Question

helium has highest ionisation energy. why is it so when it is a noble gas and cannot lose electrons??

Answer 1

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 theionisation energy (2370 kJ mol-1) is much higher than hydrogen, because the nucleus now has 2 protons attracting the electrons instead of 1.

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