Excitation and ionization potential of an atom
According to Bohr's theory, there are certain discrete orbits permitted for the motion of the electron. Electron can revolve in these orbits without radiating energy. An atom is said to be in the ground state, when its energy is least. Before an atom can emit spectral radiation, the electron in it has to be raised to a higher orbit. This process is known as the excitation of the atom. The energy required to raise an atom from its normal state into an excited state is called excitation potential energy of the atom. For example, the energy required to transfer the electron in hydrogen atom from the ground state to the first excited state = (13.6-3.4) = 10.2eV. The energy required to raise it to the second excited state = (13.6 - 1.51) = 12.09 eV. The potentials corresponding to these energies are called as the excitation potentials.
The ionisation potential is that accelerating potential which makes the impinging electron acquire sufficient energy to knock out an electron from the atom and thereby ionise the atom. For hydrogen atom, the energy required to remove an electron from first orbit to its outermost orbit(n=∞) 13.6-0=13.6eV. This energy is known as the ionization potential energy for hydrogen atom. 13.6 V is the ionisation potential of hydrogen atom.
The excitation potential and ionization potential are called as the critical potentials of the atom. The critical potential of an atom, is defined as the minimum potential required to excite a free neutral atom from its ground state to higher state.
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