# Why is magnesium hydroxide not hygroscopic?

I asked this question today and my professor was unable to provide any rationale.

I know that sodium hydroxide is hygroscopic, but why not magnesium hydroxide (at least to the same extent as $\ce{NaOH}$)? Could it be that $\ce{Mg(OH)2}$ has a higher lattice energy; the hydroxide is more closely associated with the magnesium ion and less able to react with carbon dioxide?

• Not an answer to the question, bet also a partial refutation of what has been said: aluminum is highly hydroscopic, mainly because of its high affinity for oxygen. Yet, magnesium seems to avoid the reactions of both sodium and aluminum. – BMeph Feb 3 at 15:19