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JEE Main Study Notes for Atomic Structure: Important Topics, Terms, Atomic Theory and Models
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Atomic Structure is one of the essential chapters included in the syllabus of JEE Main. Almost 2-4 questions are necessarily asked in the exam. The Atomic structure helps a lot in understanding the meaning of different terms such as Atomic mass, Atomic Radius, Size of an atom, Electronic Configuration, Orbitals, and its Structure, etc. These terms are widely used while understanding other chapters such as Periodic Table and Chemical Bonding as well. 

  • The components of Atomic Structure in JEE Main is similar to the syllabus provided by the CBSE in NCERT textbooks. Check JEE Main Chemistry Syllabus
  • To become proficient on particular topics related to Atomic Structure, the candidates need to strengthen their concepts by practicing more and more questions from different books. 
  • The students preparing for JEE can refer to the NCERT books in the beginning and later can switch to advanced level books like P. Bahadur and O. P. Tandon that includes all the important topics in detail.

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What is Atomic Structure?

What is Atomic Structure?

In the year 1803, Dalton explained a theory regarding the composition of matter. According to the theory, an atom is defined as the smallest part of matter that is a part of the chemical reaction. According to Dalton’s atomic theory, the law of conservation of mass, law of constant composition and law of multiple proportions very successfully, but are unable to come to any conclusion in many experiments. Further modern studies revealed that an atom is not indivisible but can be broken down.

  • The important parts of an atom or the fundamental particles of an atom include protons, electrons and neutrons. Other than the hydrogen atom, each atom includes these three subatomic particles. 
  • The structure of the hydrogen atom showed that it has only one electron and one proto; however, the number of neutrons present in it is 0. Sir J.J. Thomson and Goldstein respectively proved the existence of electrons and protons in which was based on the discharge tube experiments and was further confirmed by the phenomenon of radioactivity discovered by Becquerel. The neutron was discovered by James Chadwick by bombarding beryllium with rays.

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Dalton’s Atomic Theory

Dalton’s Atomic Theory 

All the objects around you, including book, pen, table, or something else constitute the matter of the universe. The matter is any substance that takes a particular space and has mass. According to Dalton, the matter is made up of very small, indivisible particles known as atoms. The concept which was proposed in 1808, was accepted for many years.

The main postulates of Dalton’s atomic theory include:

  • The matter is made up of small indivisible particles known as atoms.
  • Atoms are not created nor can they be destroyed. The chemical reaction is just a rearrangement of atoms and the same number of atoms must be present before and after the reaction.
  • Atoms are the tiniest particle of an element that takes part in a chemical reaction.
  • Atoms of the same element are similar in all respects including size, shape, and mass.
  • Atoms having different elements also have different mass, size, and shape.
  • Atoms of different elements when combined in a fixed ratio of small whole numbers will form compound atoms, called molecules.

However, according to many types of researches done by many renowned scientists and the discovery of radioactivity created confusion; it was doubted that atom was not only the tiniest indivisible particle but it also has challenging complex structures of its own and were made up of smallest particles such as electrons, protons, neutrons, etc. Almost 35 different subatomic particles are known but three particles namely electron, proton, and neutron were considered as the fundamental particles.

Existence of Electrons

Existence of Electrons

The existence of electrons in atoms was first proposed by J.J. Thomson, after experimenting with the conduction of electricity through gases at low pressures and high voltage. The e/m ratio for cathode rays is certain with the value of 1.76 x 108 C/g

Atom is electrically neutral, in case it contains negatively charged electrons then it also contains some positively charged particles. The same theory was confirmed by Goldstein in his discharge tube experiment with the perforated cathode. When high voltage was passed between the electrodes of a discharge tube it was observed that some rays came from the directions of anode that later passed through the holes in the cathode. These anode rays include positively charged particles that were formed by ionization of gas molecules through cathode rays.

The charge to mass ratio of positive charge is maximum when the tube gets filled with hydrogen gas as hydrogen is the lightest element. These positively charged particles are called protons. e/m differ based on the nature of gas taken in the discharge tube. The positive particles are positive residues of the gas left when the gas is ionized. The neutral charge particle, neutron was introduced by James Chadwick by shelling boron or beryllium with a–particles.

Practice with JEE Main Question Paper

Fundamental Particles

Fundamental Particles

The atoms are made by a combination of three fundamental particles. The charge and mass of these fundamental particles include:

Name Electron Proton Neutron
Symbol e- P N
Approximate relative mass 1/1836 1 1
Mass in kg 9.109×10–31 1.673×10–27 1.675×10–27
Mass in amu 5.485×10–4 1.007 1.008
Charge (coulomb) 1.602×10–19 1.602×10–19 0
Actual Charge 4.8 × 10–10 4.8 × 10–10 0

Atomic Models

Atomic Models

Thomson’s Atomic Model (Plum – pudding model)

Postulate: – Atoms are a sphere of positive charge in between which the number of electrons is embedded.

Limitations: – This theory was unable to satisfactorily explain the results of the scattering experiment carried out by Rutherford.

Rutherford’s Model


  • All the positive charge and mass of an atom is included in its nucleus.
  • All the electrons revolve around the nucleus in circular orbits.
  • There is a strong electrostatic attraction between nucleus and electrons

Limitations: - This theory was unable to explain the stability and electronic structure of the atom.

Some of the useful Atomic terms

Atomic Number (Z) – They are the number of protons or electrons of a neutral atom.

Mass Number (A) – It include the total number of protons and neutrons in an atom

Nucleons- Nucleons include protons and neutrons that are present in a nucleus. So, all these fundamental particles are being referred to as nucleons

Isotopes- Isotopes are atoms of the element that have the same atomic number but different mass number e.g. 1H1, 1H2, 1H3.

Isobars- Isobars are the atoms with the same mass number but different atomic numbers, e.g. 15P32 and 16S32

Isotones- They are the atoms with the same number of neutrons but different number of protons or mass number, e.g. 6C14, 8O16, 7N15

Isoelectronic- Isoelectronic are the atoms, molecules, or ions that have same number of electrons.

Nuclear isomers include – 

  • atoms having the same atomic number
  • having the same mass number but with different radioactive properties.
  • A few examples of nuclear isomers include:
  1. Uranium –X (half life 4 min) and
  2. Uranium –Z (half life 6.7 hours



Wavelength (λ) – It is the distance between two neighboring troughs or crests.

Frequency (ν) - It is the number of times a wave passes through a given point in a medium in one second. ν = c/λ

Velocity (c) = Velocity is the distance traveled by the wave in one second. c = νλ

Wave number = It is the number of wavelengths per cm.

Amplitude (a) = Amplitude is the actual height of the crest or depth of the trough. It helps in determining the intensity of the beam of light

Planck’s quantum theory

Planck’s quantum theory

  • According to this theory, the substances radiate or absorb energy discontinuously in the form of energy packets
  • The smallest packet of energy is considered as quantum and in the case light, the quantum is being referred to as photon.
  • The energy of a quantum is equally proportional to the frequency of the rays.
  • E = hv where v is the frequency of radiation and h is Planck’s constant having the value 6.626 × 10–27 erg sec or 6.626 × 10–34 J sec.
  • A body is able to radiate and absorb energy in whole number multiples of quantum hn, 2hν,3hν………..nhν. Here n is the positive integer.

Bohr’s atomic model

Bohr’s atomic model

  • According to this theory, the electrons rotate around the nucleus in circular orbits of fixed energy.
  • The electron revolves only in those orbits whose angular momentum (mvr) is an integral multiple of h/2Π.
  • The electron absorbs energy in the form of EMR, energy jumps from the minimum energy level to the maximum energy level and vice-versa.
  • The energy which is absorbed or released in an electron jump, (dE)= E2 – E1 = hν
  • The energy of stationary state oh hydrogen atom (En) = -RH (1/n2)
  • For hydrogen like species i.e. He+, Li2+ with atomic number Z
  • The velocity of an electron is equal to (2.18 ×108 ) z/n cms-1
  • Where n = 1,2,3,4…

What are the limitations of Bohr’s theory

  • This theory was unable to explain the spectra of atoms having more than one electron.
  • This theory failed to account for the splitting of the spectral line sources of a spectrum placed in a strong magnetic or electric field.
  • The dual nature of the particle and the unsure principle was not considered in the Bohr’s atomic model.

Some important topics in Atomic Structure

  • Discovery of Electron Proton and Neutron
  • Atomic models including Dalton’s atomic model, its postulates, Rutherford’s atomic model, Bohr’s atomic model, etc.
  • Dual nature of matter
  • Expression and Formula for the energy of the electron (like kinetic energy, potential energy, and total energy, etc.)
  • Quantum numbers including Principal quantum number, Magnetic Quantum Number, Electron Spin Quantum Number, etc.
  • Shapes of s, p and d orbitals

JEE Main Chemistry section carries a weightage of 30-40 % and is considered an important section in one of the most popular entrance exams. This section helps to understand a lot about chemistry and also modern physics. Almost 2 to 3 questions are asked from this section every year in the JEE Advanced and 1 to 2 questions in JEE Mains. The candidates with a good understanding of topics have higher chances of getting a good score in the JEE exams. So study harder to ace the exam!!