Chemical Formulas and Chemical Compounds

Chapter 7

Chemical Names and Formulas Section 7-1

Definitions

Chemical formula - Indicates the relative

number of atoms of each kind in a chemical

compound

Molecular compound -The chemical

formula reveals the number of atoms of each

element contained in a single molecule of the

compound

Ionic compound - Consists of a lattice of

positive and negative ions held together by

mutual attraction

• The chemical formula for an ionic compound represents

one formula unit, the simplest ratio of the compounds

positive ions (cations) and its negative ions (anions)

Monatomic Ions By gaining or losing electrons many main-

group elements form ions with noble-gas

configurations

Ions are formed from a single atom

Rather than gain or lose electrons, atoms of

carbon and silicon form covalent bonds in

which they share electrons with other atoms

Elements from the d-block form +2, +3 ions,

and in a few cases +1, +4 ions

Monatomic cations Identified simply by the element’s name

K+ Potassium cation

Mg2+ Magnesium cation

Monatomic anions

The ending of the element’s name is dropped

Then the ending –ide is added to the root name

F Fluorine

F-1 Fluoride anion

N Nitrogen

N-1 Nitride anion

Binary Ionic Compounds

The total numbers of positive and negative charges must be equal.

Compounds are composed of two elements

The positive cation is always named/written first. For simple ionic compounds, use the same naming scheme as for monatomic cations and anions

• Write the symbols for the ions side by side.

• Write the cation first

• Cross over the charges by using the absolute value

of each ion’s charge as the subscript for the other ion

• Check the subscripts and divide them by their largest

common factor to give the smallest possible whole-

number ratio of ions

• Write the formula

Steps for determining a Binary

Ionic Compound

Nomenclature – naming system When an element forms two or more cations with

different charges, use a Roman numeral to indicate an ion’s charge.

Fe+2 iron(II) Fe+3 iron (III)

Note: There is no element that commonly forms more than one monatomic anion

The Stock System of Nomenclature

Naming According to the Stock

System Write the name of the cation

Indicate the charge with Roman numeral in

parenthesis

Write the name of the anion

Oxyanions

Oxyanions are polyatomic ions that contain

oxygen

Some can combine to form more than one type

Naming Oxyanions

The name of the ion with the greater number of

oxygen atom ends in – ate

The name of the ion with one less oxygen atom

ends in –ite

The anion that has one less oxygen atom than

the –ite anion has the prefix hypo-

The anion that has one more oxygen atom than

the –ate anion has the prefix per-

Examples of Oxyanions

ClO- hypochlorite ClO2

- chlorite ClO3

- chlorate ClO4

- perchlorate

Naming Binary Molecular Compounds Use prefixes to represent how many atoms needed. # of atoms needed Prefix of Atom

1

2

3

4

5

6

7

8

9

10

mono-

di-

tri-

tetra-

penta-

hexa-

hepta-

octa-

nona-

deca-

Rules for prefix system of

nomenclature of binary molecular

compounds The element that has the smaller group number

is usually written first.

If both elements are in the same group, the element whose period number is greater is written first.

The element is given a prefix only if it has more than one atom in the molecule.

The second element is named by combining a prefix indicating the number of atoms contributed by the element

Rules for prefix system of

nomenclature of binary molecular

compounds Give the root of the name of the element and end

in –ide (see note below)

The ‘o’ or ‘a’ at the end of a prefix is usually

dropped when the word following the prefix begins

with another vowel.

For example,

monoxide instead of mono-oxide

pentoxide instead of penta-oxide

The –ide indicates that the compound contains 2

elements or is binary

Naming Acids

Binary Acids - Acids that only have 2 elements,

hydrogen and one other.

They are always prefixed with hydro- plus the

anion ending in -ic.

Hydrochloric, hydrofluoric etc.

The hydro prefix is a dead giveaway that it is a

binary acid.

KH is not an acid.

Ternary Acids

Ternary acids are acids with three elements, usually

H, O and another nonmetal.

Due to the presence of the oxygen, they are called

oxyacids and the oxygen ion that bonds to the

hydrogen is an oxyanion

Acids are water solutions containing the acid

compound.

Naming Ternary Acids

The oxyanion that ends in -ate changes the ending to –ic acid

The oxyanion the ends in –ite changes the ending to –ous acid.

Acid Formula Acid Name

H2SO4

H2CO3

HNO2

HClO3

HClO

Sulfuric acid

Carbonic acid

Nitrous acid

Chloric acid

Hypochlorous acid

Instructions for:

Writing formulas for Ionic Compounds with

Polyatomic ions

Same as Writing formulas for Ionic Compounds

but use table 2 on p 226 for the polyatomic

ions!

Oxidation Numbers Section 2

Assigning Oxidation Numbers

1. Atoms of pure elements have oxidation number zero.

2. In a binary compound, the more electronegative element is assigned the negative charge it would have as an anion.

3. Fluorine is always -1 since it is the most electronegative number.

4. Oxygen is usually -2 except for OF2 where it is +2 and H2O2 where it is -1.

5. Hydrogen is +1 in all compounds that have a higher electro negativity.

Hydrogen is -1 when with a metal.

6. The sum of all oxidation numbers in a compound is zero.

7. The sum of all oxidation numbers in polyatomic ions is the charge of the ion.

8. Rule 7 applies to covalent and ionic compounds.

Examples Oxidation Numbers

H2SO4

UF6

ClO3-1

HNO3

P4O10

N2O5

KH

S = +6

U = +6

Cl = +7

N = +5

P = +5

N = +5

H = -1

Section 7.3

Using Chemical Formulas

Definitions Formula mass - the sum of the average

atomic masses of all atoms represented in its

formula of any molecule, formula unit, or ion

Molar mass - the mass in grams of 1 mol of

a substance

(The molar mass is numerically equal to its

formula mass only in grams instead of an

amu.)

Percentage composition - The percentage

by mass of each element in a compound

Calculate Percent Composition

Percent composition =

Percent composition of Cu2S.

2 mol Cu x 63.55 g Cu = 127.10 g Cu

1 mol Cu

1 mol S x 32.07 g S = 32.07 g S

1 mol S

Percent Composition of Cu2S

Molar Mass of Cu2S

Cu 2 x 63.55 = 127.10 g

S 1 x 32.07 = 32.07 g

159.17 g

% Cu = 127.10 g Cu x 100 = 79.852 %

159.17 g Cu2S

% S = 32.07 g S x 100 = 20.15 % S

159.17 g Cu2S

Converting between Moles, Grams, and Particles

Mass of element

No. of Moles

No. of atoms

÷ molar mass X 6.02 x 1023

X molar mass ÷ 6.02 x1023

Mass of

Com-pound

No. of moles

No. of mole-cules

÷ molar mass

X molar mass ÷ 6.02 x 1023

X 6.02 x 1023

Section 7.4

Determining Chemical

Formulas

Empirical formula

Consists of the symbols for the elements combined

in a compound with subscripts showing the

smallest whole-number mole ratio of the different

atoms in the compound

Ionic compound, the formula unit is usually the

compound’s empirical formula.

Molecular compound the empirical formula does

not necessarily indicate the actual numbers of

atoms present in each molecule