Two bonding levels
Ionic between calcium and sulfate; covalent inside sulfate.
This same distinction appears in many salts containing polyatomic ions.
Ionic solid · sulfate salt
Calcium sulfate
CaSO₄
Calcium sulfate is CaSO₄, an ionic solid composed of Ca²⁺ and SO₄²⁻ in a 1:1 ratio. The sulfate ion is a covalently bonded tetrahedral polyatomic anion.
Hydrate distinction: this master represents anhydrous CaSO₄. Gypsum is CaSO₄·2H₂O, while the hemihydrate has composition CaSO₄·½H₂O.
Essentials
CaSO₄ is one Ca²⁺ ion for one SO₄²⁻ ion.
The +2 and −2 charges balance in a 1:1 formula unit. The bulk solid is an extended ionic material, not a discrete CaSO₄ molecule.
Detail
The sulfate ion has a tetrahedral sulfur–oxygen framework.
Four oxygen positions surround sulfur. Resonance and delocalization mean one localized Lewis contributor should not be treated as the complete physical picture.
Crystal structure
Hydration changes the solid-state arrangement.
Water molecules incorporated into gypsum alter coordination and crystal packing compared with anhydrous calcium sulfate.
Understand
Gypsum and anhydrite are not the same formula.
Gypsum includes two waters of crystallization for each CaSO₄ unit, while anhydrite is anhydrous. The hemihydrate contains half a water per CaSO₄ on average in its stoichiometric formula.
Material names
Gypsum and plaster materials need phase-aware naming.
Material processing can interconvert hydration states without changing the underlying Ca/S/O framework identity.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | CaSO₄ |
|---|---|
| CAS Registry Number | 7778-18-9 |
| Form represented | anhydrous calcium sulfate / anhydrite composition |
| Gypsum | CaSO₄·2H₂O |
Advanced
Push past the first model without losing the simple picture.
Is gypsum just damp CaSO₄?
No. Gypsum is the defined dihydrate CaSO₄·2H₂O.
Why can a hemihydrate have half a water?
The formula is a stoichiometric ratio averaged over the crystal: one H₂O for every two CaSO₄ units.
Is CaSO₄ a molecule?
No. Ordinary calcium sulfate solids are extended ionic materials represented by formula units.
Constituent elements