When an electron is removed from every ion of 1 mol of H − ions, 72 kJ of energy is required. In another experiment, when electrons are removed from 2 1 mole of H (one electron each) and given them

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When an electron is removed from every ion of 1 mol of H − ions, 72 kJ of energy is required. In another experiment, when electrons are removed from 2 1 ​ mole of H (one electron each) and given them to another 2 1 ​ mole of H (one electron each), 619.5 kJ of energy is found to be required. The ionisation energy (in kJ / mol ) of hydrogen is :

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The first reaction is H − ( g ) → H ( g ) + e − , requiring 72 kJ/mol. This is the negative of the electron affinity (EA) of H, so EA = − 72 kJ/mol . The second reaction is 2 1 ​ H ( g ) + 2 1 ​ H ( g ) → 2 1 ​ H + ( g ) + 2 1 ​ H − ( g ) , requiring 619.5 kJ. This can be written as 2 1 ​ ( H ( g ) → H + ( g ) + e − ) and 2 1 ​ ( H ( g ) + e − → H − ( g )) . So, 619.5 kJ = 2 1 ​ ( IE + EA ) . 1239 kJ = IE − 72 kJ . IE = 1239 + 72 = 1311 kJ/mol .

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