CBSE Class 12 Physics Notes Chapter 12 Atoms

What Are Atoms?

Atoms are made up of the same amount of negative and positive charges. In Thomson’s model, atoms were described as a spherical cloud of positive charges with embedded electrons. In Rutherford’s model, one tiny nucleus bears most of the mass of the atom along with its positive charges, and the electrons revolve around it. Rutherford’s model didn’t fulfil two explanations.

  • It could not concur with the stability of the matter as it foretells that atoms are unstable because of the electrons that revolve around the nucleus might spiral into the nucleus.
  • It is unable to explain the spectral characteristics of a line of atoms. Atoms of every element emit a specific characteristic spectrum and are stable. The spectrum is made of a line spectrum, which is an isolated parallel line.

Thomson’s model has a defect of unstable electrostatically and Rutherford’s model is electromagnetically unstable.

For more information on Rutherford’s Alpha Scattering Experiment, watch the below video

Related Topics:

NCERT Solutions for Chapter 12 Atoms

Important Questions on Chapter 12 Atoms

NCERT Exemplar for Class 12 Physics Chapter 12

CBSE Notes Class 12 Physics

Revision Notes for Atoms

Line spectrum of the Hydrogen Atom

A hydrogen atom is made up of multiple series of line spectrum which are:

  • Pfund Series
  • Brackett Series
  • Paschen Series
  • Balmer Series
  • Lyman Series

What Are the Postulates of the Bohr Atomic Model?

Niels Bohr laid the foundations of quantum mechanics, those are:

  • The hydrogen’s atom electrons revolve around in stable orbits without emitting any radiant energy.
  • In the stationary orbits, the angular momentum is the multiple of the equation
    \(\begin{array}{l}h/2\Pi\end{array} \)
    , and
    \(\begin{array}{l}L=nh/2\Pi\end{array} \)
    , where n is called the quantum number.
  • Electron makes the transition from a non-radiating orbit to another of lower energy. When this happens, a photon gets emitted of the same energy as the difference between the final and the initial states. The frequency (v) is calculated by hv=
    \(\begin{array}{l}E_{i}-E_{f}\end{array} \)

What Is De Broglie’s Hypothesis?

This states that the wavelength of electrons is

\(\begin{array}{l}\lambda =h/mv\end{array} \)
, and the whole number of wavelengths is equal to the orbits circumferencing the main orbit corresponding to the circular standing waves.

Important Questions

  1. When the alpha-particle scattering experiment is repeated using a thin sheet of solid hydrogen in place of the gold foil. (Hydrogen is a solid at temperatures below 14K.) Find out the results.
  2. Name the shortest wavelength present in the Paschen series of spectral lines.
  3. When the atom makes a transition from the upper level to the lower level when the difference of 5 eV separates two energy levels in an atom, What is the frequency of radiation emitted?

Also Read:

The De Broglie Equation and wave nature Size of the Nucleus

Frequently Asked Questions on CBSE Class 12 Physics Notes Chapter 12 Atoms


What are alpha particles?

Alpha particles are composite particles consisting of two protons and two neutrons tightly bound together.


What does the Balmer series refer to?

The Balmer series is the name given to a series of spectral emission lines of the hydrogen atom that result from electron transitions from higher levels down to the energy level with principal quantum number 2.


What is the De Broglie hypothesis?

De Broglie’s hypothesis of matter waves postulates that any particle of matter that has linear momentum is also a wave.


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