Milk of magnesia is magnesium hydroxide. What is its formula?
Mg2+ and OH-. Two hydroxides balance one magnesium: Mg(OH)2. Parentheses are required because the polyatomic hydroxide is taken twice.
What Is the Charge of Magnesium?
Magnesium commonly forms Mg2+ in introductory ionic compounds.
Magnesium forms a 2+ ion in every ionic compound: MgO in firebrick, MgCl2 in de-icing salt, and the Mg2+ at the heart of every chlorophyll molecule keeping plants green.
| Element | Magnesium |
|---|---|
| Symbol | Mg |
| Atomic number | 12 |
| Common ionic charge | +2 |
| Common ions | Mg2+ |
Visual reference
Magnesium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
Why Magnesium lets go of two electrons
Magnesium is [Ne]3s2. Both 3s electrons sit outside a neon core and leave as a pair — the second ionization energy is modest, but the third (which would break into the core) jumps to 7,733 kJ/mol. Nature never breaks the core, so magnesium is 2+ without exception.
Where Magnesium formulas usually go wrong
Writing MgCl instead of MgCl2. A single chloride only cancels half of magnesium's charge. If your formula for a magnesium compound has equal subscripts with a 1- anion, recheck it.
Worked example: Mg in a formula problem
Mg2+ in real compounds
MgCl2
Magnesium chloride
Mg2+ balances chloride ions.
MgO
Magnesium oxide
Magnesium can form oxides where total positive and negative charge balances.
Questions students ask about magnesium
What charge does magnesium have in chlorophyll?
Still 2+. A single Mg2+ sits caged in the center of the chlorophyll ring, holding the structure that captures light. Plants cannot photosynthesize without it.
Why does magnesium burn with such a bright white light?
The reaction 2Mg + O2 → 2MgO releases enormous energy as magnesium hands two electrons to oxygen. The brightness is the energy of ion formation made visible.
Is Mg2+ bigger or smaller than the Mg atom?
Much smaller — losing the entire outer shell shrinks the radius from 160 pm to 72 pm. Cations are always smaller than their parent atoms.
How do I use this charge in formulas?
Combine positive and negative ions so the total charge is zero — or let the ionic compound calculator do the crossing for you. The full chart lives on the periodic table with charges.
Where this data comes from
Charges and element facts on this page are checked against the primary references below. Found an error? Email hello@periodictablewithcharges.com — corrections ship in the next content update. See how this site is edited.