The electrons in an atom are attracted by the nucleus. When an electron is to be removed then work is done against this nuclear attraction. In other words energy is required to remove an electron from an atom. To understand the details of chemical behavior of an element we must have an indication of the energy with which an atom binds its electrons. This is obtained by the measurement of ionization potential or ionization energy. It may be defined as the energy required to remove an electron from the outermost orbit of an isolated gaseous atom in its ground state. It is expressed in electron volts (eV) or kilo calories per gram atom. In an atom, the energy required to remove first electron from a gaseous atom is called first ionization energy.
Clarification:
Ionization energy is expressed in units of joules or electron volts (eV). Ionization potential is expressed in units of volts (e.g. an ionisation potential of 5V means that 5eV of energy needs to be supplied to the electron ionize it).
('Potential' is simply the energy needed per unit of charge.)
('Potential' is simply the energy needed per unit of charge.)
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