1. CHEMISTRY CHAPTER 7
CHEMICAL FORMULAS AND CHEMICAL NAMES
SECTION 1. CHEMICAL NAMES AND FORMULAS
Significance of a Chemical Formula
A chemical formula indicates the relative number of atoms of each kind in a chemical compound.
For a molecular compound, the chemical formula reveals the number of atoms of each element contained in a single molecule of the compound.

2. Molecular Formula
example: octane — C8H18
The subscript after the C indicates that there are 8 carbon atoms in the molecule.
The subscript after the H indicates that there are 18 hydrogen atoms in the molecule.

3. Formula of an Ionic Compound
The chemical formula for an ionic compound represents one formula unit—the simplest ratio of the compound’s positive ions (cations) and its negative ions (anions).

4. Example: aluminum sulfate — Al2(SO4)3
Parentheses surround the polyatomic ion to identify it as a unit.
The subscript 3 refers to the unit.
There is no subscript for sulfur; it is understood to be 1.

5. Many main-group elements can lose or gain electrons to form ions.
Monatomic Ions
Many main-group elements can lose or gain electrons to form ions.
Ions formed from a single atom are known as monatomic ions.
example: To get to an octet, nitrogen gains 3 electrons to form N3– ions.
Some main-group elements tend to form covalent bonds instead of forming ions.
examples: carbon and silicon

6. Monatomic cations are identified using the element’s name. examples:
Naming Monatomic Ions
Monatomic cations are identified using the element’s name.
examples:
K+ = potassium ion
Mg2+ = magnesium ion
(for these there is only one type of cation formed by the element)

7. If the element can form more than one cation, a Roman numeral is used to indicate the charge.
Ex.:
Cu+ = copper (I) ion
Cu2+ = copper (II) ion

8. For monatomic anions, the ending of the element’s name is dropped, and the ending -ide is added to the root name.
examples:
F– = fluoride ion
N3– = nitride ion
O2- = oxide ion
Others: Cl-, Br-, I-, S2-, P3-

9. Common Monatomic Ions

11. Visual Concepts

12. Binary Ionic Compounds
Compounds composed of 2 elements are known as binary compounds.
In a binary ionic compound, the total numbers of positive charges and negative charges must be equal.
Example: magnesium bromide
Ions: Mg2+, Br–
To balance one Mg2+, 2 Br- are needed
Chemical formula: MgBr2

13. How to figure out a formula: “crossing over” method
1) Write the symbols for the ions, cation first. Example: aluminum oxide
Al3+ O2–
2) Cross over the charges by using the absolute value of each ion’s charge as the subscript for the other ion (subscripts are always positive numbers).

14. 3) Rewrite without the charges: Al2O3
(Do not write “1” as a subscript.)
4) If necessary, change the subscripts to give the lowest possible whole number ratio.
5) Check that the charges balance: (2 × 3+) + (3 × 2–) = 0

15. Example 2: zinc chloride
Zn2+ Cl-
Zn2+1Cl-2
ZnCl2

16. Example 3: iron (II) oxide
Fe2+ O2-
Fe2+2O2-2
Fe2O2
FeO
(lowest whole number ratio)

17. Naming Binary Ionic Compounds
For cations that have only one possible charge, the name simply comes from the ions.
Ex.: ZnCl2 =
Al2S3 =
MgO =

18. Naming Binary Ionic Compounds
For cations that have only one possible charge, the name simply comes from the ions.
Ex.: ZnCl2 = zinc chloride
Al2S3 = aluminum sulfide
MgO = magnesium oxide

19. 2. The Stock System is used to name compounds where the cation can have more than one charge.
The system uses a Roman numeral to indicate an ion’s charge.

20. Examples Cu2S Cu+ copper (I) sulfide CuS Cu2+ copper (II) sulfide FeF2
formula
cation
name
Cu2S
Cu+
copper (I) sulfide
CuS
Cu2+
copper (II) sulfide
FeF2
Fe2+
FeF3
Fe3+
PbCl2
PbCl4

21. Examples Cu2S Cu+ copper (I) sulfide CuS Cu2+ copper (II) sulfide FeF2
formula
cation
name
Cu2S
Cu+
copper (I) sulfide
CuS
Cu2+
copper (II) sulfide
FeF2
Fe2+
iron (II) fluoride
FeF3
Fe3+
iron (III) fluoride
PbCl2
Pb2+
lead (II) chloride
PbCl4
Pb4+
lead (IV) chloride

22. An older naming system uses:
-ous ending for ions with lower charge
-ic ending for ions with higher charge
Latin names for some elements

23. Examples: formula cation name Stock system name SnCl2 Sn2+
stannous chloride
tin (II) chloride
SnCl4
Sn4+
stannic chloride
tin (IV) chloride
CuO
cupric oxide
Cu2O
CoF2
cobaltous fluoride
AuCl3
auric chloride

24. Examples: formula cation name Stock system name SnCl2 Sn2+
stannous chloride
tin (II) chloride
SnCl4
Sn4+
stannic chloride
tin (IV) chloride
CuO
Cu2+
cupric oxide
Cu2O
Cu+
CoF2
Co2+
cobaltous fluoride
AuCl3
Au3+
auric chloride

25. Examples: formula cation name Stock system name SnCl2 Sn2+
stannous chloride
tin (II) chloride
SnCl4
Sn4+
stannic chloride
tin (IV) chloride
CuO
Cu2+
cupric oxide
Cu2O
Cu+
cuprous oxide
CoF2
Co2+
cobaltous fluoride
AuCl3
Au3+
auric chloride

26. Examples: formula cation name Stock system name SnCl2 Sn2+
stannous chloride
tin (II) chloride
SnCl4
Sn4+
stannic chloride
tin (IV) chloride
CuO
Cu2+
cupric oxide
copper (II) oxide
Cu2O
Cu+
cuprous oxide
copper (I) oxide
CoF2
Co2+
cobaltous fluoride
cobalt (II) fluoride
AuCl3
Au3+
auric chloride
gold (III) chloride

27. Compounds Containing Polyatomic Ions
Naming: as with binary compounds, give the cation first, followed by the anion.
Ex.: NH4Cl ions: NH4+, Cl- name:
NaOH ions: Na+, OH-
name:

28. Compounds Containing Polyatomic Ions
Naming: as with binary compounds, give the cation first, followed by the anion.
Ex.: NH4Cl ions: NH4+, Cl- name: ammonium chloride
NaOH ions: Na+, OH-
name: sodium hydroxide

29. Formulas: use the crossing over method
Formulas: use the crossing over method. If more than one of the ion, use parentheses.
Ex. 1: sodium nitrate Na+ NO3-
NaNO3
Ex. 2: aluminum sulfate
Al3+ SO42-
Al2(SO4)3

30. Understanding Formulas for Polyatomic Ionic Compounds

31. Naming Binary Molecular Compounds
Unlike ionic compounds, molecular compounds are composed of individual covalently bonded units, or molecules.
As with ionic compounds, there is also a Stock system for naming molecular compounds (covered in Section 2).

32. One system of naming molecular compounds is based on the use of prefixes. Examples:
CCl4: carbon tetrachloride (tetra- = 4)
CO: carbon monoxide (mon- = 1) CO2: carbon dioxide (di- = 2)
The prefix “mono” is not used for the first element.
Ex.: PCl3 = phosphorus trichloride, not monophosphorus trichloride.

33. Prefixes for Naming Covalent Compounds

34. The first element is usually the one with the lower group number.
Ex.: nitrogen and oxygen – NO, not ON.
If the two elements are in the same group, the higher period element comes first.
Ex. – SO2, not O2S.

35. The name of the second element has its ending changed to -ide (just as for ionic compounds).
If the element name begins with a vowel, an “a” or “o” at the end of the prefix is omitted.
Ex.: N2O4 = dinitrogen tetroxide
(not “tetraoxide”)

36. Some compounds that have names ending in “-ide” are not binary; they contain polyatomic ions with names ending in “-ide”:
OH- =
CN- =
O22- = peroxide

37. Some compounds that have names ending in “-ide” are not binary; they contain polyatomic ions with names ending in “-ide”:
OH- = hydroxide
CN- = cyanide
O22- = peroxide

38. Practice:
CO2 CO
CCl4
SO3
As2O5

39. Practice:
CO2 carbon dioxide
CO carbon monoxide
CCl4 carbon tetrachloride
SO3 sulfur trioxide
As2O5 diarsenic pentoxide

40. Covalent-Network Compounds
Some covalent compounds do not consist of individual molecules.
Instead, each atom is joined to all its neighbors in a covalently bonded, three-dimensional network.

41. Subscripts in a formula for covalent-network compound indicate smallest whole-number ratios of the atoms in the compound.
Examples:
SiC, silicon carbide
SiO2, silicon dioxide
Si3N4, trisilicon tetranitride

42. Covalent-Network Compounds
Silicon carbide
SiC
commons.wikimedia.org

43. Silicon dioxide (silica)
SiO2
utas.edu.au

44. An acid is a compound that produces H+ when dissolved in water.
Acids, Bases, and Salts
An acid is a compound that produces H+ when dissolved in water.
Example: HCl → H+ + Cl- (hydrochloric acid)
Most acids used in laboratories are either binary acids or oxyacids.

45. Binary acids are acids that consist of two elements, usually hydrogen and a halogen.
Oxyacids are acids that contain hydrogen, oxygen, and a third element (usually a nonmetal).

46. In the laboratory, the term acid usually refers to a solution in water of an acid compound rather than the acid itself.
example: hydrochloric acid refers to a water solution of the molecular compound hydrogen chloride, HCl

47. Many polyatomic ions are produced by the loss of hydrogen ions from oxyacids.
Examples:
sulfuric acid H2SO4 sulfate
nitric acid HNO3 nitrate
phosphoric acid H3PO4
phosphate

48. The naming of an acid depends on its anion.
Anions ending in –ide
(= monatomic anions):
Replace “-ide” with “-ic acid” and add the prefix “hydro.”
Ex. HCl – the anion is chloride
name = hydrochloric acid
Others:
HF =
HBr =
HI = hydriodic acid

49. The naming of an acid depends on its anion.
Anions ending in –ide
(= monatomic anions):
Replace “-ide” with “-ic acid” and add the prefix “hydro.”
Ex. HCl – the anion is chloride
name = hydrochloric acid
Others:
HF = hydrofluoric acid
HBr = hydrobromic acid
HI = hydriodic acid

50. 2. Anions ending in –ate
(which are polyatomic):
Replace “-ate” with “-ic acid”
Ex. H2CO3 → 2H+ + CO32-
carbonic acid carbonate

51. There may be an additional change back toward the element name:
Anions ending in –ate (cont.)
There may be an additional change back toward the element name:
Ex. H2SO4 → 2H+ + SO42-
sulfuric acid sulfate
H3PO4 → 3H+ + PO43-
phosphoric acid phosphate

52. 3. Anions ending in –ite:
Replace “-ite” with “-ous acid”
Ex. HNO2 → H+ + NO2-
nitrous acid nitrite
HClO → H+ + ClO-
hypochlorous acid hypochlorite

53. Prefixes and Suffixes for Oxyanions and Related Acids 751515

54. A base is a compound that produces OH- when dissolved in water.
Bases
A base is a compound that produces OH- when dissolved in water.
Ex NaOH → Na+ + OH-
Bases – naming
Bases containing OH- are named by naming the metal first, then “hydroxide” (the usual rule for an ionic compound).
Ex: NaOH =

55. A base is a compound that produces OH- when dissolved in water.
Bases
A base is a compound that produces OH- when dissolved in water.
Ex NaOH → Na+ + OH-
Bases – naming
Bases containing OH- are named by naming the metal first, then “hydroxide” (the usual rule for an ionic compound).
Ex: NaOH = sodium hydroxide

56. 2. Other bases have individual names.
A common base that does not contain OH is ammonia, NH3
When dissolved in water, ammonia produces OH-:
NH3 + H2O → NH4+ + OH-
Many bases in biology are nitrogen-containing compounds that act in a similar manner. (Ex. – the four bases in DNA)

57. Salts
Definition #1: An ionic compound composed of a cation and the anion from an acid is often referred to as a salt.
Definition #2: An ionic compound in which the cation is anything but H+, and the anion is anything but OH- or O2-.

58. examples:
Table salt, NaCl, contains the anion from hydrochloric acid, HCl.
Calcium sulfate, CaSO4, is a salt containing the anion from sulfuric acid, H2SO4.

59. Salt

60. In an acidic salt, one or more of the hydrogens of the acid remains.
Ex. 1: H2CO3 =
replace both H’s: Na2CO3 =
replace only one: NaHCO3 =
sodium hydrogen carbonate (sodium bicarbonate), an acidic salt

61. In an acidic salt, one or more of the hydrogens of the acid remains.
Ex. 1: H2CO3 = carbonic acid
replace both H’s: Na2CO3 =
sodium carbonate
replace only one: NaHCO3 =
sodium hydrogen carbonate (sodium bicarbonate), an acidic salt

62. Ex. 2: H3PO4 =
replace all H’s: Na3PO4 =
replace only one: NaH2PO4 =
sodium dihydrogen phosphate
replace two: Na2HPO4 =
disodium hydrogen phosphate

63. Ex. 2: H3PO4 = phosphoric acid
replace all H’s: Na3PO4 =
sodium phosphate
replace only one: NaH2PO4 =
sodium dihydrogen phosphate
replace two: Na2HPO4 =
disodium hydrogen phosphate

64. SECTION 2. OXIDATION NUMBERS
An oxidation number is a number that represents the charge on an atom in a compound or polyatomic ion.
For a pure element, the oxidation number is zero.
Examples: all atoms in sodium, Na, oxygen, O2, phosphorus, P4, and sulfur, S8, have oxidation numbers of zero

65. For a monatomic ion, the oxidation number is the same as the charge.
Ex. – K2S: the K is present as K+ and has an oxidation number of +1. The S is present as S2- and has an oxidation number of -2.

66. For an atom in a molecular compound or polyatomic ion, the oxidation number is not the actual charge. Instead, it is a “bookkeeping” device, in which electrons in a covalent bond are assigned to the atom having greater attraction for electrons
( = electronegativity).

67. Rules for assigning oxidation numbers (p. 232):
F is most electronegative and is always -1.
Oxygen is almost always -2.
Hydrogen is -1 with metals but +1 with more electronegative elements.
(continued)

68. The algebraic sum of the oxidation numbers of all atoms in a neutral compound is equal to zero.
The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.

69. In a binary molecular compound, the more electronegative element is written second, and assigned a charge equal to the charge it would have as an anion.
The first element is then given an oxidation number so that the total of the oxidation numbers of all atoms is:
zero (for a compound)
the charge (for a polyatomic ion)

70. Many nonmetals can have more than one oxidation number:

71. Earlier we saw the prefix system for naming molecular compounds
Earlier we saw the prefix system for naming molecular compounds. There is also a Stock system using Roman numerals to indicate oxidation numbers.
The oxidation number of the first element is indicated by a Roman numeral. The second element is given an -ide ending and has no prefix.

72. Example: SO3
Oxygen as an anion is -2, so all the oxygens give 3 x (-2) = -6.
To give an overall total of zero, the S is given an oxidation number of +6.
Name = sulfur (VI) oxide

73. Example with a polyatomic ion: NO2-
Oxygen as an anion is -2.
If we call the oxidation number of the nitrogen x, the total of the oxidation numbers can be written:
x + 2(-2) = -1 (the charge on NO2-)
Solving, x = = +3

74. Rules for Assigning Oxidation Numbers 75155

75. Sometimes the –ous and –ic endings are used for molecular compounds:
What are the prefix system names?
ox. # of N
name
Stock system name
N2O NO

76. Sometimes the –ous and –ic endings are used for molecular compounds:
What are the prefix system names?
ox. # of N
name
Stock system name
N2O +1 NO +2

77. Sometimes the –ous and –ic endings are used for molecular compounds:
What are the prefix system names?
ox. # of N
name
Stock system name
N2O +1
nitrous oxide NO +2
nitric oxide

78. Sometimes the –ous and –ic endings are used for molecular compounds:
What are the prefix system names?
ox. # of N
name
Stock system name
N2O +1
nitrous oxide
nitrogen (I) oxide NO +2
nitric oxide
nitrogen (II) oxide

79. Sometimes the –ous and –ic endings are used for molecular compounds:
What are the prefix system names?
dinitrogen monoxide, nitrogen monoxide
ox. # of N
name
Stock system name
N2O +1
nitrous oxide
nitrogen (I) oxide NO +2
nitric oxide
nitrogen (II) oxide