I saw somewhere that all metal ions except for the alkali metals and alkaline earth metals will hydrolyze to increase acidity. If I'm interpreting things correctly, sodium ions, for example, will not hydrolyze because sodium hydroxide is strong, thus sodium ions will not react with water to form sodium hydroxide and hydrogen ions. But I was told that magnesium will not hydrolyze, but magnesium isn't on the strong bases list that was given. By the way, I'm a grade 12 student, so I would appreciate if you kept things down to earth. Can someone clarify please?


1 Answer 1


So the ability of a metal cation to hydrolyze water molecules to form acidic solutions is based on the charge of the ion and the size of the atom. The larger atoms will stabilize charge better (i.e. it spreads it out over a larger space). Unstable cations form hydroxides or oxides with water molecules and release hydronium ions.

Estimations of the acidity for metal cations can be found with the equation below,

$pka = 15.14 - 88.16(\frac{Z^2}{r})$

Z being the charge of the ion and r being the atomic radius. I bold the word estimation cause this isn’t a perfect system and I want to make that clear. However, better equations do exist.

Electronegativity does play a role too. The more electronegative a metal cation is, the more acidic it will be.

I would recommend taking a look at this link for more information

  • $\begingroup$ Thank you for your answer. To clarify, group one and two metal ions have low charges --> a low Z value --> a low pKa value, thus they don't hydrolyze? $\endgroup$
    – Sam Liu
    Oct 8, 2020 at 2:34
  • $\begingroup$ And would you also mind confirming that magnesium ions will not hydrolyze? $\endgroup$
    – Sam Liu
    Oct 8, 2020 at 2:35
  • $\begingroup$ @SamLiu See chemistry.stackexchange.com/a/41678/43057 for an example with magnesium $\endgroup$
    – ManRow
    Oct 8, 2020 at 8:22
  • $\begingroup$ @ManRow ahh ok thank you, that cleared it up $\endgroup$
    – Sam Liu
    Oct 8, 2020 at 19:51

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