C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 1

Chapter 5: Molecules and Compounds

Read Chapter 5

Check for MasteringChemistry due dates.

Pure Substances and Mixtures:

Pure substances have one invariable composition (elements and compounds)

Mixtures have a variable composition. Homogeneous mixtures (solutions) mix

uniformly throughout the mixture (coffee, salt water, air) while Heterogeneous

mixtures are not uniform, they vary in texture and have regions of different

composition (soil, pencil).

Law of Constant Composition: Joseph Proust (1754-1826)

All samples of a pure compound have the same proportions of their elements.

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 2

Chemicals:

The properties of chemicals (pure substances) vary widely and greatly depend on

how they are organized. Table salt is sodium chloride, but its properties are very

different than elemental sodium or elemental chloride. We have learned about

elements such as where to find metals or nonmetals on the periodic table, their

symbols and names, as well as compounds which have a fixed ratio of two or more

different elements. Now we will study a how to write out chemical formulas and

name them (nomenclature).

Chemical Formula:

Chemicals are represented by element symbols and subscripts following those

elements, indicating the number of atoms. By convention, a subscript of 1 is

omitted. Parentheses are used when grouping more than one of the same polyatomic

ions together.

Example 1: How many atoms of each and atoms total are in the following chemicals?

a) H2O b) NH4Cl c) Al2(SO4)3 d) C4H10S e) Cholesterol, C27H45OH

Generally, the cation or more metallic element comes first followed by the anion or

more nonmetallic elements. When dealing with organic formulas the order is

usually, CxHyothers as listed in the table below.

The anion hydroxide is one exception to this general order: OH-1

Example 2: Write the formulas when the following atoms make a compound.

Remember to write the more metallic element first, except for organic

compounds write CxHyothers.

a) 4 chlorine and 1 carbon

b) 3 oxygen, 2 iron (rust)

c) 1 cobalt, 3 bromine

d) 8 carbon, 18 hydrogen (octane)

e) 2 oxygen, 4 hydrogen, 2 carbon

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 3

Chemical Bonds:

Compounds and elements may have covalent bonds (molecules)

Covalent bonds share electrons and are found in molecules made up when two

or more nonmetals combine.

Examples: elemental oxygen, O2; compound sucrose table sugar, C12H22O11

Compounds may have ionic bonds (formula units)

Ionic bonds transfer electrons and are found in compounds made up from

combining cations (metals, positively charged) with anions (nonmetals,

negatively charged).

Examples: table salt, sodium chloride, NaCl; rust, iron(III) oxide, Fe2O3

Chemical Formulas: Structural Formula will show the specific connections between atoms within a

molecule and gives more information than a chemical formula alone.

Example: formula = C2H6O, condensed structural formula = CH3CH2OH

or structural formula =

Empirical Formula gives the relative number of atoms of each element in a

compound.

Molecular Formula gives the actual number of atoms of each element in a

molecule of the compound.

Molecular N2O4 Empirical NO2

Comparison microscopic views for methane, CH4:

Note: the empirical and molecular formulas of CH4 are the same.

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 4

Acids and Bases (Arrhenius’ Definition):

Acids are substances that increase H+1 ions in water. The cation is H+1.

Bases are substances that increase OH-1 ions in water. The anion is often OH-1.

Nomenclature:

Elements: Elements may be atomic or molecular.

(Know your elements by name and symbol)

Most elements are written in an atomic form

and given the element name: Cu-copper,

He-Helium, Zn-zinc

Seven elements are diatomic and are called by

their element name:

H2-hydrogen, N2, O2, F2, Cl2, Br2, I2

A couple elements are polyatomic:

P4 is phosphorus, sulfur can be S, S2, or S8 (most common pure form)

Some elements form more than one molecule/structure (allotropes)

O2 is oxygen, O3 is ozone; C(graphite), C(diamond), C60 (buckminsterfullerene)

Common names

Some molecules have been around so long they go by a common name:

H2O-water, NH3-ammonia, CH4-methane, SiH4-silane

Binary molecules (Made up from nonmetallic elements)

CO2, P2O4, CCl4

Mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca

Prefix (not mono) element name prefix root of element plus ide

Carbon dioxide

Drop the a before an o, pentoxide, not pentaoxide.

Organic molecules (carbon based)

Alkanes (CnH2n+2)

Methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane

C5H12, hexane C6H14, heptane C7H16, octane C8H18, nonane C9H22,

decane C10H22

Organic molecules have functional groups that help identify properties.

Combinations of carbon based organic molecules are enormous.

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 5

Ionic compounds (write cation name then anion name)

In the chemical formula, the sum of positive charges must

equal the sum of negative charges. If possible, reduce the

subscripts to the smallest whole number ratios.

Cations with known oxidation state of metal (Type I)

Group 1A (+1), 2A (+2), Al and Ga (+3), Zn and Cd (+2), Ag (+1)

Name of ion is identical to the name of the atom for cations

Variable oxidation state of metal (Type II)

Transition metals and metals below the nonmetal on the right have a

variable oxidation state that must be indicated by Roman Numerals in

parenthesis (this method is what I expect you to learn.

Fe+3, iron (III); Fe+2, iron (II); Cu+1, copper (I); Sn+4, tin (IV)

An alternative method differentiates from the higher oxidation number

and lower oxidation number using the old form of the name and ic or

ous as an ending respectively. (you should be aware of this method, but

it will not be on a test)

Fe+3, ferric Fe+2, ferrous; Cu+2, cupric; Cu+1, cuprous; Sn+4, stannic;

Sn+2, stannous.

Elemental Anions

Group VA (-3); VIA (-2), VIIA (-1)

Name of the element root followed by ide.

N-3, nitride; S-2, sulfide, Br-1, bromide

Polyatomic cations and anions

Memorize the polyatomic ions. Be able to evaluate ion names,

formulas, and charges following some basic rules.

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 6

Hydrates

Ionic compounds can be chemically attached to a small number of water

molecules in a solid form. CuSO4.5H2O, copper (II) sulfate pentahydrate

Acids

Binary acids-H+ cation and an anion that ends with ide.

Hydro root of element ic acid. H2S, hydrosulfuric acid

Ternary oxyacids-H+1 cation with and anion ending in ate or ite

ate changes to ic acid, do not use hydro

H2SO4 becomes sulfuric acid

ite changes to ous acid, do not use hydro

H2SO3 becomes sulfurous acid

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 7

Formula Mass:

Formula mass is the sum of the weighted average atomic masses of all the atoms in

the chemical formula. Other names that apply include… molar mass, formula

weight, atomic mass/weight, molecular mass, molecular weight. This is the mass in

grams for one mole (6.022 x 1023particles).

Nomenclature Practice:

Fill in the table with formulas and names of the ionic compounds.

Cl* CO3

-2 PO4-3 OH-1

Na+1

Zn*

Sn+4

NH4+1

Al*

H+1

*predict the known oxidation number

C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e | 8

Nomenclature Practice:

Names Formulas

Chlorine

Aluminum sulfide

Sodium carbonate

Carbon dioxide

Nickel (II) chloride

methane

Ammonium phosphate

Hydrosulfuric acid

Sulfuric acid

Sulfurous acid

Cobalt (II) chloride hexahydrate

Nomenclature Practice:

H3PO4

H2SO4 (aq)

H2O

HBr (g)

HBr (aq)

MgSO4·7H2O

CCl4

KMnO4

CuSO4

Al(HCO3)3

Formula Mass practice: solve for the formula masses for the species above.