Rules for Assigning Oxidation Numbers to Elements
Oxidation numbers are bookkeeping numbers. They allow chemists to do things such as balance redox (reduction/oxidation) equations. Oxidation numbers are positive or negative numbers, but don’t confuse them with positive or negative charges on ions or valences.
Oxidation numbers are assigned to elements using these rules:
Rule 1: The oxidation number of an element in its free (uncombined) state is zero — for example, Al(s) or Zn(s). This is also true for elements found in nature as diatomic (two-atom) elements
and for sulfur, found as:
Rule 2: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example:
Rule 3: The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion. This rule often allows chemists to calculate the oxidation number of an atom that may have multiple oxidation states, if the other atoms in the ion have known oxidation numbers.
Rule 4: The oxidation number of an alkali metal (IA family) in a compound is +1; the oxidation number of an alkaline earth metal (IIA family) in a compound is +2.
Rule 5: The oxidation number of oxygen in a compound is usually –2. If, however, the oxygen is in a class of compounds called peroxides (for example, hydrogen peroxide), then the oxygen has an oxidation number of –1. If the oxygen is bonded to fluorine, the number is +1.
Rule 6: The oxidation state of hydrogen in a compound is usually +1. If the hydrogen is part of a binary metal hydride (compound of hydrogen and some metal), then the oxidation state of hydrogen is –1.
Rule 7: The oxidation number of fluorine is always –1. Chlorine, bromine, and iodine usually have an oxidation number of –1, unless they’re in combination with an oxygen or fluorine.
These rules give you another way to define oxidation and reduction — in terms of oxidation numbers. For example, consider this reaction, which shows oxidation by the loss of electrons:
Notice that the zinc metal (the reactant) has an oxidation number of zero (rule 1), and the zinc cation (the product) has an oxidation number of +2 (rule 2). In general, you can say that a substance is oxidized when there’s an increase in its oxidation number.
Reduction works the same way. Consider this reaction:
The copper is going from an oxidation number of +2 to zero. A substance is reduced if there’s a decrease in its oxidation number.
Chemistry Glossary
Archimedes Principle
A principle discovered by the Greek mathematician Archimedes which states that the volume of a solid is equal to the volume of water it displaces.
Chemistry Glossary
atomic number
The number of protons in the nucleus of an atom.
Chemistry Glossary
Bohr model
A model of atomic structure developed by Niels Bohr, a Danish scientist. In this model, electrons occur in orbits of differing energy levels around the nucleus of an atom.
Chemistry Glossary
condensation
The change in the physical state of matter from a gaseous state to a liquid state.
Chemistry Glossary
deposition
The change in the physical state of matter from a gaseous state to a solid state without ever becoming a liquid. The reverse of sublimation.
Chemistry Glossary
electrolytes
Substances that can conduct electricity either in the molten state or when dissolved in water.
Chemistry Glossary
electron configuration notation
A method used by chemists to represent electrons in bonding and chemical reactions.
Chemistry Glossary
electronegativity
A measure of an atom’s strength to attract a bonding pair of electrons to itself.
Chemistry Glossary
energy level diagram
A method used by chemists to diagram the electrons for an atom (including orbitals and subshells) in bonding and chemical reactions.
Chemistry Glossary
heterogeneous mixture
A mixture whose composition varies from position to position within a sample.
Chemistry Glossary
homogeneous mixture
A mixture whose composition is the same from position to position within a sample.
Chemistry Glossary
isotopes
Atoms of the same element that have varying numbers of neutrons.
Chemistry Glossary
mass number
The sum of the protons and neutrons in a particular isotope; also called atomic weight.
Chemistry Glossary
nonelectrolytes
Substances that do not conduct electricity in the molten state or when dissolved in water.
Chemistry Glossary
nuclear fission
A nuclear reaction in which an atom’s nucleus splits into smaller parts.
Chemistry Glossary
nuclear fusion
A process in which lighter nuclei of atoms join together into a heavier nucleus; essentially the opposite of nuclear fission.
Chemistry Glossary
nuclear reaction
Any reaction that involves a change in nuclear structure.
Chemistry Glossary
periodic table
A table that displays all known chemical elements in an arrangement that is based on the properties of the elements; changes over time as new elements are discovered.
Chemistry Glossary
quantum mechanical model
A model of atomic structure that is based on mathematics and can be used to explain observations made on complex atoms.
Chemistry Glossary
radioactivity
The spontaneous decay of an unstable nucleus in an atom.
Chemistry Glossary
SI system
A worldwide measurement system that is based on the older metric system. The SI comes from the French Systeme International.
Chemistry Glossary
sublimation
The change in the physical state of matter from a solid state to a gaseous state without ever becoming a liquid (such as dry ice).
Chemistry Glossary
valence electrons
The electrons in the outermost energy level of an atom, the farthest away from the nucleus.
