# CBSE Class 12 Physics Notes Chapter 12 Atoms

## What are Atoms?

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

• It could not concur the stability of the matter as it predicts that atoms are unstable because of the electrons which are revolving 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 characteristics spectrum and are stable. The spectrum is made of line spectrum which is an isolated parallel line.

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

### 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 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 $h/2\Pi$, and $L=nh/2\Pi$ where n is called the quantum number.
• Electron makes the transition from a non-radiating orbit to another of lower energy. When this is happening, 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=$E_{i}-E_{f}$

### What is De Broglie’s Hypothesis?

This states that the wavelength of electrons is $\lambda =h/mv$ 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?