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Arrange the following species in decreasing order of first ionisation energy: K, Mg, Mg²+, Na.

1. Mg²+ > Mg > Na > K
2. K > Na > Mg²+ > Mg
3. K > Mg > Na > Mg²+
4. K > Na > Mg > Mg²+

Correct answer: Mg²+ > Mg > Na > K

Solution

First ionisation energy (removing one electron): Mg²+ is already a cation with high nuclear charge and small size — its 'first' ionisation energy (to form Mg³+) is very high. Among neutral atoms, K < Na < Mg (general periodic trend). Therefore the order is Mg²+ > Mg > Na > K.

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