Structure of an Atom

Atomic structure refers to the structure of an atom containing nucleus (at the centre) in which there are protons (positively charged) and neutrons (neutral) and electrons (negatively charged) revolving around the nucleus.

One of the most important and fundamental chapters of Class 11 NCERT syllabus is Structure of an atom. In this chapter, you get to learn about the constituents of an atom, various atomic theories, their drawbacks, and the experiments which were done to establish those theories. Once you have read the chapter, you need to solve the important questions Class 11 chemistry chapter 2 - Structure Of An Atom has to offer. Knowing the answers to all these questions will help you form a solid understanding of the topic. This page compiles for you all the important concepts covered in the chapter from where questions usually come in the Boards as well as other competitive exams after Class 12. 

Atomic Structure: An Overview

The history of atomic structure and quantum mechanics reiterates only one name who first proposed that matter is made up of atoms; the name is Democritus. The entire class of chemical reactions, bonds and their physical properties are deeply correlated with the atomic structure. After Democritus, it is John Dalton who proposed the first scientific theory of atomic structure in 1800. The discovery of atomic structure led to many other inventions like subatomic particles. These are all the fundamental particles which compose the structure of a matter. 

Describe The Atomic Structure In Detail

The structure of Atom comprises of its nucleus and the organization of the electrons around it. The primary structure of an atom consists of protons, electrons and neutrons. The nucleus of the atom is composed of protons and neutrons. The nucleus of the atom is surrounded by the electrons. The total number of protons in the nucleus is termed as the atomic number. 

There are equal numbers of protons and electrons in the neutral atoms. But, in order to increase their stability atoms may gain or lose electrons. As a result, a charged entity is found which is called an ion. 

Because of the different number of protons and electrons atoms of different elements have distinguished atomic structures. The above phenomenon results in the unique characteristics of different elements. 

What is Dalton’s Atomic Theory?

The English Chemist John Dalton proposed the theory that every matter is composed of atoms which are indivisible and indestructible. The postulates of Dalton’s theory are given in the following:

  • All the matters are composed of atoms.

  • Atoms are indivisible and indestructible.

  • Each and every atom comprise of its own constant mass that differs from element to element.

  • During a chemical reaction, atom get rearranged. 

  • There is neither creation nor destruction of atoms, the only transformation from one form to another can be possible. 

The Laws of chemical reactions was successfully explained by Dalton’s atomic theory which includes the Law of conservation of mass, Law of constant properties, Law of multiple proportions and Law of reciprocal proportions. 

Drawbacks of Dalton’s Atomic Theory:

  • Existence of isotopes was not explained by the theory.

  • Appropriate explanation of the structure of the atom was missing.

  • Later on the discovery of the presence of particles inside the atom proved that the atoms are divisible. 

What was Thomson’s Atomic Model?

The English chemist Sir Joseph John Thomson was better known for his discovery of electrons for which he got the Nobel Prize. The following experiment was performed by him to invent electron:

Cathode Ray Experiment:

A tube of glass with two openings, one for the vacuum pump and other for the intake through which a gas is pumped in is present is taken.   

(Image to be added soon)

A partial vacuum is maintained by the vacuum pump inside the glass chamber. A high voltage power supply is linked with the help of electrodes i.e. Cathode and Anode is placed inside the glass tube. 


  • There were rays transmitted from the cathode towards the anode when a high voltage power supply is connected. The ‘fluorescent spots’ present on the ZnS screen confirms the above-mentioned fact. These rays were termed as ‘Cathode Rays’. 

  • The cathode rays get deviated towards the positive electrode when an external electric field is projected. But they are in a straight line in the absence of electric field.

  • They seem to rotate when rotor blades are fixed in the path of the cathode rays. The fact of composing of cathode rays by particles of a certain mass has been proved by the fact. 

  • Compiling all the pieces of evidence, Thomson concluded that cathode rays are composed of negatively charged particle called ‘electrons’. 

  • The charge to mass ratio (e/m) was found by applying the electric and magnetic field upon the cathode ray (electrons). (e/m) for electron: 17588 X 1011 e/bg

Mullikin found the charge of the electron by the help of this ratio through an oil drop experiment. Charge of e- = 1.6 x 10-16 C and Mass of e- = 9.1093 x 10.31 kg

Thomson described the structure of the atom as a positively charged sphere into which negatively charged electrons were entrenched on the basis of his experiment. The popular name of Thomson’s model is “plum pudding model” because it can be seen as a plum pudding dish where the pudding means the positively charged atom and the plum pieces stand for the electrons. 

Drawbacks of Thomson’s Model:

Rutherford invented another subatomic particle called ‘nucleus’ by modifying the atomic structure of J.J. Thomson. He has performed the Alpha Ray Scattering Experiment:


  • A very thin gold foil comprising of 1000 atoms thick is taken.

  • The gold foil supposed to be bombarded by Alpha rays (doubly charged Helium He2+).

  • ZnS screen is located behind the gold foil. 


  • Rutherford concluded that a maximum portion of the atom is empty because most rays went through.

  • Few rays got reflected as there might be a repulsion of its positive charge with some other positive charge inside the atom.

  • It was observed that 1/1000th of rays got sharply deflected as a result of a very strong positive charge in the centre of the atom. This positive charge was termed as ‘nucleus’.

What Is Rutherford’s Atomic Structure?

  • Most of the storage and mass of the atom is concentrated in the nucleus.

  • The shape of the atom is spherical.

  • Nucleus is revolved around by the electrons in a circular orbit. 

What Are The Subatomic Particles?

The subatomic particles are:


The positive charge of Proton is 1e which is near about 1.602 x 10-19. The mass of a proton is nearly 1.672 x 10-24. Electrons are 1800 times lighter than protons. The atomic number of the element is equal to the number of protons.


Mass of the neutron is 1.674 x 10-24. Neutrons are electrically neutral subatomic particle. Isotopes are the elements with the same number of protons but a different number of neutrons. 


An electron consists of charge of -1e i.e. -1.602 x 10-19. The mass of the same is around 9.1 x 10-31. Electrons are neglected at the time of calculating the mass of an atom due to the relatively negligible mass of electrons. 

There are various arguments in the theories of the atomic structure but the basic structure of the atom consists of protons, neutron and electrons. The nucleus is present in the centre of the atom.

FAQ (Frequently Asked Questions)

1. What is The Atomic Structure of Isotopes?

The nucleus of an atom is comprised of Nucleons. A nucleon can either be positively charged or negatively charged. Each element consists of a unique number of protons in it. This is described by a unique atomic number. Various atomic structures of an element can exist which vary in the total number of nucleons. Isotopes are defined as the variants of elements having a different nucleon number (also known as the mass number). Hence, the isotopes of an element are the elements having the same number of protons but a different number of neutrons. The chemical symbol of the element, the atomic number of the element and the mass number of the isotope describe the atomic structure of an isotope. The isotopes of hydrogen are protium, deuterium and tritium. The isotopes of an element are differed by the stability also. But, isotopes generally depict similar chemical behaviour owing to the fact that they carry the same electronic structures. 

2. What is The Atomic Structure of Hydrogen?

Protium is the most abundant isotope of hydrogen on the planet earth. The atomic number and the mass number of protium are 1 and 1 respectively. There is one proton, one electron and no neutrons (total number of neutrons = mass number – atomic number) in the element. 

3. What is The Atomic Structure of Carbon?

There are two stable isotopes of carbon namely 12C and 13C. The abundance of 12C is 98.9%. It is composed of 6 protons, 6 electrons and 6 neutrons. The electrons are segregated into two shells and the outermost shell (valance shell) contains four electrons. The ability of carbon to form a variety of chemical bonds with different elements is because of the tetravalency.

4. What is The Structure of An Oxygen Atom?

The three stable isotopes of Oxygen are 18O, 17O and 16O. But the most abundant isotope in nature is oxygen-16. The atomic number of this isotope is 8 and the mass number is 16. That means it is composed of 8 protons and 8 neutrons. Out of 8 electrons, 6 electrons in an oxygen atom fill-up the valence shell. 

5. What Were The Limitations of Rutherford Atomic Model?

If the electrons revolve around the nucleus, energy will be spent by them and that would be against the strong force of attraction from the nucleus. Gradually they will lose all their energy and will fall into the nucleus. Hence, the stability of the atom is not explained. According to Rutherford, the spectrum would be a continuous spectrum. But in reality, it is line spectrum.