Transcript Chapter 7: Chemical Formulas and Chemical compounds

Chapter 7: Chemical Formulas and
Chemical Compounds
Section 2: Oxidation Numbers
Overview
• We will list the rules for assigning oxidation
numbers.
• We will be able to give the oxidation number for
all the elements in a compound.
• We will name binary molecular compounds using
oxidation numbers and the stock system.
Oxidation Numbers
• A way to indicate the distribution of electrons among
bonded atoms in a molecular compound or a polyatomic
ion.
• Also known as an atom’s oxidation state.
• This is not the same as an atom’s charge!
• Oxidation numbers are helpful in some cases in naming a
compound.
• Later we will use them in balancing equations.
How to Assign Oxidation Numbers
• Atoms by themselves or bonded to themselves have an
oxidation state of 0.
• Hydrogen has an oxidation number of +1 and when
bonded to a metal -1.
• Fluorine is always a -1.
• Oxygen is -2 unless in H2O2 (-1) or OF2 (+2).
• The most electronegative atom has an oxidation number
equal to its charge as an ion. (Closest to F)
• The charge of an individual ion is its oxidation number.
• The sum of the oxidation numbers must equal the charge
of the compound.
Binary Molecule Examples
• HF
• Both have fixed values
H = +1
F = -1
• Cl2
• Bonded to Self
Cl = 0
• CS2
• Sulfur is more electronegative S = -2
• There are two sulfur atoms so their oxidation totals to
-4. The carbon atom must equal this charge to make
the molecule’s total oxidation 0. C = +4
Polyatomic Ion Examples
• SO4-2
• Oxygen is more electronegative & fixed value.
O = -2
• There are four oxygen atoms, so the total negative charge due to
oxygen is -8.
• The total charge needs to equal -2 so sulfur must account for
that.
S = +6
• SO3-2
• Oxygen is more electronegative & fixed value.
O = -2
• There are three oxygen atoms, so the total negative charge due
to oxygen is -6.
• The total charge needs to equal -2 so sulfur must account for
that.
S = +4
Molecular Compounds
w/ Polyatomic Ions
• H2CO3
• CO3 has a -2 charge so that is its oxidation number.
• Hydrogen is always +1. H = +1
• Oxygen is the most electronegative atom & fixed
value. O = -2
• Total oxidation number of the three oxygen atoms is -6
• So the oxidation number of carbon and -6 need to
equal -2.
C = +4
Using Oxidation States to Name
• We already do this in the stock system for ionic
compounds.
• The prefix system for molecular compounds can
be modified using the oxidation numbers to the
stock system.
• NCl3
• Nitrogen trichloride
• Nitrogen (III) chloride