Reference for all common iodine oxidation states (-1, 0, +1, +3, +5, +7) with example compounds, identification steps, and a compound-to-state lookup.
Use the Iodine Oxidation States
Atomic #53IIodine
Atomic Mass
126.904 u
Group
17 (VIIA)
Period
5
Block
p-block
Electronegativity
2.66 (Pauling)
Oxidation States
-1, 0, +1, +3, +5, +7
Iodine has six common oxidation states. Click a state card to see its
compounds, uses, and how to identify it.
Oxidation State −1
The -1 state is by far the most common and thermodynamically stable oxidation state for iodine. It occurs when iodine gains one electron to complete its 5p6 valence shell, achieving a noble-gas configuration like xenon. In this state, iodine exists as the iodide ion (I-) in ionic compounds or as a covalent partner in hydrogen iodide (HI). Iodide is the biologically relevant form of iodine used by the thyroid gland to synthesize hormones.
How to Identify This State
Look for I bonded to metals (NaI, KI, CaI2) or to hydrogen (HI). In HI: H is +1, molecule is neutral → I = -1. In KI: K is +1, compound is neutral → I = -1. In CaI2: Ca is +2, 2I total must be -2 → each I = -1.
Example Compounds
Formula
Name
Assignment
Notes
NaI
Sodium Iodide
+1 + I = 0 → I = -1
Used in iodized salt, medical imaging (radiocontrast), and as a pharmaceutical. Key source of dietary iodine.
KI
Potassium Iodide
+1 + I = 0 → I = -1
Administered in thyroid-protection emergencies (radiation exposure). Also used in Lugol's solution.
HI
Hydrogen Iodide
+1 + I = 0 → I = -1
Strong acid in aqueous solution (hydroiodic acid). Reducing agent in organic synthesis.
CH3I
Iodomethane
C is -2, 3H at +1, I: -2+3+I=0 → I = -1
Methylating agent in organic synthesis. Also used as a soil fumigant (now restricted).
Common Uses
Dietary iodine supplement (iodized salt)
Thyroid hormone synthesis (T3 and T4)
Medical imaging: radioiodine (I-131) for thyroid cancer diagnosis and treatment
Synthesis precursor in organic and pharmaceutical chemistry
Oxidation State 0
Elemental iodine is assigned oxidation state 0 by convention — bonds between identical atoms carry no oxidation state contribution. At room temperature, iodine exists as a shiny blue-black diatomic solid (I2) that sublimes readily to a characteristic violet vapor. The I–I bond is the weakest of the halogen-halogen bonds, making I2 a milder oxidizing agent than Cl2 or Br2.
How to Identify This State
Any pure elemental I2 — solid, dissolved in solution (brown-yellow), or vapor (violet) — carries oxidation state 0 by definition. No arithmetic is needed.
Example Compounds
Formula
Name
Assignment
Notes
I2 (solid)
Molecular Iodine (solid)
Elemental → I = 0
Shiny blue-black solid; sublimes at 184 °C. Standard laboratory reagent, used in starch tests and titrations.
I2 (solution)
Iodine in KI solution
Elemental → I = 0
I₂ dissolves in aqueous KI to form I₃⁻ (triiodide), giving the familiar brown color of Lugol's solution. Iodine itself remains at 0; the charge is on the complex.
I2 (vapor)
Iodine Vapor
Elemental → I = 0
Distinctive violet color. Used to visualize organic compounds on TLC plates by staining.
Common Uses
Starch-iodine indicator in analytical chemistry (turns blue-black)
Lugol's solution (I₂ in KI) as antiseptic and thyroid function test
TLC plate visualization (iodine chamber)
Iodometric titration standard (with thiosulfate)
Oxidation State +1
The +1 state is the lowest positive oxidation state of iodine. It requires iodine to lose one valence electron and is stabilized when iodine bonds to a more electronegative halogen (fluorine or chlorine) or to oxygen in hypoiodite/hypoiodous acid. Iodine monochloride (ICl) and iodine monofluoride (IF) are the main interhalogen examples. Hypoiodous acid (HOI) and hypoidodite (IO-) appear transiently in aqueous oxidation reactions.
How to Identify This State
In ICl: Cl is -1 (more electronegative), molecule neutral → I + (-1) = 0 → I = +1. In HOI: O is -2, H is +1, molecule neutral: I + (+1) + (-2) = 0 → I = +1. In IO-: O is -2, ion charge -1: I + (-2) = -1 → I = +1.
Example Compounds
Formula
Name
Assignment
Notes
ICl
Iodine Monochloride
Cl = -1; I + (-1) = 0 → I = +1
Red-brown liquid; electrophilic iodination reagent in organic synthesis (Wohl-Ziegler type reactions).
IF
Iodine Monofluoride
F = -1; I + (-1) = 0 → I = +1
Unstable interhalogen; disproportionates readily to I₂ and IF₅. Mainly of theoretical interest.
HOI
Hypoiodous Acid
H = +1, O = -2; I + 1 - 2 = 0 → I = +1
Weak, unstable acid formed in basic solutions of I₂. Short-lived oxidizing intermediate in iodination reactions.
IO-
Hypoiodite Ion
O = -2; I + (-2) = -1 → I = +1
The conjugate base of HOI, formed when I₂ is added to alkaline solution. Disproportionates quickly to IO₃⁻ and I⁻.
Common Uses
Electrophilic iodination of aromatic compounds (ICl as reagent)
Intermediate in iodine redox cycles in aqueous solution
Disinfection byproduct studies in drinking water treatment
Halogen exchange reactions in synthesis
Oxidation State +3
The +3 state is less common but accessible, appearing primarily in interhalogen compounds with fluorine and in the short-lived iodous acid (HIO2) or iodite ion (IO2-). Iodine trifluoride (IF3) is the classic +3 interhalogen. The +3 state requires engagement of one 5d orbital and is less stable than either +1 or +5 for iodine, making iodous acid difficult to isolate.
How to Identify This State
In IF3: F is -1 (3 atoms × -1 = -3), neutral molecule → I + (-3) = 0 → I = +3. In HIO2: H = +1, 2O × (-2) = -4; I + 1 - 4 = 0 → I = +3. In ICl3: Cl = -1 (3 atoms × -1 = -3), neutral → I = +3.
Example Compounds
Formula
Name
Assignment
Notes
IF3
Iodine Trifluoride
3(-1) + I = 0 → I = +3
Yellow solid; only stable below -28 °C; a strong fluorinating agent and Lewis acid.
ICl3
Iodine Trichloride
3(-1) + I = 0 → I = +3
Orange-yellow solid; used as a Lewis acid catalyst and electrophilic chlorinating/iodinating agent. Disproportionates above 101 °C.
HIO2
Iodous Acid
(+1) + 2(-2) + I = 0 → I = +3
Unstable oxyacid; only detected as a transient intermediate in iodine oxidation chemistry.
IO2-
Iodite Ion
2(-2) + I = -1 → I = +3
Conjugate base of iodous acid; metastable in aqueous solution, rapidly disproportionating to iodate and iodide.
Common Uses
Fluorination reactions (IF₃ as selective fluorinating agent at low temperature)
Lewis acid catalysis (ICl₃) in organic synthesis
Mechanistic intermediate in iodine oscillation reactions (Briggs-Rauscher)
Study of halogen bond formation and interhalogen chemistry
Oxidation State +5
The +5 state is iodine's most stable positive oxidation state in aqueous solution. It appears in iodic acid (HIO3), the iodate ion (IO3-), iodine pentafluoride (IF5), and iodyl compounds. Iodate is the dominant form of iodine in the ocean and is produced in the troposphere by photochemical oxidation of I2. HIO3 is a moderately strong acid and a useful primary standard in analytical chemistry.
How to Identify This State
In HIO3: H = +1, 3O × (-2) = -6; I + 1 - 6 = 0 → I = +5. In IO3-: 3O × (-2) = -6, ion charge -1: I + (-6) = -1 → I = +5. In IF5: F = -1, 5 atoms × (-1) = -5; neutral molecule → I = +5.
Example Compounds
Formula
Name
Assignment
Notes
HIO3
Iodic Acid
(+1) + 3(-2) + I = 0 → I = +5
Stable white solid; strong oxidizing acid; used as a primary standard in titrations. Moderately strong acid (pKa ≈ 0.77).
KIO3
Potassium Iodate
K = +1, 3(-2) + I = -1 → I = +5
Used to iodize salt in many countries. Also the reagent in iodometric back-titrations.
Dominant iodine species in seawater (at 50–90 nmol/L). Reduced to iodide in reducing marine environments.
Common Uses
Iodometric titration primary standard (KIO₃)
Salt iodization programs (KIO₃, especially in tropical climates where KI oxidizes)
Fluorinating agent in synthesis (IF₅)
Environmental tracer for ocean chemistry and atmospheric iodine cycling
Oxidation State +7
The +7 state is the maximum oxidation state of iodine and requires formal loss of all seven valence electrons. It is found in periodic acid (HIO4 or H5IO6), the periodate ion (IO4- or IO65-), and iodine heptafluoride (IF7). Periodate is a powerful and selective oxidant widely used in organic synthesis to cleave vicinal diols. IF7 is the only known molecular compound of iodine in the +7 state without oxygen.
How to Identify This State
In HIO4: H = +1, 4O × (-2) = -8; I + 1 - 8 = 0 → I = +7. In IO4-: 4(-2) + I = -1 → I = +7. In IF7: F = -1, 7 atoms × (-1) = -7; neutral → I = +7.
Example Compounds
Formula
Name
Assignment
Notes
HIO4
Metaperiodic Acid
(+1) + 4(-2) + I = 0 → I = +7
Strong oxidizing acid; cleaves vicinal diols (Malaprade reaction). Used in carbohydrate and glycol analysis.
NaIO4
Sodium Periodate
Na = +1, 4(-2) + I = -1 → I = +7
Standard reagent for periodate oxidations in organic and biochemistry labs (e.g., RNA 3'-end labeling).
IF7
Iodine Heptafluoride
7(-1) + I = 0 → I = +7
Pentagonal bipyramidal geometry (AX₇); extremely reactive fluorinating agent; the only AX₇ halogen fluoride.
IO4-
Periodate Ion
4(-2) + I = -1 → I = +7
Tetrahedral, analogous to sulfate. Selectively oxidizes 1,2-diols to aldehydes/ketones in aqueous media.
Common Uses
Periodate oxidation of vicinal diols in carbohydrate chemistry (Malaprade reaction)
RNA and glycoprotein end-labeling in biochemistry
Selective cleavage of sugars for structural analysis
IF₇ as a powerful, selective fluorinating agent in materials chemistry
Select a compound from the list to see the oxidation state of iodine
with a step-by-step calculation.
Select a compound to see the oxidation state calculation.
Oxidation state of I:
Step-by-step
Oxidation State Summary
State
Stability
Key Example
Notes
-1
Most Common
NaI / KI
Iodide; gains 1e⁻ to complete 5p shell; found in all ionic iodides and HI; essential for thyroid hormones.
0
Elemental
I2
Diatomic solid at room temperature; sublimes to violet vapor; assigned 0 by convention.
+1
Uncommon
ICl / HOI
Interhalogen with Cl or F; hypoiodous acid in aqueous redox; transient electrophilic iodinating agent.
+3
Rare
IF3 / ICl3
Interhalogen with F or Cl; iodous acid is very unstable. Less stable than +1 or +5 due to poor orbital matching.
+5
Stable
HIO3 / IO3-
Most stable positive state; iodate is the dominant form in ocean and atmosphere; KIO₃ used in salt iodization.
+7
Strong oxidant
NaIO4 / IF7
Maximum state; periodate is a selective diol-cleaving reagent; IF₇ is the only AX₇ halogen fluoride.
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Summary
Iodine (I, atomic number 53) is a halogen in Group 17 with the ground-state electron configuration [Kr] 4d10 5s2 5p5. Its seven valence electrons give it a strong tendency to gain one electron (-1), but unlike lighter halogens, the available 5d orbitals let iodine expand its octet and reach positive oxidation states all the way to +7. This makes iodine the most versatile halogen in oxidation-state chemistry. The -1 state dominates in ionic iodides and biological systems; the positive states appear in interhalogen compounds, oxyacids (hypoiodous, iodous, iodic, periodic acid), and their salts.
How it works
Click an oxidation state card (-1, 0, +1, +3, +5, or +7) 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 iodine compound and see the oxidation state of I explained step by step.
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Switch between the Explorer and Compound Lookup tabs using the tab bar.
Use cases
Students revising oxidation state rules and halogen chemistry for exams.
Chemistry teachers preparing reference materials on Group 17 elements.
Researchers checking the oxidation state of iodine in a specific reagent or pharmaceutical.
Analytical chemists distinguishing iodide (-1), iodate (+5), and periodate (+7) in solution.
Learners comparing oxidation-state trends across the halogens (F, Cl, Br, I).
Lab workers selecting the correct iodine oxyacid or salt for a synthesis.
Frequently Asked Questions
Iodine commonly exhibits -1, 0, +1, +3, +5, and +7. The -1 state is by far the most common and is found in all simple iodide salts (NaI, KI) and HI. Zero is assigned to elemental I2. The positive states (+1, +3, +5, +7) appear in interhalogen compounds and iodine oxyacids: hypoiodous acid HOI (+1), iodous acid HIO2 (+3), iodic acid HIO3 (+5), and periodic acid HIO4 (+7).
Fluorine has no d orbitals available in its valence shell and is the most electronegative element, so it cannot expand its octet or reach positive oxidation states. Iodine, being a larger, less electronegative Period 5 element, has accessible 5d orbitals that allow octet expansion and stabilize the +5 and +7 states when bonded to highly electronegative oxygen or fluorine.
Assign O = -2 (three atoms, total -6). The ion has charge -1: I + 3(-2) = -1 → I - 6 = -1 → I = +5. The iodate ion therefore carries iodine in the +5 state.
Fluorine is -1 (most electronegative element). Seven F atoms total -7. The molecule is neutral: I + 7(-1) = 0 → I = +7. IF7 is one of the few examples of iodine in its maximum oxidation state.
In NaI, iodine is directly bonded to the less electronegative sodium and holds the bonding electrons, giving -1. In NaIO3, iodine is bonded to three highly electronegative oxygen atoms, which pull electron density away from iodine, forcing it into the positive +5 state. The presence of oxygen is the key structural difference.
No. The -1 state (iodide) is thermodynamically most stable under normal conditions. The +7 state in periodate is strongly oxidizing and represents the highest accessible state. Iodate (+5) is kinetically stable in water and is the dominant form in the iodine cycle in seawater.
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