Excitation and Ionization Energy and Potential

Excitation and Ionization Energy

The minimum energy required to excite an electron from the ground state of an atom to any excited state is called excitation energy. Thus, the energy required to excite an electron in the ground state to the first excited state is called the first excitation energy and so on. The excitation energy of an energy state is the difference between the energy of the excited state and that of the ground state. For example, for the hydrogen atom, the ground state energy(energy of the first orbit) is E₁ = -13.6 eV and energy of the second orbit is E₂ = -3.4 eV. This means to excite hydrogen atom, the energy required to be given to it is E₂ – E₁ = -3.4 + 13.6 = 10.2 eV. Hence, 10.2 eV is the first excitation energy of the hydrogen atom. Similarly, the II, the III excitation energy of hydrogen atom are E₃ – E₁, E₄ – E₁ etc.

Ionization Energy

Exciting an electron in an atom from its ground state (i.e. n = 1 state) to the infinite state (i.e. n \(\alpha \) state) is called ionization. Thus, the energy required to excite an atom from its ground state to the infinite state is called ionization energy. The ionization energy of an atom, in fact, is the difference of energy when it is in infinite state and in the ground state. If E₁ and E_{\(\infty \)} represent the energy of the atom when it is in the ground state and in the infinite state respectively, than the ionization energy of the atom is given by E_{\(\infty \)} – E₁ = 0 + 13.6 = 13.6 eV.

Excitation and Ionization Potential

Excitation Potential

By definition, if an electron is accelerated by 1 volt of potential difference, it acquires 1 eV of energy. So if the electron is accelerated through a p.d. of 10.2 volts, it acquires 10.2 eV energy. If such an extra electron collides with a ground state hydrogen atom, the hydrogen atom may be excited to the first excited energy state. Hence, the 10,2 volt of potential difference is the first excitation potential for a hydrogen atom. Second excitation potential for the hydrogen atom is given by (E₃ in eV – E₁ in eV) volt and so on.

Thus, the excitation potential of an energy of an atom is the potential, which is required for an electron to jump from the ground state to any one of its excited states.

Ionization Potential

The potential difference through which the extra electron is to be in acceleration in order for it to cause the ionization of an atom is called the ionization potential of the atom. For example, for the hydrogen atom, the ionization energy is 13.6 eV. By definition of 1 eV, an electron acquires 13.6 eV energy when it is accelerated through a potential difference of 13.6 volts.

Thus, ionization potential is the minimum potential to be applied in order to remove the electron completely from its ground state to infinity.

Emission Spectra

When the excited atoms make transitions from the excited state to the lower lying energy levels, then the emission spectra is obtained. Emission spectra are classified into continuous, line and a band spectrum visible from hot solid is an example of the continuous spectrum. A continuous spectrum is produced by incandescent solids, liquids, and compressed gasses. Line spectra are discontinuous lines produced by excited atoms and ions as they fall back to the lower energy level.

Absorption spectra

Absorption spectra are obtained when electrons are taken from lower energy states to the higher energy states. Various absorption series are Lyman, Balmer, Paschen, Bracket, and Pfund.

Limitations of Bohr’s Theory of Hydrogen Atom

1. Elliptical orbits are possible for the electron orbits, but Bohr’s theory does not tell us why only elliptical orbits are possible.
2. Bohr’s theory does not explain the spectra of only simple atoms like hydrogen but fails to explain the spectra of multi-electron atoms.
3. The fine structure of certain spectral lines of hydrogen could not be explained by Bohr’s theory.
4. It does not explain the relative intensities of spectral lines.
5. This theory does not account for the wave nature of electrons.

reference

Manu Kumar Khatry, Manoj Kumar Thapa et al. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.