3. 1. Prevent confusing one
compound for another
Consider these compounds:
FeO and Fe2O3
Iron (II) oxide and iron (III) oxide
CO2 and CO
carbon dioxide and carbon monoxide
4. 2. Formulas may be too
complex to say.
For carbon dioxide …
… you can easily say CO2
But would you like to say
C6H4OCOCH3CO2H
when you could say aspirin
Or, even acetylsalicylic acid
5. 3. Families of compounds
have similarities in names
and properties
The calcium carbonate in
Tums is an antacid …
… any carbonate compound
will react with any acid.
6. Chemical names …
Prevent confusing one
compound for another
Keep from having to say
complex formulas
Group compounds into
families
7. The convention for writing
formulas of simple
inorganic compounds:
The element with the
positive oxidation
number is written first.
8. A binary compound
contains only two
kinds of elements
H2O and HCl
CO2 and N2O5
NaCl and BiF3
10. Naming a compound with
only two nonmentals…
1. Write the name of the first
element along with the
appropriate prefix.
2. Write the name of the second
element, modified to end in ide,
along with the appropriate prefix.
14. Use mono- sparingly
The prefix mono- should only
be used to prevent confusion
… as in CO and CO2
Carbon monoxide and
carbon dioxide
15. You can write the formula from the
name. The prefix tells the subscript.
Nitrogen trichloride
NCl3
Carbon tetrachloride
CCl4
Diphosphorous pentoxide
P 2O 5
16. Naming Binary
Compounds
Containing a Metal
and a Nonmetal
17. The Stock System
Named for Alfred Stock, a
German chemist of the nineteenth
century.
Add a Roman Numeral after the
name of the positive element to
indicate its oxidation number.
18. Using the Stock System:
1. Write the name of the positive
element
2. Add a Roman numeral
corresponding to the oxidation
number of the positive element (if
necessary)
3. Write the name of the negative
element, modified to end in -ide
19. When is the Roman Numeral
Necessary?
When the metal has more than one
oxidation number.
Metals in groups I A, II A, and III B,
do not need Roman numerals.
Others metals have several oxidation
numbers, and need Roman numerals.
20. What is an oxidation number?
An oxidation number is a
number that tells us how an
element combines with other
elements.
For ions, the
oxidation number is
the ionic charge.
21. We can find oxidation
numbers in tables or we
can predict them using
the periodic table.
22. What does the Roman Numeral do?
The Roman numeral indicates the
oxidation number on a single metal
atom, and differentiates between
several possible compounds.
Consider: FeO and Fe2O3
Both contain iron and oxygen
But, both cannot be iron oxide …
23. … so we use a Roman
numeral to differentiate
between the two
compounds.
24. First, determine the oxidation
number of iron in FeO …
… by starting with the negative
element, find the oxidation number
of the positive element.
Since iron has an oxidation
number of +2, FeO is named
iron (II) oxide.
25. Likewise, for Fe2O3 …
First, determine the oxidation number
of iron in Fe2O3 …
… by starting with the negative
element, find the oxidation
number of the positive element.
Since iron’s oxidation number is +3,
Fe2O3 is named iron (III) oxide.
26. Remember …
Only add a Roman Numeral if it is
needed.
It is needed when the metal has more
than one positive oxidation number.
If the element has only one positive
oxidation number, you do not need a
Roman numeral.
28. The sum of the oxidation
numbers on all the atoms in
a compound must equal
zero. Consider iron (II) oxide.
Oxygen is –2
+2 –2 = 0 and iron is +2.
FeO The sum is 0.
29. Now consider iron (III) oxide
In iron (III) oxide,
+3 -2 the iron is +3 and
Fe2O3 the oxygen is –2.
These don’t add up to zero.
30. Now consider iron (III) oxide
But, since there
+6 -6 = 0
+3 -2 are two Fe atoms
Fe2O3 and three O atoms,
we can multiply to
2x3=6 get the totals.
3 x –2 = -6 Now the sum is
zero.
31. Look at it another way:
+6 –6 = 0
+6
-6
+3 +3 -2 -2 -2
Fe2O3 = Fe Fe O O O
33. The Crisscross Method
Simple but effective,
…most of the time.
Be aware of the potential
problems with this method.
34. Suppose you are writing the
formula for copper (II) chloride
First, write down
the symbols:
Cu Cl
35. Then write the oxidation
numbers in copper (II) chloride
The oxidation Get the
number of oxidation
copper comes number of
from the +2 -1
chlorine
name. Cu Cl from the
periodic table.
36. Crisscross the numbers for the
formula of copper (II) chloride
When you crisscross,
ignore the signs.
+2 -1
Cu1 Cl 2
37. Clean up the subscripts in the
formula of copper (II) chloride
Subscripts of 1 are invisible.
(don’t even put the 1)
+2 -1
Cu1 Cl 2
38. Now you have the formula of
copper (II) chloride
Cu Cl 2
39. When is the “crisscross
method” a problem?
When all the
subscripts are divisible
by a number other than
1.
40. Look at chromium (VI) oxide
The
oxidation +6 -2
number
of Cr isCr O
+6 Oxygen is
always -2
41. Look at chromium (VI) oxide
Now, crisscross the oxidation
numbers. +6
-2
-2
Cr2 O6
When you crisscross, both
subscripts are divisible by 2.
42. Look at chromium (VI) oxide
Divide each subscript by 2.
+6 -2
Cr__ O__
2 6
2 2
43. Look at chromium (VI) oxide
This is the correct formula
Cr O3
Remember, 1’s are invisible.
44. Remember: reduce the
subscripts to their
smallest whole-number
values.
Unless there’s a really
good reason not to.
47. The “ic/ous” method …
… is an archaic method, but still in
use today by the chemical industry
… uses the –ic or –ous suffixes on
the name of the metal.
… may use the Latin root
… uses prefixes like hypo- & per-
48. Name and Lower Ox. Higher Ox.
Latin Root Number Number
iron ferrous ferric
ferrum Fe 2+
Fe 3+
copper cuprous cupric
cuprum Cu +
Cu 2+
mercury mercurous mercuric
Hg22+ Hg2+
