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Define ionization enthalpy. How does it vary along the period and down the group in the periodic table? Explain.

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The amount of energy required to remove an electron from an isolated gaseous atom in the ground state is called ionization enthalpy. 

  • Ionization enthalpy increases along the period.

Reason: This is because along the period atomic size decreases and effective nuclear charge increases.

  • Ionization enthalpy decreases down the group.

Reason: This is because atomic size increases down the group due to the increase in number of shells. The shielding effect increases and hence effective nuclear charge decreases.

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