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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S3.1 - The periodic table - Classification of elements

3.1.1 Periodic Table Structure 3.1.2 Periodic, Group and Electron Configuration 3.1.3 Periodicity of Elements 3.1.4 Group Trends 3.1.5 Metallic to Non-Metallic Oxide Behaviour 3.1.6 Oxidation States 3.1.7 Ionization Energy Trends Exceptions (AHL) 3.1.8 Transition Element Properties (AHL) 3.1.9 Transition Element Oxidation States (AHL) 3.1.10 Colour and Transition Element Compounds (AHL)

Oxidation States

Specification Reference S3.1.6

Quick Notes

  • The oxidation state (or oxidation number) shows how many electrons an atom has lost or gained in bonding.
  • Oxidation state of a free element = 0 (e.g. O2, Na, Cl2).
  • Common rules:
    • Group 1: always +1
    • Group 2: always +2
    • Oxygen: –2, except in peroxides (–1)
    • Hydrogen: +1, except in metal hydrides (–1)
    • Fluorine: always –1
  • The sum of oxidation states in a compound = 0. In a polyatomic ion = charge of the ion.
  • Oxidation states are shown using + or – signs followed by Arabic numerals (e.g. +2, –1).
  • Roman numerals may be used in naming oxyanions (e.g., nitrate(V), sulfite(IV)), though generic names like “nitrate” are still acceptable.

Full Notes

Oxidation Numbers

Oxidation numbers help track electron transfer in reactions. It is straightforward to see how atoms have lost or gained electrons when ions get formed, however it can be harder to see how atoms have lost or gained electron density when dealing with molecules.

Example Carbon combustion

Carbon is oxidised to form carbon dioxide when combusted. However, no ions get formed, meaning it isn’t immediately clear how electrons are involved.

IB Chemistry diagram showing oxidation of carbon to carbon dioxide in combustion.

To help, we consider each atom to have an ‘imaginary’ charge, described as its oxidation number (or state).

Rules for Assigning Oxidation States

Element / Case Oxidation State
Uncombined elements (e.g., O2, N2, Fe) 0
Group 1 metals +1
Group 2 metals +2
Oxygen (usual) −2
Oxygen in peroxides (O22−) −1
Oxygen in OF2 +2
Hydrogen (usual) +1
Hydrogen in metal hydrides (e.g. NaH) −1
Neutral compound Sum of oxidation states = 0
Polyatomic ion Sum of oxidation states = charge of ion

Using these rules, we can see how carbon gets oxidised from an oxidation state of 0 in C(s) to +4 in CO2(g).

IB Chemistry diagram showing electron transfer and change in oxidation state of carbon from 0 to +4.

An increase in oxidation number (gets more positive) means oxidation has occurred. A decrease in oxidation number (gets more negative) means reduction has occurred.

Worked Example

Assign oxidation states in H2SO4 (sulfuric acid).

  1. H = +1 (there are 2 H, total +2).
  2. O = −2 (there are 4 O, total −8).
  3. The total charge must be 0, so S must be +6 to balance the equation:
    2(+1) + S + 4(−2) = 0 → S = +6.

Special Cases to Know

Compound Type Oxidation State
Metal hydrides H = –1 (e.g., NaH)
Peroxides O = –1 (e.g., H2O2)

Why Is the Oxidation State of Elements Zero?

In pure elements, atoms are not bonded to different atoms, so there’s no transfer of electrons. Therefore, each atom has equal pull on its own electrons → oxidation state = 0 (e.g., O2, Cl2, Fe).

Using Roman Numerals

When naming compounds, Roman numerals are used to show the oxidation state of transition metals and other elements that can have variable oxidation states.

Naming Conventions for Oxyanions

Oxyanions are negatively charged ions that contain a central atom (usually a non-metal) bonded to oxygen atoms. Their names are based on the oxidation number of the central atom, which can vary.

Systematic Naming (IUPAC):

Traditional (Common) Naming:

Naming Patterns:

Summary