Write the formula for calcium phosphate, the mineral in bone.
Ca2+ and PO4 3-. Lowest common multiple of 2 and 3 is 6: three Ca2+ (+6) balance two phosphates (-6), giving Ca3(PO4)2.
What Is the Charge of Phosphorus?
Phosphorus commonly forms P3- P3+ P5+ in introductory ionic compounds.
Facing a metal, phosphorus gains three electrons to form the phosphide ion P3- (as in Ca3P2). Facing oxygen, it flips to +3 in phosphites and +5 in phosphates — and the phosphate backbone of your DNA holds phosphorus at +5 right now.
| Element | Phosphorus |
|---|---|
| Symbol | P |
| Atomic number | 15 |
| Common ionic charge | -3, +3, +5 |
| Common ions | P3-, P3+, P5+ |
Visual reference
Phosphorus element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron count behind Phosphorus's charge
Phosphorus is [Ne]3s2 3p3, three electrons short of argon — hence -3 with metals. But its 3p and 3s electrons are also loosely enough held that oxygen strips their share away, producing the +5 state that dominates biology and fertilizer chemistry.
The classic Phosphorus error
Mixing up phosphide (P3-), phosphite (PO3 3-), and phosphate (PO4 3-). All three end in a 3- charge, which tempts students to treat them as interchangeable. Only phosphide is a bare ion; the other two are polyatomic groups with different oxygen counts and different phosphorus oxidation states (+3 and +5).
Worked example: P in a formula problem
P3- and P3+ and P5+ in real compounds
Na3P
sodium phosphorus compound
Sodium ions can balance P3- in simple ionic examples.
CaP2
calcium phosphorus compound
Calcium ions can balance common negative Phosphorus ions.
Questions students ask about phosphorus
What is the charge of phosphorus in DNA?
+5. Each phosphate group linking the sugar backbone is a PO4 unit; the negative charges on its oxygens are why DNA migrates toward the positive electrode in gel electrophoresis.
Why does white phosphorus burst into flame in air?
Its strained P4 tetrahedra react eagerly with O2, driving phosphorus from oxidation state 0 up to +5 in P4O10. The stored strain energy, not any ionic charge, makes it pyrophoric.
Is the phosphorus in fertilizer P3-?
No — fertilizer phosphorus is phosphate (P at +5), usually as ammonium phosphate or superphosphate. Phosphide (P3-) is actually used in rodent poison, where it releases toxic phosphine gas.
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.