Reference for all common arsenic oxidation states (-3, 0, +3, +5) with example compounds, identification tips, and a compound-to-oxidation-state lookup.
Atomic #33AsArsenic
Atomic Mass
74.922 u
Group
15 (VA)
Period
4
Block
p-block
Electronegativity
2.18 (Pauling)
Oxidation States
-3, 0, +3, +5
Arsenic has four common oxidation states. Click a state card to see its compounds,
uses, and how to identify it.
Oxidation State −3
Arsenic reaches its lowest oxidation state of -3 when it gains three electrons to complete its 4p shell (4p3 → 4p6). This parallels nitrogen and phosphorus in Group 15. It occurs when arsenic bonds to elements less electronegative than itself: principally metals and, in some cases, hydrogen.
How to Identify This State
Look for As bonded to metal cations in arsenide salts (e.g., GaAs, Na3As) or to hydrogen in arsine (AsH3). Assign the metal or H oxidation states first, then solve for As. If the metal is +3 (e.g., Ga3+) and the compound is neutral with 1:1 ratio, As = -3.
Example Compounds
Formula
Name
Assignment
Notes
GaAs
Gallium Arsenide
+3 + As = 0 → As = -3
III-V semiconductor used in LEDs, laser diodes, and solar cells. One of the most important compound semiconductors.
Na3As
Sodium Arsenide
3(+1) + As = 0 → As = -3
Ionic arsenide salt. Reacts with water to release arsine (AsH3).
AsH3
Arsine
3(+1) + As = 0 → As = -3
Colorless, extremely toxic gas. Used in the semiconductor industry for vapor-phase epitaxy (MOCVD/MOVPE).
InAs
Indium Arsenide
+3 + As = 0 → As = -3
Narrow-bandgap semiconductor used in infrared detectors and high-speed transistors.
Common Uses
Semiconductor fabrication (GaAs, InAs, AlAs devices)
Infrared detectors and photodetectors
High-electron-mobility transistors (HEMTs)
Dopant precursor (arsine) in silicon chip production
Oxidation State 0
Elemental arsenic is assigned an oxidation state of zero by convention — no electron transfer occurs in element-to-element bonds. The most common allotrope is gray (metallic) arsenic, a brittle, steel-gray solid with a layered rhombohedral structure. Yellow arsenic (As4 molecules, analogous to P4) and black arsenic also exist but are less stable.
How to Identify This State
Any pure elemental arsenic — gray arsenic, yellow As4, or black arsenic — carries oxidation state 0 by definition. No arithmetic needed.
Example Compounds
Formula
Name
Assignment
Notes
As (gray)
Gray Arsenic
Elemental → As = 0
Most stable allotrope; semi-metallic; sublimes at 887 °C at 1 atm. Historically used in pigments and poisons.
As4
Yellow Arsenic
Elemental → As = 0
Tetrahedral As4 molecules analogous to P4; very reactive and toxic; forms by rapid condensation of arsenic vapor.
As (black)
Black Arsenic
Elemental → As = 0
Amorphous or polymeric form; less reactive than yellow; intermediate between gray and yellow.
Common Uses
Alloying element in lead-acid batteries (hardening lead)
Historical use in wood preservatives (CCA) and pigments
Standard reference material in analytical chemistry
Research into 2D arsenene (analogous to graphene)
Oxidation State +3
The +3 state is the most characteristic and kinetically accessible oxidation state of arsenic, reflecting the growing inert-pair effect in Period 4. Arsenic(III) compounds are pyramidal due to the lone pair on arsenic. As2O3 is the most important +3 compound — historically one of the most notorious poisons ("inheritance powder") and now a medicine for acute promyelocytic leukemia.
How to Identify This State
In AsCl3: Cl = -1, three atoms total -3, molecule neutral → As = +3. In As2O3: O = -2 (3 atoms × -2 = -6 per formula unit), 2 As per formula unit neutral → 2As + (-6) = 0 → As = +3. In H3AsO3: 3(+1) + 3(-2) + As = 0 → As = +3.
Example Compounds
Formula
Name
Assignment
Notes
As2O3
Arsenic Trioxide (Arsenious Oxide)
2As + 3(-2) = 0 → As = +3
Also written As4O6. Used to treat acute promyelocytic leukemia (APL). Historically a major poison.
AsCl3
Arsenic Trichloride
3(-1) + As = 0 → As = +3
Fuming liquid; reacts violently with water. Used as a precursor in synthesis of organoarsenic compounds.
H3AsO3
Arsenious Acid
3(+1) + 3(-2) + As = 0 → As = +3
Weak, triprotic acid formed when As2O3 dissolves in water. Arsenite (AsO33-) is its conjugate base.
AsF3
Arsenic Trifluoride
3(-1) + As = 0 → As = +3
Used as a fluorinating agent in organofluorine chemistry.
Common Uses
Cancer treatment (As2O3 for APL leukemia)
Synthesis of organoarsenic pharmaceuticals and agrichemicals
Glass decolorizing (As2O3 removes green tint from iron)
Wood preservative precursors (historical CCA treatment)
Oxidation State +5
The highest common oxidation state for arsenic. It requires engagement of the 4d orbitals to accommodate 10 electrons around the central atom. Arsenic(V) is more oxidizing than phosphorus(V) because the extra stability of the 4s2 inert pair makes arsenic less willing to reach +5 without a strong oxidant. In water at neutral pH, As(V) exists as the arsenate anion (H2AsO4- / HAsO42-), while As(III) exists as un-ionized H3AsO3 — a key difference for water treatment.
How to Identify This State
In H3AsO4: 3(+1) + 4(-2) + As = 0 → As = +5. In As2O5: 5(-2) per As gives 2As + 5(-2) = 0 → As = +5. In AsO43-: 4(-2) + As = -3 → As = +5.
Example Compounds
Formula
Name
Assignment
Notes
H3AsO4
Arsenic Acid
3(+1) + 4(-2) + As = 0 → As = +5
Triprotic acid; used historically as a wood preservative (CCA); structurally analogous to phosphoric acid.
As2O5
Arsenic Pentoxide
2As + 5(-2) = 0 → As = +5
Deliquescent solid; dissolves in water to give H3AsO4. Strong oxidizing agent.
AsO43-
Arsenate Ion
4(-2) + As = -3 → As = +5
Mimics phosphate (PO43-) in biochemistry; binds to phosphate-handling enzymes, explaining arsenic toxicity at the metabolic level.
Ca3(AsO4)2
Calcium Arsenate
3(+2) + 2As + 8(-2) = 0 → As = +5
Historical insecticide used on cotton fields and orchards; now banned or restricted in most countries.
Common Uses
Wood preservation (copper chrome arsenate, CCA)
Semiconductor dopant sources (n-type doping)
Analytical reference standards for arsenic speciation
Historical insecticides and herbicides (now largely replaced)
Select a compound from the list to see the oxidation state of arsenic
with a step-by-step calculation.
Select a compound to see the oxidation state calculation.
Oxidation state of As:
Step-by-step
Oxidation State Summary
State
Stability
Key Example
Notes
-3
Common
GaAs
Arsenic bonded to metals; gains 3 electrons. Central to III-V semiconductor compounds.
0
Elemental
As (gray)
Assigned by convention to all allotropes of elemental arsenic.
+3
Very Common
As2O3
Most kinetically stable oxidation state due to the inert-pair effect; pyramidal geometry.
+5
Common
H3AsO4
Fully oxidized; requires strong oxidant; arsenate mimics phosphate in enzymes.
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Summary
Reference for all common arsenic oxidation states (-3, 0, +3, +5) with example compounds, identification tips, and a compound-to-oxidation-state lookup.
How it works
Click an oxidation state card (-3, 0, +3, or +5) to open its detail panel.
The detail panel shows a description, example compounds, step-by-step assignment, and identification tips.
Use the Compound Lookup tab to select a known arsenic compound and see the oxidation state of As 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 arsenic chemistry for exams.
Chemistry teachers preparing reference materials on Group 15 elements.
Researchers quickly checking the oxidation state of arsenic in a specific reagent or mineral.
Toxicologists and environmental chemists distinguishing arsenite (+3) from arsenate (+5) in water analysis.
Learners comparing oxidation-state stability trends down Group 15 (N, P, As, Sb, Bi).
Frequently Asked Questions
Arsenic commonly exhibits -3, 0, +3, and +5. The -3 state appears in arsenides such as GaAs and Na3As. Zero is assigned to elemental arsenic (gray allotrope, As4 or polymeric). The +3 state is found in arsenious oxide (As2O3), arsenic trichloride (AsCl3), and arsenious acid (H3AsO3). The +5 state is the most oxidized and appears in arsenic pentoxide (As2O5) and arsenic acid (H3AsO4).
As you go down Group 15, the inert-pair effect becomes increasingly significant. The 4s2 electron pair in arsenic is harder to oxidize than the phosphorus 3s2 pair. As a result, As(III) compounds such as As2O3 are more kinetically stable in many environments than the fully oxidized As(V) compounds, unlike phosphorus where +5 dominates. In strongly oxidizing conditions (e.g., acidic HNO3) As can be taken to +5.
Assign known states first: H = +1 (three atoms, total +3) and O = -2 (four atoms, total -8). The molecule is neutral, so As + 3(+1) + 4(-2) = 0 → As + 3 - 8 = 0 → As = +5.
Chlorine is -1. Three Cl atoms total -3. The molecule is neutral: As + 3(-1) = 0 → As = +3.
In GaAs (gallium arsenide), gallium is +3 (Group 13 metal). For the compound to be neutral: +3 + As = 0 → As = -3. Arsenic is more electronegative than gallium, so it formally holds the bonding electrons, giving it the negative oxidation state.
Arsenite refers to As(III) species — principally the AsO3^3- ion or H3AsO3 (arsenious acid). Arsenate refers to As(V) species — principally the AsO4^3- ion or H3AsO4 (arsenic acid). The distinction is critical in toxicology and water chemistry: arsenite is generally more acutely toxic than arsenate because it binds more readily to sulfhydryl groups in proteins.
