons were discovered in 1932. Until then, only electrons and protons were thought to contribute to an atomic structure. Then, neutrons were found from Rutherford’s alpha scattering experiment. He found that the mass of an atom was more than calculated. He discovered there was some mass at the center of the atom contributing to the overall mass of the atomic structure. When the alpha particles reverted at 180 degrees, his hypothesis turned to the surety.


Neutron was discovered by the bombardment of the beryllium atom with the alpha particles. The alpha particles were released from a radioactive source that yielded penetrating particles but non-ionizing radiation. Scientists experienced some neutral energies that confirmed the existence of a neutral ion, having a similar mass to that of a proton at the center of the atomic structure. However, the neutral energy was more than that of the photonic energy.

James Chadwick was the scientist who finally discovered neutrons. He stated a fundamental matter, neutral in energy, and named its neutron. He was able to derive the accurate mass of the particle from the conservation of energy phenomenon. He found that the mass of the neutron was equivalent to the mass of a proton.

MN = 1.00866 u = 1.6749 X 10-27 kg

After this discovery, it was proved that the nucleus of an atom now has three fundamental particles – electrons, protons, and neutrons. Out of these, electrons and protons exist in pairs and are called nucleons. Neutrons led to the concepts of atomic number and atomic mass of an atom. It also leads to isotopes, isobars, and isotones because of the radioactivity of the atomic structure.

Generally, the following notations are used to denote an atomic number, atomic mass, and neutron number-

  • Z – Atomic Number = number of protons/ electrons
  • N – Neutron Number = Number of Neutron
  • A – Atomic Mass Number = Z + N = Total number of protons and neutrons

Neutrons are calculated from the difference between a mass number and the number of protons in an atom. For example, it is represented as 23592 U in an atom, which means Uranium 235 has 235 nucleons, 143 neutrons, and 92 protons. If the number of neutrons and protons is the same of different atoms, then those atoms are called isodiaphers.

Thorium - 234 = 90 Th 144
No of protons (atomic number Z ) = 90
Mass number = no of neutrons + no of protons
Mass number (denoted by A) = 234 and no of protons = 90
No of neutrons = 234 - 90 = 144
Difference between neutrons and protons = 144 -90 = 54
and Uranium-238 = 92 U 238
No of protons (atomic number Z) = 92
No of neutrons = 146
Difference between neutrons and protons = 146 - 92 = 54
From the above calculations, we see the number of neutrons and protons are the same in both atoms. Hence, they are isodiaphers.


Similar atoms having different numbers of neutrons are known as isotopes. For example, consider Uranium atoms 23592 U and 23992 U. Both these two atoms contain 143 and 147 neutrons, respectively. Therefore, if an extra neutron is added to them, it may change the properties of the atom.

There are two types of isotopes – radioactive and stable. Radioactive isotopes are those that are unstable to exist in nature upon itself. They spontaneously break into daughter elements releasing energy and alpha, beta, and gamma particles. Stable isotopes are the ones that exist stably in nature without breaking.


The elements having the same number of nucleons are known as isobars. For example, the series of atoms in the atomic table - 4016S, 4017Cl, 4018Ar, 4019K, and 4020Ca, have the same mass number and are termed as isobars. It may be noticed, even though they contain the same number of particles inside the nucleus, they have different numbers of protons and neutrons.


The atoms that have the same number of neutrons are called isotones. Consider the atoms - 3616S, 3717Cl, 3818Ar, 3919K, and 4020Ca. These have 20 neutrons each. Therefore, they have known isotones as they have the same number of neutrons.

Example 1
Chlorine-37 and Potassium-39
Chlorine (Cl)
No of protons = 17
No of neutrons = 37 - 17 = 20
Potassium (K)
No of protons = 19
No of neutrons = 39- 19 = 20
Since the neutrons are the same, therefore, Cl and K are isotones.

Example 2
32 Ge 76 and 34 Se 78
Germanium (Ge)
No of neutrons = 76 - 32 = 44
Selenium (Se)
No of neutrons = 78 - 34 = 44
Since the neutrons are the same, Ge and Se are isotones.


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