Helium has a structure 1s2. The electron is being removed from the same orbital as in hydrogen's case. It is close to the nucleus and unscreened. The value of the ionisation energy (2370 kJ mol-1) is much higher than hydrogen, because the nucleus now has 2 protons attracting the electrons instead of 1.
Why the second ionization energy is higher than the first?
The second ionization energy of Mg is larger than the first because it always takes more energy to remove an electron from a positively charged ion than from a neutral atom. The third ionization energy of magnesium is enormous, however, because the Mg2+ ion has a filled-shell electron configuration.
Why do halogens have the highest ionization energy?
Atoms are more stable when they have full valence shells. But compared to halogens they are harder to remove electrons from because halongens have 7 electrons in their valence shell so they are not as close to the state of completion as noble gases. Halogens have one less proton than their adjacent noble gas.
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