The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion is called ionisation energy or first ionisation energy.
Ionisation energy required to remove the second electron from a monovalent cation is called second ionization energy.
Similarly, the energy required to remove the third electron from a divalent cation is called the third ionisation energy.
Ionisation energy depends on:
(i) Atomic size
The greater the atomic size, the lesser the force of attraction. Since the electrons of the outermost shell lie further away from the nucleus, thus make their removal easier,i.e., ionisation energy required is less.
(ii) Nuclear charge: The greater the nuclear charge, greater is the attraction for the electrons of the outermost shell.
Therefore, the electrons in the outermost shell are more firmly held because of which greater energy is required to remove the electrons.
Basically, it is a measure of how hard the nucleus is holding the electrons.
After loss of the first electron the size of atom is decreased and after second and third electron removal the atom shrink in size and become smaller in size
Electrons which are closer to the nucleus are strongly held by nucleus thus energy required to lose electron is more.smaller the size stronger the hold on electrons by the nuclear charge.
Hence because of small size, the nuclear charge is strong and therefore it required more energy to remove the electron.
Because of smaller size, the third ionisation energy is higher than others.