Discovery of Neutron, Isotopes, Isobars and Isotones

It is a remarkable fact the existence of neutron was not discovered until 1932. The general atomic imagination of the time was protons and electrons. Through Rutherford’s alpha scattering experiments, it was found out that the Atomic mass number A of an element is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of an atom was concentrated in a very tiny space at the center of the atom. The alpha particles that took a 180-degree turn stand as a proof to this.

Until 1930, a few electrons were thought to coexist with the protons in the dense nucleus but the immense amount of energy required to sustain such a system was way beyond the atomic energies. If we take the size of a Hydrogen atom as 0.2 nano meters, then the electron confinement energy is 38eV which is the correct magnitude for atomic electrons. But if the electron were to coexist with the protons in the nucleus, the electron confinement energy is approximately 250Mev! Many magnitudes huger than the 38eV.

Who discovered the Neutron?

A breakthrough came when it was shown that the bombardment of Beryllium with alpha particles from a radioactive source yielded penetrating but non-ionizing radiation. Such a neutral radiation confounded the scientists since the only known neutral radiation than were photos. The neutral radiation had it been a photon would exit the beryllium atom with far more energy than it actually does.

The answer to this puzzle was provided by James Chadwick who boldly stated that this was a new type of fundamental particle which is neutral and he called them Neutrons. From conservation of energy and momentum, he was able to derive with considerable accuracy the mass of this new particle. He found that the mass of a neutron was very close to that of a proton.

MN = 1.00866 u = 1.6749 X 10-27 kg

So now the nucleus had another resident, and the proton-neutron pair was called a Nucleon. The discovery of Neutron led to a better understanding of atomic mass and atomic number also with isotopes which is what radioactivity is based on!

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

So now the elements of the periodic table had a new form of representation;

For example, the nucleus of a Uranium atom is represented by a 23592 U which means that one atom of Uranium 235 contains 235 nucleons, of which 92 are protons and the rest 143 are neutrons.

Isotopes, Isobars and Isotones

Isotopes are variants of a particular element with different a different number of neutrons. For example, the two isotopes of Uranium are, 23592 U and 23992 U. You will see here that the number of protons is the same in both the isotopes but they contain 143 and 147 neutrons respectively. The presence of an extra neutron significantly changes the behavior of that particular atom. There are two different types of isotopes, stable and radioactive. Stable isotopes are one that can exist in its free state without breaking down spontaneously. Radioactive isotopes are ones that are too unstable to sustain itself and they spontaneously break down into two lighter daughter elements with the emission of particles such as alpha, beta and gamma rays.

Isobars are elements that have the same number of nucleons (sum of protons and neutrons). The series of elements with 40 Mass number serves as a good example; 4016S, 4017Cl, 4018Ar, 4019K, and 4020Ca. The nucleus of all the above mentioned elements contain the same number of particles in the nucleus but contain varying numbers of protons and neutrons.

Isotones are atoms that have the same neutron number but different proton number. For example, 3616S, 3717Cl, 3818Ar, 3919K and 4020Ca are all isotones of 20 since they all contain 20 neutrons.

Nuclear energy and Radioactivity relies on the unstable isotopes of heavy elements to tap the explosive power of the nucleus. The discovery of one fundamental particle unlocked such amazing doors for humanity! Come fall in love with learning at BYJU’s.


Practise This Question

A famous relation in physics relates moving mass m to the rest mass m0 of a particle in terms of its

speed v and the speed of light c. (This relation first arose as a consequence of special theory of relatively

given by Albert Einstein). A boy recalls the relation almost correctly but forgets where to put the constant 

c. He writes m=m0(1v2)12. Guess where to put the missing c?