Revision: Atoms and Nuclei >> Atoms Physics Science (English Medium) Class 12 CBSE

When an atomic gas or vapour at low pressure is excited, it emits radiation of certain wavelengths. The emitted radiation, when analysed with a spectroscope, shows a series of bright lines on a dark background. This type of spectrum is called an emission line spectrum.

When white light from a source passes through an atomic gas or vapour, the gas absorbs radiation of certain wavelengths. As a result, dark lines appear in the otherwise continuous spectrum. This type of spectrum is called an absorption spectrum.

Dark spectral absorption lines are the lines seen in a continuous spectrum at the frequencies absorbed by the atoms of a rarefied gas.

When an atom absorbs a photon having precisely the same energy as that required for an electron in a lower energy state to make a transition to a higher energy state, the process is called absorption.

OR

Absorption is the process in which an atom takes in a photon whose energy exactly matches the energy needed for an electronic transition from a lower level to a higher level.

The various lines in atomic spectra are produced when electrons jump from a higher energy state to a lower energy state, and photons are emitted. These spectral lines are called emission lines.

OR

Emission lines are the spectral lines produced when electrons fall from higher energy states to lower energy states and emit photons.

If the distance between the alpha-particle and the nucleus is rr, then the electrostatic force between them is given by:

F = \[\frac {1}{4πε_0}\] ⋅ \[\frac{2e\cdot Ze}{r^2}\]

• Bohr accepted Rutherford’s nuclear model but modified it using quantum ideas.
• Classical mechanics and electromagnetism could not explain atomic-scale behaviour fully.
• Only certain orbits are allowed for the electron in the hydrogen atom.
• These orbits have definite total energy.
• Electron transitions between energy levels lead to photon emission.
• The energy of the hydrogen atom is negative because the electron is bound to the nucleus.

• Ground state energy of hydrogen = -13.6 eV.
• Ionisation energy of hydrogen in the ground state = 13.6 eV.
• Energy required for first excitation = 10.2 eV.
• Energy required for second excitation = 12.09 eV.
• The energy of a free electron is 0 eV.

• An atom emits radiation when it moves from a higher energy state to a lower energy state.
• The energy difference appears as a photon.
• Because the quantum numbers are integers, only discrete frequencies are emitted.
• These give rise to emission lines.
• If atoms absorb photons of the exact required energy, dark absorption lines appear in a continuous spectrum.