Interactive reference for all common phosphorus oxidation states (-3 to +5) with example compounds, identification tips, and a compound-to-oxidation-state lookup.
Atomic #15PPhosphorus
Atomic Mass
30.974 u
Group
15 (VA)
Period
3
Block
p-block
Electronegativity
2.19 (Pauling)
Oxidation States
-3, 0, +1, +3, +5
Phosphorus has five common oxidation states. Click a state card to see its compounds,
uses, and how to identify it.
Oxidation State −3
Phosphorus achieves its lowest oxidation state of -3 when it gains three electrons to complete its 3p shell (3p3 → 3p6). This parallels nitrogen's -3 state in ammonia. It occurs when phosphorus bonds to elements less electronegative than itself, principally hydrogen and metals.
How to Identify This State
Look for P bonded to H only (phosphine) or P bonded to metals in phosphide salts (e.g., Na3P). Assign H = +1 and metal cation oxidation states, then solve for P.
Example Compounds
Formula
Name
Assignment
Notes
PH3
Phosphine
+1×3 + P = 0 → P = -3
Colorless, toxic gas. Spontaneously flammable in air when impure. Used in semiconductor doping.
Na3P
Sodium Phosphide
3(+1) + P = 0 → P = -3
Ionic phosphide salt. Reacts with water to release PH3.
Ca3P2
Calcium Phosphide
3(+2) + 2P = 0 → P = -3
Used in signal flares; hydrolyzes to Ca(OH)2 and PH3.
AlP
Aluminium Phosphide
+3 + P = 0 → P = -3
Wide-bandgap semiconductor; also used as a pesticide (releases PH3 in moisture).
Common Uses
Semiconductor fabrication (phosphide compounds)
Fumigants and rodenticides (metal phosphides that release PH3)
Organic synthesis intermediates
Flame chemistry research
Oxidation State 0
Phosphorus in its elemental form is assigned an oxidation state of zero by convention — no electron transfer occurs when an element bonds to itself. The most stable allotrope is white phosphorus (P4 tetrahedra), with red and black phosphorus as less reactive polymorphs.
How to Identify This State
Any pure elemental phosphorus — P4 molecules, red phosphorus, black phosphorus — carries oxidation state 0. No calculation needed.
Example Compounds
Formula
Name
Assignment
Notes
P4
White Phosphorus
Elemental → P = 0
Waxy solid; highly reactive; ignites spontaneously in air above 34 °C. Used in incendiary devices and synthesis.
Pred
Red Phosphorus
Elemental → P = 0
Polymeric network; much less reactive than white phosphorus. Used in match heads and flame retardants.
Pblack
Black Phosphorus
Elemental → P = 0
Most stable allotrope; layered structure similar to graphite; semiconducting properties.
Common Uses
Match heads (red phosphorus)
Flame retardants
Synthesis of phosphorus compounds
Emerging 2D materials research (black phosphorus)
Oxidation State +1
The +1 state is uncommon but genuine. It arises when phosphorus retains two P-H bonds (each hydrogen +1) while also bonding to oxygen. The classic example is hypophosphorous acid (H3PO2), where two of the three "H" atoms are directly bonded to phosphorus rather than to oxygen.
How to Identify This State
In H3PO2: two P-H bonds (+1 each = +2 total from H) and one P-OH group with one P=O. Counting: 2(+1 for P-H) + 1(OH → -2+1) + 1(P=O → -2) + overall charge 0 → P = +1. Watch for the structural formula — H bonded to P, not O.
Example Compounds
Formula
Name
Assignment
Notes
H3PO2
Hypophosphorous Acid
2(+1) + (-2) + (-2+1) + P = 0 → P = +1
Strong reducing agent; monobasic acid (only one ionizable OH). Used to reduce metal salts and as an antioxidant.
Reduction of diazonium salts to arenes (Balz-Schiemann route)
Antioxidant in food and polymers
Pharmaceutical synthesis
Oxidation State +3
In the +3 state phosphorus has lost three electrons (or formed three bonds to more electronegative atoms). The 3p3 half-filled configuration gives it moderate stability here. Phosphorus(III) compounds are commonly pyramidal due to the lone pair on phosphorus.
How to Identify This State
In PCl3: Cl = -1, three Cl atoms total -3, molecule neutral → P = +3. In H3PO3: note one P-H bond (H counted +1 directly on P) and two P-OH groups, plus one P=O. Structure: +1 (P-H) + 2(-1 from OH-net) + (-2 from P=O) + P = 0 → P = +3.
Example Compounds
Formula
Name
Assignment
Notes
PCl3
Phosphorus Trichloride
3(-1) + P = 0 → P = +3
Reactive liquid; important precursor to phosphorus(V) compounds and organophosphorus chemistry.
H3PO3
Phosphorous Acid
P-H + 2(OH) + P=O → P = +3
Dibasic acid (not tribasic — one H is on P, not O); mild reducing agent.
PBr3
Phosphorus Tribromide
3(-1) + P = 0 → P = +3
Used in organic chemistry to convert alcohols to alkyl bromides.
PF3
Phosphorus Trifluoride
3(-1) + P = 0 → P = +3
Toxic gas; ligand in organometallic chemistry; similar pi-acid behavior to CO.
P4O6
Phosphorus Trioxide
P-O-P network with +3 P → P = +3
Cage structure with six bridging oxygens; reacts violently with water to give H3PO3.
Common Uses
Manufacture of organophosphate pesticides and nerve agents precursors
Synthesis of phosphite ester stabilizers in polymers
Converting alcohols to alkyl halides (PBr3, PCl3)
Ligand in metal carbonyl analogues (PF3)
Oxidation State +5
The highest common oxidation state for phosphorus. It requires access to 3d orbitals (unavailable to nitrogen) to accommodate 10 electrons around the central atom. Phosphorus(V) compounds are the most oxidized and thermodynamically stable form in many environments, including biological systems.
How to Identify This State
In H3PO4: 3(+1 from H) + 4(-2 from O) + P = 0 → P = +5. In PCl5: 5(-1) + P = 0 → P = +5. In PO43-: 4(-2) + P = -3 → P = +5.
Example Compounds
Formula
Name
Assignment
Notes
H3PO4
Phosphoric Acid
3(+1) + 4(-2) + P = 0 → P = +5
Triprotic acid; used in fertilizers, food acidulant (cola drinks), rust treatment, and detergents.
PCl5
Phosphorus Pentachloride
5(-1) + P = 0 → P = +5
Strong chlorinating and Lewis acid reagent; trigonal bipyramidal geometry.
P4O10
Phosphorus Pentoxide
Formal +5 P in cage structure
Powerful desiccant; reacts vigorously with water to give H3PO4.
PO43-
Phosphate Ion
4(-2) + P = -3 → P = +5
Central to biochemistry — backbone of DNA/RNA, ATP energy currency, bone mineral (hydroxyapatite).
POCl3
Phosphoryl Chloride
3(-1) + (-2) + P = 0 → P = +5
Versatile reagent; makes phosphate esters; used in synthesis of nerve agent precursors and pharmaceuticals.
Common Uses
Fertilizers (phosphoric acid, phosphate salts)
DNA/RNA backbone and ATP (phosphate groups)
Desiccants (P4O10)
Flame retardants (organophosphate esters)
Food acidulants (H3PO4 in colas)
Select a compound from the list to see the oxidation state of phosphorus
with a step-by-step calculation.
Select a compound to see the oxidation state calculation.
Oxidation state of P:
Step-by-step
Oxidation State Summary
State
Stability
Key Example
Notes
-3
Common
PH3
Phosphorus bonded to H or metals; gains 3 electrons like nitrogen.
0
Elemental
P4
Assigned by convention to all allotropes of elemental phosphorus.
+1
Uncommon
H3PO2
Present in hypophosphorous acid and hypophosphite salts; good reducing agents.
+3
Common
PCl3
Pyramidal geometry with lone pair; typical of P(III) halides and H₃PO₃.
+5
Very Common
H3PO4
Most oxidized state; possible due to 3d orbital expansion; dominates biology.
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Summary
Interactive reference for all common phosphorus oxidation states (-3 to +5) with example compounds, identification tips, and a compound-to-oxidation-state lookup.
How it works
Click an oxidation state card (-3, 0, +1, +3, or +5) to open its detail panel.
The detail panel shows a description, example compounds, common uses, and identification tips.
Use the Compound Lookup tab to select a known phosphorus compound and see the oxidation state of P explained step by step.
Click any formula badge to copy it to your clipboard.
Switch between the Explorer and Compound Lookup tabs using the tab bar.
Use cases
Students revising oxidation state rules and phosphorus chemistry for exams.
Chemistry teachers preparing reference materials on Group 15 elements.
Researchers quickly checking the oxidation state of phosphorus in a specific reagent.
Anyone working through acid-base or redox problems involving phosphoric or phosphorous acids.
Learners comparing the reactivity and stability of different phosphorus oxidation states.
Frequently Asked Questions
Phosphorus commonly exhibits -3, 0, +1, +3, and +5. The -3 state appears in phosphides and phosphine (PH3). Zero is assigned to elemental phosphorus (P4). The +1 state occurs in hypophosphorous acid (H3PO2). The +3 state is found in phosphorous acid (H3PO3) and phosphorus trichloride (PCl3). The +5 state is the most oxidized, found in phosphoric acid (H3PO4) and phosphorus pentachloride (PCl5).
Nitrogen has no available d orbitals in its valence shell (2s 2p only), so it is capped at +5 only in special cases like NO3-. Phosphorus, in Period 3, has accessible empty 3d orbitals that allow it to expand its octet and accommodate 10 electrons around the central atom, enabling the +5 state in stable compounds like PCl5 and H3PO4.
Assign known oxidation states first: H = +1 (three H atoms, total +3) and O = -2 (four O atoms, total -8). The molecule is neutral, so the sum must be zero. P + 3(+1) + 4(-2) = 0 → P + 3 - 8 = 0 → P = +5.
Chlorine is -1. With three Cl atoms the total is -3. The molecule is neutral, so P + 3(-1) = 0 → P = +3.
Hydrogen is assigned +1 in compounds with non-metals. With three hydrogen atoms (total +3) and the molecule being neutral, P + 3(+1) = 0 → P = -3. Phosphorus is more electronegative than hydrogen, so it formally holds the bonding electrons.
It is less common than +3 or +5 but genuine. Hypophosphorous acid (H3PO2) contains P in the +1 state. The structure has two P-H bonds (each H counts as +1) and two P-O bonds to the OH groups, leading to P + 2(+1) + 2(-2) + 2(+1) = 0 → P = +1. H3PO2 is a moderately strong reducing agent precisely because P can be oxidized from +1 to higher states.
