Why Mg ion+2 is smaller than Mg

Rise in charge reduced the size due to rise in attraction between electron and nucleus.

Explanation:

Ion of magnesium is smaller than the atom of magnesium. As the nuclear charge increases, the attraction of electron to the nucleus increases in charged ions form of any element. Due to this increase in charge of magnesium ion,  the size of ion reduces

No. of electrons in Mg = 12

No. of proton in Mg = 12

Where as

No. of electrons in Mg2+ = 12

No. of proton in Mg2+ = 10

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