Structure of Atom Class 11 Notes

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Structure of Atom Class 11 Notes

All matter in the universe is made up of atoms – it is the building block of everything. For students of Science stream, knowledge of atoms, its structure and constituent particles is of utmost importance. The chapter of class 11 on Structure of an Atom, which is part of the class 11 chemistry syllabus, thus aims to familiarise students with this topic. Struggling with this chapter? Here are some easy to understand Structure of Atom class 11 notes that will help you in grasping all the concepts in this chapter!

What are Subatomic Particles?

The word atom is derived from the Greek word “a-tomio”, meaning  “non-divisible”. Atoms are the most crucial thing in building blocks of matter, and therefore it is required to understand the structure of the atom and its essential parts i.e., protons, neutrons, and electrons. The chapter of class 11 on the structure of an atom covers all the essential topics in detail.


In the year 1850, Michael Faraday experimented with studying the electrical discharge in cathode ray discharge tubes. Under this experiment, the pressure of different gases was adjusted with glass tubes’ help, and a high voltage was applied across the electrodes. As a result, the current started flowing from the negative electrode to the positive electrode, and it was called cathode ray particles. The experiment showed that cathode rays’ behaviour was like that of negatively charged particles and concluded that cathode rays consist of negatively charged particles called electrons. Thus, it was derived that electrons are the primary constituent of all the atoms. You can refer to the chapter of class 11 on the structure of an atom for further clarification.

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Protons and Neutrons

An electrical discharge in the cathode ray tube led to the discovery of canal rays that were positively charged. These positively charged particles had multiple fundamental electrical charge units and were behaving opposite to that of cathode rays. Thus, the lightest and smallest positive ion obtained from hydrogen was called a proton, and those particles that had mass higher than that of protons was called neutrons. You can also refer to the chapter of class 11 on structure of an atom for getting further clarification on this topic.

Also Read: Class 11 Hydrogen Notes

Atomic Models

Different atomic models were proposed by different scientists to explain charged particles’ movement and behaviour in an atom. While some models explained the stability of atoms, some failed to do so. It is necessary to understand the behaviour of elements in terms of physical and chemical properties and how electromagnetic radiations are emitted and absorbed by the atoms. Let us have a look at some popular Atomic Models covered in Structure of Atom class 11 notes.

Thomson Model of Atom

In 1898, scientist J.J. Thomson proposed that an atom has a spherical shape where the positive charge is distributed uniformly. Because of this, this model was also called plum pudding, raisin pudding, or watermelon model. This model visualized the watermelon of a positive charge that had seeds like electrons embedded into it. 

J.J. Thomson’s atom model explained the overall neutrality of the atom while detailing how an atom is uniformly distributed over the other atom. Go through the chapter of class 11 on the structure of an atom for more details.

What is Charge to Mass Ratio of Electron?

By using the cathode ray tube, British physicist J.J. Thomson applied the electrical and magnetic field perpendicular to each other to find the ratio of electrical charge to the mass of the electron. During the experiment, he finds out that it is possible to bring back the electron to the predefined path by balancing the electrical and magnetic field strength. He also concluded that the mass of the particle affects the electrical and magnetic field.  

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Rutherford’s Nuclear Model of Atom

Scientist Rutherford and his students conducted the alpha particle scattering experiment by directing the high energy alpha particles from a radioactive source to a thin gold metal foil. When the alpha particles were struck on the fluorescent zinc sulphide screen around the gold foil, it produced a tiny flash of light. 

The experiment showed that most of the alpha particles left undeflected, and only a small fraction of particles were deflected. Rutherford concluded that most of the atom’s space is not utilized and left empty, and the electrostatic force of attraction holds the nucleus and electrons. You can also refer to the chapter of class 11 on structure of an atom for more details.

What is Atomic Number and Mass Number?

A nucleus has a positive charge because of protons, and the total number of protons in an atom is known as the atomic number. It is essential because it is unique for every atom in an element. To calculate an atomic number, calculate the number of protons in the nucleus of an atom or the number of electrons present in a neutral atom. 

A nucleon is a combination of protons and neutrons present in the nucleus. The total number of nucleons in an atom is called a mass number of the atom. The mass number equals the number of protons and neutrons.

What are Isobars and Isotopes?

Isobars are the atoms with similar mass numbers but have a different atomic number, while Isotopes have the same proton numbers, but different neutron numbers. The difference between the isotopes happens because of the presence of different neutron numbers present in the nucleus. The chapter of class 11 on the structure of an atom provides comprehensive knowledge on these topics.

Developments leading to Bohr’s Model of Atom

Wave Nature of Electromagnetic Radiation

Thermal radiations consist of electromagnetic waves that vary in frequency and wavelength. In 1870, James Maxwell gave a comprehensive explanation that other electrical and magnetic fields are produced when electrically charged particles are moved with acceleration. These magnetic fields, when transmitted in the form of a wave, are called electromagnetic radiation or electromagnetic waves.

Unlike the other waves that require a medium to move, electromagnetic waves do not require any medium and can move in a vacuum. When these electromagnetic radiations differ in terms of wavelengths, they form the electromagnetic spectrum. The chapter of class 11 on the structure of an atom covers all the topics about electromagnetic waves and radiations.

Also Read: Class 11 Redox Reactions

Planck’s Quantum Theory

When an ideal body uniformly emits and absorbs the radiation of all the frequencies, it is called a black body and the radiation from such a body is called as black body radiation. German Physicist Max Planck in 1900 given the first concrete explanation on how black body radiation occurs and its effect. 

Based on this, he suggested that atoms and molecules can emit and absorb the energy in discrete quantities and not necessarily in a continuous manner. He named the smallest quantity of the energy as a quantum that can emit and absorb the energy in the form of electromagnetic radiation and named the theory as Planck’s Quantum theory.   

What is the Photoelectric Effect?

The photoelectric effect is an experiment performed by scientist H. Hertz in 1887. In the experiment, the electrons were ejected when metals like potassium, rubidium, caesium, etc. were exposed to a light beam. According to this experiment, there was no time lag between the striking of the light beam and the ejection of the electrons. 

The number of electrons ejected was proportional to the brightness of the light. Thus, a photoelectric effect shows the emission of electrons when it gets hit by electromagnetic radiation.

Also Read: Class 11 Chapter 10 S Block Elements

Bohr’s Model

The first person to explain the general features of the structure of an atom of hydrogen quantitatively was Neils Bohr. He developed this model in 1913, using the concept of quantisation of energy developed by Planck. The model is based on a structure that is similar to the solar system – the electrons move around a dense nucleus in circular paths or orbits. it also mentions that the energy of an electron does not undergo any changes although it may move from a lower to a higher stationary state or vice versa.

Developments towards Quantum Mechanical Model of Atom

The shortcomings of the bohr’s model necessitated the development towards a more acceptable atomic model. Some important developments in this regard were as follows.

Dual Behavior Of Matter

In 1924, de Broglie, a French physicist proposed that just like radiation, matter should also display dual behavior – both particle and wave-like properties. it meant that electrons should also have both momentum and wavelength, just like photons. This led to him developing a formula of the relations between wavelength and momentum of any particle of matter.

Heisenberg Uncertainty Principle

As a consequence of radiation and the dual behavior of matter, in 1927  a German physicist Werner Heisenberg developed his uncertainty principle. This principle states that it is not possible to simultaneously determine the exact momentum and exact position of an electron.

Quantum Mechanical Model of Atom

Quantum mechanics is the branch of science that takes into account the dual behavior of matter. It deals with studying the motions of microscopic objects that possess both particle-like and wave-like properties. The quantum mechanical model of the atom is developed through the application of the Schrödinger equation to the atoms. Some of the main features of this model include:

  • The energy of electrons within an atom is limited to certain specific values
  • The quantized energy level is a result of the wavelike properties of the electrons
  • The exact position and exact momentum of an electron cannot be determined simultaneously

Structure of Atom- Practice Questions

The chapter of class 11 on the structure of an atom is relatively easy if you work on practice questions as part of your preparation. Below are some vital question sets for practice:

  1. What is Bohr’s model for hydrogen atoms?
  2. What does the negative electronic energy for hydrogen atoms mean?
  3. What is the difference between electromagnetic radiations and waves?

Structure of Atom Class 11 Notes – Solved Questions

Question 1: How many protons and neutrons are there in the nuclei 13C6?

Carbon mass number = 13
=> atomic number of carbons = no. of proton in one atom of carbon = 6
=> total number of neutrons in 1 atom of carbon = mass number – atomic number 
=> 13-6 = 7

Question 2: Write the possible values of n, l, and ml, if the electron is in one of the 3d orbitals.
Value of n = 3, Value of l = 2, and Value of ml = -2, -1, 0, 1, 2
You can go through the chapter of class 11 on the structure of an atom for more practice questions.

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