CBSE Class 11 Chemistry Revision Notes Chapter 2

CBSE Chemistry Notes for Class 11 Chapter 2: Structure of Atom

The Revision Notes from Extramarks offer a concise guide for Class 11 Chemistry Chapter 2: Structure of Atom which contains diagrammatic representations of all the important models, basic structures of atoms and their properties. Students will have an easier time compiling the vast amount of information in the chemistry discipline with the help of this guide.

CBSE Chemistry Notes for Class 11 Chapter 2 – Structure of Atom – Free Download

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Class 11 Chemistry Chapter 2 Structure Of Atom – In a Nutshell

The lessons in Class 11 Chemistry cover more than just states of matter; they also include advanced concepts like radioactivity, atomic models, and a deeper understanding of particles. 

Chapter 2: The Structure of Atom is vital for students to learn because it includes complex ideas based on the atomic model. They learn about atoms, electrons, neutrons, protons, and other topics. Additionally, the chapter covers shell configuration and how elements complete their shells. It also describes atom valency and the methods used by scientists to find these particles.

Class 11 Chapter 2: The Structure of Atom Notes from Extramarks covers these ideas with clarity to make understanding easier.

Important Topics Covered Under Chemistry Class 11 Chapter 2 Notes

Given below are the topics included in Extramarks Chemistry Class 11 Chapter 2 which will help you to give precise answers to questions.

  • A basic introduction to atoms
  • What are atoms?
  • What are the subatomic particles of an atom?
  • Other basic definitions of subatomic particles
  • Thompson model
  • Rutherford’s model
  • Atomic mass and atomic number
  • What are electromagnetic radiations?
  • Electromagnetic spectrum
  • Planck’s quantum theory
  • Spectrum
  • Bohr’s model
  • Quantum mechanical model
  • Probability distribution of 1s and 2s

Chemistry Notes for Class 11 Chapter 2 – Revision Notes


Whether it’s reactions or bonds with other elements, chemistry is all about understanding three particles thoroughly. Extramarks Notes contain the definition, discovery, and properties of the given particles:

  • Proton (positively charged)
  • Neutron
  • Electron(negatively charged)

The cathode ray discharge tubes that contributed to the understanding of electrons will be covered in more detail. Along with a clearly labelled diagram that depicts every component of the CRT (Cathode Ray Tube), the discovery process is also described. The compressed points of the properties of electrons are given in Extramarks Class 11 Chapter 2 Notes. In order to retain information longer, every aspect is presented in a simple and straightforward manner.

Other topics included in the notes are:-

  • The charge of an electron
  • The charge-to-mass ratio of an electron
  • Origin of anode and cathode rays

Important Note: An electron has a charge of 1.602176 x 10 -19 Coulombs. Students need to remember this value as it is needed in solving numerical questions.

The distinction between the terms cathode and anode is that while electrons flow out of the cathode, they flow into the anode terminal.

  • The charge on the cathode is negative
  • The charge on the anode is positive


This section contains the definition of the three fundamental particles written concisely. 

  • Proton- a stable subatomic particle with a positive electrical charge, which is equal to and opposite to that of an electron. 
  • Neutron- a subatomic particle found in the nucleus of every atom except simple hydrogen. Neutrons carry no electrical charge.
  • Electron- a subatomic particle that is negatively charged and can be either free or bound to a nucleus.

Detailed information on every particle, as well as its electric charge and structure, is provided in Extramarks Class 11 Chapter 2 Revision Notes.

Thomson Model

One of the pioneering researchers to propose the atomic model was Thompson. He has found that an atom contains electrons. 

Extramarks Class 11 Chapter 2 Notes present this experiment in a concise manner with a diagram, making it easier to understand.

In the diagram, a sphere with combined positive and negative charges can be seen. As the charges cancel each other out because they are present in the same location, the model demonstrated the overall neutrality of an atom. 

Therefore, it was assumed that since there is a balance maintained between the charges, the element would not react or disintegrate and remain intact. However, there were some limitations to this model, which were revealed with time and by other researchers.

Rutherford’s Model

A scientist named Rutherford proposed an atomic model that addressed Thompson’s shortcomings. The calculations and observations in this model are different from those in Thompson’s earlier model.

The Rutherford experiment was titled “scattering.” It is so because the neutron’s central location prevents some particles from being sent in opposite directions.

The following conclusions are supported by this experiment:

  • The neutron must be a large particle.
  • Particles are deflected by a hollow space around them.

It was later discovered that Rutherford’s atomic model had some shortcomings, just like the earlier model. 

Any charged body rotating around a particle is known to continuously emit energy. Additionally, because of the attractive forces present, it is likely to enter the core all at once and strike a neutron.

If this is the case, the neutron will eventually collide with the electrons that are revolving around it. However, doing so would invalidate the reality of the things in our environment and disprove the proposed theory.

Atomic Number and Mass Number 

The atomic number and mass number are two of Physical Chemistry’s fundamental terms. The brief definitions of these terms are:

  • The atomic number, which is symbolised by Z, is the total number of protons (capital).
  • The mass number is denoted by the letter A and is equal to the sum of protons and neutrons (capital).

Based on this, the elements are categorised into:

  • Isobars
  • Isotones
  • Isoelectronic and
  • Isotopes 

In the Extramarks Class 11 Chapter 2 Notes, it is explained in a tabular format for clear understanding. Hydrogen, oxygen, and a few other common element isotopes are highlighted in this section. In addition, isobars and other atomic properties are also shown in tables to make data representation and memorising information simpler. 

Electromagnetic Radiations

The energy forms known as electromagnetic radiation are all around us and can take many different forms, including:

  • Microwaves
  • X-rays
  • Gamma rays and many more

Sun rays are considered to be electromagnetic radiation as well. 

With this in mind, Extramarks has created condensed Chapter 2 Class 11 Notes for students who want a quick review of the important points for each topic.

The wave propagation diagram explains the following:

  • As field scientists Max Planck and Huygens suggested, light can have both a particle and a wave nature.
  • Using a mathematical formula, the relationship between the frequency and wavelength of light is demonstrated.
  • The Planck’s constant, denoted by the letter l, is also used and discussed in these equations.
  • Here, Avogadro’s number (N), which is also written in mathematical notation, will also be mentioned.

Some related terminologies of the wave nature of light:

  • Amplitude
  • Frequency
  • Time
  • Velocity
  • Wavelength
  • Wavenumber

Electromagnetic Spectrum

The entire range of light radiation is referred to as the spectrum. The majority of the light in this universe, from x-rays to radio waves to gamma rays, is invisible to the human eye. The colours that make up a rainbow make up visible light. To identify each one, the spectrum is also labelled according to wavelength. 

Extramarks’ Class 11 Chemistry Notes for Chapter 2 also include:

  • Electromagnetic theory
  • Failure of the approach due to two experiments called, Black body radiation and Photoelectric effect

Helpful guides with relevant points are explained with diagrams in an easy-to-understand manner. 

Planck’s Quantum Theory

The term “quantum” refers to the movement of energy in the form of tiny packets. The word photon, which describes light, is also used to refer to quantum in this context.

  • The energy in each quantum α frequency of radiation.
  • Energy will be in whole number quanta.
  • Explanation of black body radiation.
  • Description of the photoelectric effect.

Extramarks Notes contain the description of the given two cases and several more with their respective equations and observations.

When the threshold frequency is-

  • Higher than the frequency of incident light.
  • Smaller than the frequency of incident light.

The dual nature of the light theory—that it exists as both a particle and a wave—is found to be true. Students can clarify their doubts with the guidelines given on the relationship between kinetic energy and the frequency of light. This will help them produce better answers with solid explanations.


The spectrum is the continuous band or wavelength of light, whether visible to the human eye or not.

It also includes:

  • The continuous emission spectrum shows how light disperses into seven colours using a prism. It is referred to as a “continuous emission spectrum” because there are no gaps in it.
  • With discontinuous emission spectra, there are gaps and omissions in the wavelength distribution.

Bohr’s Model

Scientist Niels Bohr’s model and its effect on other atomic models are explained in Extramarks Revision Notes for Class 11 Chapter 2. 

The postulates given for this model are:

  • The atom is made up of smaller particles that either carries a positive charge (proton), a negative charge (electron) or are neutral in charge (neutron).
  • Every particle has a specific charge, which is measured in Coulombs.
  • In increasing order, starting with the innermost orbit, electrons travel in orbits with natural number numbers.
  • Every orbit has a different energy level, and an equation is used to express this energy of electrons while taking the Rydberg constant for calculation.
  • Bohr’s orbit is a formula for calculating radii during the stationary state of atoms in orbits. 
  • Electrons have a fixed amount of energy and angular momentum. They maintain their current energy level while rotating in a fixed orbit.

Quantum Mechanical Model

There are several new electronic configuration concepts presented under this heading in Extramarks Class 11 Chapter 2 Revision Notes.

Heisenberg’s uncertainty principle and the dual nature of radiation serve as the main foundations for the quantum mechanical model. This important principle has a diagrammatic representation, an explanation, and meaning. 

The Aufbau Principle and Pauli’s Exclusion Principle, along with other topics, are also covered in the Revision Notes in detail.

Quantum Mechanical Model of Atom

Quantum mechanics is a crucial subject that combines Physics and Chemistry. It examines the motion of tiny particles with dual nature, like light. The fundamental equation of quantum mechanics was created by Schrodinger, a scientist and pioneer. 

Short explanations of this model are written based on the given topics in the Extramarks Revision Notes.

  • Probability Density
  • Concept of Orbital
  • Quantum Numbers(principal, azimuthal, spin, magnetic)
  • Angular momentum

 These notes will also give you a clear understanding of:

  • Shells
  • Orbitals
  • Nodes 
  • Nodal planes

This topic, which is thought to be the most crucial in Atomic Structure Chemistry, is covered in an easy-to-memorise format with the help of diagrams, tables, and charts.

Relation Between Various Models and Bohr’s Theory

A collection of equations is available related to the following topics in Extramarks Revision Notes.

  • Rydberg’s equation
  • Bohr’s model
  • Velocity of electron
  • Photoelectric effect
  • Radius of orbit
  • De-Broglie’s equation
  • Heisenberg’s uncertainty principle

Probability Distribution of 1s and 2s

This topic can be better understood with the help of graphs. These six graphs are given in the Extramarks Revision Notes, which show the probability of finding electrons in the 1s and 2s Orbitals.