Neutrons, Isotopes, Isotones and Isobars

A neutron is a subatomic particle holding no charge. This particle was discovered by James Chadwick in 1932 where he observed that when beryllium was bombarded with the alpha particles neutral radiation was emitted. The application of principles of conservation of energy and momentum stated that the bombardment of beryllium with alpha particles led to extremely penetrating radiations that could not be deflected by an electrical or magnetic field were not protons. They were neutrons because neutrons are chargeless particles, and do not get deflected by an electric or magnetic field.

mp  (mass of proton) = 1.6726231 x 10 ^ - 27 kg

The Neutrons Have Slightly Higher Mass than the Protons Given by,

mn (mass of neutron) = 1.6749286 x 10 ^ - 27 kg


In nuclear physics, isodiaphers refer to the set of elements having different numbers of protons (atomic number) and neutrons and mass number (no of neutrons + no of protons) however they have the same difference between the number neutrons and protons and neutrons excess are same.

To determine if the atoms are isodiaphers, we use the formula,

N - P 


Where P is the no of protons and N is the number of neutrons that can be calculated by the difference in mass number (A) of an atom and the number of protons in that atom. 

If the difference of N- P in each atom comes out to be the same, then those two atoms are considered as isodiaphers.

Isobars and Isotones


The set of elements having the same number of nucleons. Where nucleons are protons or neutrons. For example, 40 Sulphur, 40 Chlorine , 40 Argon, 40 Potassium and 40 Calcium are all isobars.


The two or more atoms or nuclei having the same number of neutrons are called isotones. For example, 36S, 37Cl, 38Ar, 39K and 40 Ca nuclei are isotones as they all comprise 20 neutrons.

Isotones Examples

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

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

Ge and Se are isotones.

Isodiaphers Examples

The atoms of different elements have the same difference of the neutrons and protons.


Thorium - 234  =  90 Th 144

No of protons (atomic number Z ) = 90

Since 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…(1)

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….(2)

As you can see in eq(1) and eq(2) that the difference between the neutrons and protons for Thorium-234 and Uranium-238 are the same. Hence we can say that these two elements are isodiaphers.

Example 2

9 F 19 and Sodium 11 Na 23


No of protons = 9 

and number of neutrons = 10 (19 - 9)

Difference  = 10 -9 = 1


No of protons = 11  and

number of neutrons = 12 (23 - 11)

So the difference will be  = 12  - 11  = 1

Here, we can see that the difference is the same, i.e. 1.

Hence fluorine and sodium are isodiaphers. 

Isomer and isotope


Isomer is a Greek word, which means having equal share or part.

In nuclear physics, any two or more nuclei that possess the same number of neutrons and protons and mass number, however, exist in different energy states and have different radioactive properties.

It can also be said that the nuclei exist in any of several energy states for a measurable period of time.

For example, two nuclear isomers of Cobalt-58 are 58Co and 58mCo

Where 58Co  is a lower energy isomer has a half-life of 71 days and the high energy isomer is 58mCo (here m is for metastable which means 58mCo tends to remain in the state of equilibrium) having a half-life of 9 hours undergoes gamma decay further to form 58Co.

Gamma Decay

It is the stage that occurs when a nucleus is in an excited state and has too much energy to be stable, only energy is emitted however the number of protons remains the same. 


In the radioactive decay, the half-life is the duration of time following which there is a 50% chance that the atom will undergo nuclear decay.


Isotopes are the set of atoms or nuclei that have the same number of protons however different numbers of neutrons.

For example, Carbon-12 and Carbon-14 have 6 protons in each, however, have 6 and 8 neutrons respectively.

FAQ (Frequently Asked Questions)

Q1: What Do You Mean by Isoelectronic? State Some Examples.

Ans: Isoelectronic refers to the set of atoms, ions, or molecules having the same number of valence electrons and the same electronic structure. Since these elements have the same electronic configuration that’s why they possess similar chemical attributes.


  1. Chlorine ion  (Cl -)  and Argon atom (Ar) both have the same number of electrons with the same electronic configuration.

  2. A water molecule (H2O)  and hydronium ion (H3O+) have 8 valence electrons.

  3. N ^ 3- , O^ 2- , F^- , Ne, Mg ^2+, Al ^3+ : they all have electronic configurations given by,  1s^2 2s^2 2p^6.

  4. S^2- , K^ +, Ca^ 2+ and Sc ^ 3+ : they all have electronic configuration given by [Ne] 3s^2 3p^6.

Q2: What is the Mirror Nucleus in Physics?

Ans: Mirror nucleus is an atomic nucleus that comprises a number of protons and a number of neutrons that are mutually exchanged in comparison with another nucleus where the number of protons of the first element (Za) equals the number of neutrons of the second element (Nb) and the number of protons of the second element (Zb) equals the number of neutrons in the first element (Na), such that the mass number is the same given by,

Mass Number (A) = Na + Za = Nb + Zb

For example, an atom nitrogen-15 consists of seven protons and eight neutrons is the mirror nucleus of oxygen-15, containing eight protons and seven neutrons.