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NEET Chemistry Atomic Structure

Atomic Structure PDF Notes, Important Questions and Synopsis


Dalton’s atomic theory:
Dalton’s atomic theory is based on the law of conservation of mass and the law of definite proportions. He also proposed the law of multiple proportions.

Thomson’s model of atom:

  • It is also known as watermelon model or plum pudding model.
  • According to this model, the atom is a positive sphere in which negative charges are distributed.

Rutherford alpha scattering experiment:

  • This experiment was performed to understand the structure of an atom.
  • This model concludes that most of the part of an atom is empty and each atom consists of a heavy positively charged nucleus.
Fundamental particles of an atom:


         Mass (amu)






         JJ Thomson










Bohr’s model of atom:

  • Electrons orbit the nucleus in orbits which have a set size and energy.
  • The lowest energy is found in the smallest orbit.
  • Radiation is absorbed or emitted.

Electromagnetic radiation:

  • Wavelength: It is the distance between successive points of equal phases of a wave.
  • Frequency: It is the number of cycles or oscillations or vibrations of wave motion in unit time.
  • Velocity: It is the distance travelled by the wave in one second.
  • Wave number: It is defined as the number of waves in unit wavelength.

Heisenberg’s uncertainty principle:
The uncertainty principle says that we cannot measure the position and momentum of the particle with absolute precision.

Bohr–Sommerfield model:

  • According to this model, the path of an electron around the nucleus is an ellipse with the nucleus at one of its foci.
  • The angular momentum of an electron in a closed elliptical path is also quantised.

An orbital is a three-dimensional region in which the probability of finding an electron is maximum.

Quantum numbers:
Principal quantum number:

  • It is denoted by n.
  • n = 1, 2, 3, 4 … ∞
  • n = 1 K shell; n = 2 L shell
  • n = 3 M shell; n = 4 N shell
  • As the value of ‘n’ increases, the energy of the electron increases.

Azimuthal or angular quantum number:

  • It is denoted by l.
  • The values of l are from 0 to (n-1)
  • l = 0 s subshell; l = 1 p subshell
  • l = 2 d subshell; l = 3 f subshell
  • The value of ‘l’ signifies the shape and energy level of subshells in a major energy shell.

Magnetic quantum number:

  • It is denoted by ml.
  • The values of ml lie from –l to +l.
  • The values of ml signify the possible number of orientations of subshells.

Spin quantum number:

  • It is denoted by ms.
  • The values of ms are +1/2 and −1/2.
  • The values of ms signify the direction of rotation or the spin of an electron in its axis during motion.
Shape of orbitals:


              Shape of orbital






              Double dumbbell


              Diffused shape


Pauli’s exclusion principle:

  • It is impossible for two electrons of an atom to have all its four quantum numbers same.
  • Maximum number of electrons in a shell can be 2n2.
  • Maximum number of electrons in a subshell can be 2 only.

Aufbau principle:

  • Orbitals with lower energy level are filled first.

(n+l) rule:

  • The subshell with lower values of (n+l) possesses lower energy.
  • If (n+l) for two orbitals are the same, the one with lower value of n possesses lower energy and should be filled first.

A substance whose molecules, ions or atoms have unpaired electrons is paramagnetic.

Molecules, ions or atoms having no unpaired electrons are diamagnetic.