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7.2 - Oxidation Numbers
Transcript of 7.2 - Oxidation Numbers
UF6 : Fluorine has an oxidation number of -1. We multiply by the amount of atoms there are. So fluorine's oxidation number is -6. This is a molecular compound, so the sum of the oxidation numbers has to equal to 0. Therefore, the uranium atom has an oxidation number of +6.
7.2 - Oxidation Numbers
In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers are assigned to the atoms of compound or ion.
Oxidation numbers don't have an exact physical meaning.
Oxidation Numbers for Formulas and Names
Rules for assigning oxidation numbers
There are nine certain rules for assigning oxidation numbers.
1. Atoms in a pure element have an oxidation number of zero.
Ex: Na, O2, P4
2. The more electronegative element in a binary molecular compound is assigned the number equal to the negative charge it would have as an anion. The less electronegative atom is assigned the number equal to the positive charge it would have as a cation.
3. Flourine has an oxidation number of -1 in all of its compounds because it is the most electronegative element.
4. Oxygen has an oxidation number of -2 in almost all compounds. There are two exceptions: with peroxides like H2O2 which is -1 and when it is with compounds including fluorine, which is +2.
5. Hydrogen has an oxidation number of +1 in all compounds that have elements more electronegative than it. It has an oxidation number of -1 in compounds with metals.
6. The algebraic sum of the oxidation numbers of all atoms in a neutral compound is equal to zero.
7. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
8. Although rules 1-7 apply to covalently bonded atoms, oxidation numbers can also be assigned to atoms in ionic compounds.
9. A monoatomic ion has an oxidation number equal to the charge of the ion.
H2SO4 : Sulfur and oxygen are more electronegative than hydrogen, so it has an oxidation number of +1. Oxygen has an oxidation number of -2 because it does not fit into any of the exceptions. The sum has to equal to zero,. There is only one sulfur atom so its oxidation number is +6.
ClO3- : This is a polytaomic ion, so the sum must equal to its charge, which is -1. Oxygen has a charge of -2. There are 3 atoms of oxygen, so it is -6. The ion must have a charge of -1, therefore, chlorine's oxidation number is +5.
Many nonmetals can have more than one oxidation number.
These numbers can sometimes be used like ionic charges to determine formulas.
Roman numerals can be used to denote ionic charges in the Stock System for naming ionic compounds.
The Stock System is based on oxidation numbers. It can also be used with a prefix system for naming binary molecular compounds.
Using the prefix system, the name for N2O would be dinitrogen monoxide (the prefixes indicate the amount of atoms) and not Nitrogen (I) oxide.