Hard water contains Ca2+. Why does soap scum form?
Soap anions (long-chain carboxylates, 1- each) pair two-to-one with Ca2+, forming an insoluble calcium salt — the scum. Water softeners swap Ca2+ for Na+, whose 1+ soap salts stay dissolved.
What Is the Charge of Calcium?
Calcium commonly forms Ca2+ in introductory ionic compounds.
Calcium forms a 2+ ion, Ca2+, everywhere it appears: CaCO3 in limestone and seashells, Ca3(PO4)2 in your skeleton, CaCl2 melting sidewalk ice. It is the fifth most abundant element in your body, all of it as Ca2+.
| Element | Calcium |
|---|---|
| Symbol | Ca |
| Atomic number | 20 |
| Common ionic charge | +2 |
| Common ions | Ca2+ |
Visual reference
Calcium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
Why Calcium lets go of two electrons
Calcium is [Ar]4s2. Both 4s electrons depart to expose the argon core — the first two ionization energies are affordable (590 and 1,145 kJ/mol), but the third leaps to 4,912 kJ/mol because it would tear into the noble-gas core. So calcium is 2+, never 1+ or 3+.
Where Calcium formulas usually go wrong
Writing calcium phosphate as CaPO4. Phosphate is 3-, calcium is 2+; they do not cancel one-to-one. Cross the charges: Ca3(PO4)2. Any calcium formula must make total + equal total -.
Worked example: Ca in a formula problem
Ca2+ in real compounds
CaCl2
Calcium chloride
Ca2+ balances chloride ions.
CaO
Calcium oxide
Calcium can form oxides where total positive and negative charge balances.
Questions students ask about calcium
Is the calcium in milk metallic calcium?
No — it is dissolved Ca2+ bound to casein proteins and phosphate. Metallic calcium would react violently with the water in milk. Supplements likewise contain calcium salts, not the metal.
What charge does calcium have in cement?
2+, mainly in calcium silicates. When water hits cement, Ca2+ and silicate ions rearrange into the interlocking crystals that make concrete hard.
Why do Ca2+ and Mg2+ behave so similarly?
Same group, same 2+ charge — both make water 'hard' and form insoluble carbonates. Calcium's larger radius (100 pm vs 72 pm) makes its salts generally a bit less soluble, which is why limescale is mostly CaCO3.
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.