Isotones definition is given as any of either two or more species of nuclei or atoms that contain a similar neutron count. Since the nucleus of this species of chlorine has 17 protons and 20 neutrons, while the nucleus of this species of potassium has 19 protons and 20 neutrons, potassium-39 and chlorine-37 are isotones.
Two nuclides are said to be isotones if they contain similar neutron number N, but with different proton numbers Z. For example, carbon-13, and boron-12 nuclei both have 7 neutrons, and so they are called isotones. In the same way, 37Cl, 36S, 39K, 38Ar, and the 40Ca nuclei are all isotones of 20 since they all hold 20 neutrons. About its similarity to the Greek word for "same stretching," the word "isotope" was created by German physicist K. Guggenheimer by changing the letter "p" in "isotope" from "proton" to "neutron."
Observationally, the largest numbers of the stable nuclides there exist for isotones 50 (which are five: 88Sr, 86Kr, 90Zr, 89Y, and 92Mo) and 82 (which are six: 139La, 138Ba, 141Pr, 140Ce, 144Sm, and 142Nd). The neutron numbers where there are no stable isotones are given as 19, 21, 35, 39, 45, 61, 89, and 115 or even more. In contrast, the proton numbers, where there are no stable isotopes, are given as 43, 61, and 83 or even more.
This is related to the nuclear magic numbers, which are the number of nucleons that form complete shells within the nucleus. For example, 2, 8, 28, 50, 82. Not more than one stable nuclide contains a similar odd neutron number, except for 1 (it means 2H and 3He), 5 (which means 9Be and 10B), 55 (the 97Mo and 99Ru), and also 107 (the 179Hf and 180mTa). 27 (50V), 65 (113Cd), 81 (138La), 85 (147Sm), and 105 are the odd neutron numbers of a primordial radionuclide and a stable nuclide (176Lu). Neutron numbers, where there exist two primordial radionuclides, are given as 88 (151Eu and 152Gd) and 112 (187Re and 190Pt).
About the discovery of Neutron, Isotopes, Isobars, and Isotones
It’s a remarkable fact that the neutron existence was not discovered until 1932. Protons and electrons are the most general atomic imagination of the time. Through the alpha scattering experiments of Rutherford, it was found that the Atomic mass number, which is ‘A’ of an element, is a bit greater than twice the atomic number, which is ‘Z’ for the most atoms and that importantly all the mass of an atom was concentrated in a tiny space at the center of the atom. The alpha particles, which took a turn of 180-degree stand as proof of this.
Until 1930, some of the electrons were thought to coexist with protons in the dense nucleus, whereas the huge amount of energy needed to sustain such a type of system was way beyond atomic energies. If we take the Hydrogen atom size as 0.2 nanometers, then the electron confinement energy is given as 38eV, which is the exact magnitude for the atomic electrons. Whereas, if the electron were to coexist with protons present in the nucleus, the electron confinement energy is approximately given as 250Mev! Several magnitudes are higher than the 38eV.
What is Neutron?
James Chadwick has provided an answer to this puzzle, who boldly stated this was a new fundamental particle type, that is neutral, and he called them Neutrons. From the conservation of momentum and energy, he was able to derive with considerable accuracy this new particle’s mass. He also found that a neutron’s mass was closer to that of a proton.
MN = 1.00866
U = 1.6749 x 10-27 Kg
So, the nucleus had another resident now, and the proton-neutron pair was known as a Nucleon. The Neutron discovery led to a better understanding of atomic number and atomic mass also with the isotopes; that is what the radioactivity is based on.
N – Neutron Number = Number of Neutron
Z – Atomic Number = means, the number of protons or electrons
A – Atomic Mass Number = Z + N = it means, the total number of protons and neutrons
Hence, the elements of the periodic table now had a new form of representation;
For example, the Uranium atom nucleus can be represented by a 23592 U, which means one atom of Uranium 235 comprises 235 nucleons, where 92 are the protons, and the rest of 143 is said to be neutrons.
What are Isotopes?
Isotopes are the variants of a specific element with a different number of neutrons. For example, the two isotopes of Uranium are given as 23992 U and 23592 U. We can also notice that the number of protons is similar in both the isotopes, but they have 147 and 143 neutrons, respectively. An extra neutron presence significantly changes the behaviour of that specific atom. There exist two different types of isotopes, radioactive and stable. Radioactive isotopes are defined as the ones that are more unstable to sustain themselves, and they break down into two lighter daughter elements spontaneously with the particle emission, such as the rays of alpha, beta, gamma. Stable isotopes are the ones that can exist in their free state without spontaneously breaking down.