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Amount of energy required to excite an electron of an atom from the lower energy state to its next higher energy state is defined as:
(A) Ionization potential
(B) Electron affinity
(C) Critical potential
(D) Reduction potential

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Hint: Ionization potential and critical potential both are energy required to remove an electron from a neutral gaseous atom present in its ground state.

Complete step by step solution:
Let us understand the different types of potentials mentioned in the question one by one.
Ionization potential: It is also called the ionization energy. It refers to the amount of energy that needs to be supplied in order to remove an electron from the outermost shell/valence shell of an isolated gaseous atom to infinity. It is generally an endothermic process. Its value can be calculated by using the formula:
$ \triangle E=13.6\times \cfrac { { Z }^{ 2 } }{ { n }^{ 2 } } eV$
Where ‘n’ is the principal quantum number for the valence shell and ‘Z’ is the number of protons present in the nucleus of the atom i.e. its atomic number.
Electron affinity: It refers to the amount of energy released when an electron is added to a neutral gaseous atom in order to form an anion. It can be negative, zero or positive. The more negative it is, the more stable will be the anion and the electron addition process will be more favourable. It decreases with the increase in: the atomic size, screening effect. It increases with the increase in effective nuclear charge.
Critical potential: It is also called resonance potential. It refers to the energy supplied to an atom in order to remove an electron from a lower energy state (orbit) to the next higher energy state (orbit). It can be calculated using the formula:
$ \triangle E=13.6\times { Z }^{ 2 }(\cfrac { 1 }{ { n }_{ 2 }^{ 2 } } -\cfrac { 1 }{ { n }_{ 1 }^{ 2 } } )eV$
Where ‘${ n }_{ 1 }$’ is the principal quantum number for the lower energy state and ‘${ n }_{ 2 }$’ is the principal quantum number for the higher energy state. ‘Z’ is the number of protons present in the nucleus of the atom i.e. its atomic number.
Reduction potential: It is the measure of the tendency of a chemical species to gain electrons and hence get reduced. Its value is calculated by considering hydrogen as the electrochemical reference which is assigned a reduction potential of zero. Greater (more positive) the value of reduction potential of a species, more easily it will get reduced.

Therefore, the amount of energy required to excite an electron of an atom from the lower energy state to its next higher energy state is defined as (C) Critical potential.

Note: Do not get confused between the ionisation potential and the critical potential. In ionisation potential the electron is removed from the outermost shell to an infinite distance such that we get a cation. In critical potential, the electron that is removed is still present in the atom but in the next higher energy state.
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