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Nuclear Structure - Composition and Size for JEE

What is Nuclear Structure Composition and Size?

Last updated date: 19th Mar 2023
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Atomic nuclei contain protons and neutrons, while the electron clouds around the nuclei contain electrons. Protons are positively charged and are very close to the nucleus of an atom. Solid energy is a type of interaction that binds protons and neutrons together. In addition to strong forces, protons will separate the nucleus. It is predicted that 99.9% of the entire mass of an atom is enfolded in the nucleus.  Consequently, the nucleus size is 10,000. times lesser in comparison to the atom’s size. Henceforth, the nucleus utilizes a minute space in the atom which is composed of neutrons as well as protons.

What are the Elements in a Nucleus?

The size of the nucleus or the radius of the nucleus ranges between 1 to $10\times 10^{-14}~m$. This is less in contrast to an which is ${{10}^{-10}}\,m$ in size. Hence, the position of the nucleus is inside the atomPrimarily, the nucleus contains protons that acquire a positive charge and neutrons that are free of any electrical charge. Nucleus is surrounded by an electron cloud containing electrons. 

Structure of an atom

Structure of Atom 

Image depicts the structure of an atom comprising positively charged proton and negatively charged electron particles, and also the neutrons that possess no charge.

We learnt about the composition of the nucleus. Now, let us learn some important properties of electrons, protons and neutrons.


  • The electron is recognized as a crucial particle of an atom.

  • It moves around the nucleus and acquires a negative charge of magnitude $1.6\times {{10}^{-19}}\,C$.

  • Rest mass of an electron is $9.1\,\times \,{{10}^{-31}}kg$.


  • The proton number in a neutral atom is equivalent to the number of electrons.

  • They reside inside the nucleus and have a mass of $1.6\times {{10}^{-27}}\,kg$.

  • They are 1836 times heavier than the mass of an electron.

  • They are positive charged particles of magnitude the same as an electron i.e.$1.6\times {{10}^{-19}}\,C$.


  • It is neutral in charge and resides inside the nucleus.

  • It has a mass approximately equal to that of protons and 1839 times heavier than electrons.

  • They have non-zero magnetic moments.


Atomic Number and the Mass Number

The nuclear structure composition can be demonstrated by the atomic number (Z) and the mass number (A). The number of protons $\left( p \right)$ inside the nucleus gives the atomic number which is indicated by Z. In neutral atoms, the number of electrons (e) is equal to the number of protons, so the atomic number can be also represented by the number of electrons in the atoms. Therefore, Z = p or Z = e 

Mass number (A) is represented by the total number of protons and neutrons inside the nucleus. We have to add the number of protons and the neutrons in the nucleus of an atom to get its mass number i.e.,


Comparison of Electron, Proton and Neutron

Let us learn the difference among Electron, Proton and Neutron on several parameters.



Relative mass


Relative charge


$5.45\times {{10}^{-4}}$


Revolves around nucleus 





Inside the nucleus





Inside the nucleus


Classification of Nuclei

The categorisation is done on the account of the proton number and the total nucleons residing in the nucleus. Primarily, it can be classified in three types: isotopes, isotones, and isobars.

1. Isotopes

Atoms of an element having the same number of protons but different numbers of neutrons are known as isotopes. As the number of protons in a nucleus denotes its atomic number, isotopes are elements with the same atomic numbers but different mass numbers.

For example, protium \[\left( {}_{1}^{1}H \right)\] , deuterium \[\left( {}_{1}^{2}H \right)\] and tritium \[\left( {}_{1}^{3}H \right)\] are the isotopes of hydrogen as they have the same atomic number, \[1\] but different mass numbers \[1,\,2\,\] and \[3\]respectively.

2. Isobars

Atoms of different elements have the same mass number, but different atomic numbers are termed as isobars. As the atomic number is equal to the number of protons, isobars will have different numbers of protons but the same number of neutrons. 

For example, carbon \[\left( {}_{6}^{14}C \right)\] and nitrogen \[\left( {}_{7}^{14}N \right)\] both have same mass number, 14 but different atomic number, 6 and 7 respectively. Carbon and nitrogen both are isobars.

3. Isotones

The nuclei of two or more atoms possessing the same number of neutrons are known as isotones. To understand better, let us take an example of chlorine \[\left( {}_{17}^{37}C \right)\] and argon \[\left( {}_{18}^{38}C \right)\]. 

Number of neutrons = Mass number - Atomic number

Number of neutrons in \[\left( {}_{17}^{37}C \right)=\,37\,-\,17\,\,=\,20\,\]

Number of neutrons in \[\left( {}_{18}^{38}C \right)=\,38\,-\,18\,\,=\,20\,\]

Both chlorine \[\left( {}_{17}^{37}C \right)\] and argon \[\left( {}_{18}^{38}C \right)\] have same number of neutrons, 20 and are isotones.


In this article a comparative study of the particle’s electrons, protons, and neutrons with their properties are formulated in the table. We conclude this article with information about negatively charged particles moving around the nucleus and neutron and proton reside in the nucleus along with that we also discussed about nucleus and its composition, elements in the nucleus, and types of nuclei i.e. isotopes, isotones and isobars were briefly discussed with proper examples.

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FAQs on Nuclear Structure - Composition and Size for JEE

1. What is the importance of nuclear composition of an atom for the JEE exam?

Nuclear composition of an atom is an important topic for all the exams be it JEE or your boards exams also. With proper preparation and revision of this topic you can easily secure 10 - 12 marks in JEE exam. MCQ related to this topic are frequently asked in each year's JEE exam. Nuclear composition of an atom is an important topic if you are planning to do higher studies in Nuclear physics.

2. What are the different types of nuclei?

Depending upon the relative number of nucleons, nuclei are divided into three types: isotopes, isobars and isotones. Isotopes have the same number of protons but different number of neutrons, isobars have equal mass number while in isotrons the number of neutrons is same but number of protons are different.  The periodic table of elements includes 118 known species of atoms, and each of these exists (either naturally or synthetically) in several versions with differing numbers of neutrons, giving rise to a total of about 3,000 different atomic nuclei.

3. What is the size of the nucleus and how is it determined?

Accurate size of a nucleus cannot be predicted but it is in the range of 1 to $10\times 10^{-14}~m$. It is evaluated through the scattering of electrons by the nucleus. Moreover, it is measured in atomic spectra. We can calculate the size of the nucleus, by obtaining the point of closest approach of an alpha particle. By shooting alpha particles of kinetic energy 5.5 Mev, the point of closest approach was estimated to be about \[4\,\times \,{{10}^{-14}}\,m\]. As the repulsive force acting here is Coulomb repulsion, there is no contact.