Chapter 7

•Significance of A Chemical Formula• Indicates the relative number of atoms of

each kind in a chemical compound• Reveals the number of atoms of each

element contained in a single molecule

• Example: C8H18

↑ ↑ The subscripts indicate the number of

atoms in a molecule of octane

Chemical Names and Formulas

Monatomic ions – ions formed from a single atom.

In naming anions, drop the elements ending and add “ide”.

Ex. Oxygen to Oxide

Monatomic Ions

Binary compounds – compounds composed of two different elements.

Binary Ionic Compounds

Na+ + Cl- NaCl

Ca2+ + F- CaF2

Ca2+ + N3- Ca3N2

Nomenclature – the naming system of binary ionic compounds involves combining the names of the compound’s positive and negative ions.

Naming Binary Ionic Compounds

Al2O3

Name of cation Name of anionaluminum oxide

The Stock System of Nomenclature

CuCl2

Name of cation + Roman numeral indicating charge

Name of anioncopper(II)chloride

Oxyanions – polyatomic ions containing oxygen.

Naming Binary Molecular Compounds

Number Prefix

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-

10 deca-

P4O10

Prefix needed if there is more than one atom

+

Name of less – electronegative element

Prefix indicated by the number of atoms contributed by more-electronegative element

Root name of more-electronegative element + ide

+

tetraphosphorus decoxide

• Each atom is joined to all its neighbors in a covalently bonded, 3-D network.

• The subscripts in a formula indicates the smallest whole-number ratio of the atoms in the compound.

Covalent-Network Compounds

Acid – a molecular substance that when dissolved in water creates a solution with hydrogen ion activity.

Binary acids – acids consisting of two elements.

Oxyacids – acids containing hydrogen, oxygen, and a third element.

Salt – an ionic compound composed of a cation and the anion from an acid.

Acids and Salts

• The oxidation state of any element such as Fe, H2, O2, P4, S8 is zero (0).

• The more electronegative element is assigned an negative oxidation number of it’s charge.

• The less electronegative element is assigned a positive oxidation number of it’s charge.

• The oxidation state of oxygen in its compounds is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1. When in a compound with halogens, such as OF2, the oxidation number for O is +2.

• The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1.

Oxidation Numbers

• The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states zero.

• The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.

HClO3

-2

-6

+1

+1 +5

+5

H = +1, Cl = +5, O = -2

Using Oxidation Numbers for Formulas and Names

Formula Prefix System Stock System

PCl3 phosphorus trichloride

phosphorus (III) chloride

PCl5 phosphorus pentachloride

phosphorus (V) chloride

The Stock System is based on oxidation numbers and can be used as an alternative way to the prefix system.

Using Chemical FormulasFormula Masses

Finding the formula mass for H2O

Step 1: Find the average atomic mass for hydrogen and oxygen

Step 2: Use the conversion table to change atoms to atomic mass units (amu)

Average atomic mass of H: 1.01 amu

Average atomic mass of O: 16.00 amu

2 H atoms

H atoms

1.01 amu= 2.02 amu

1 O atoms

O atoms

16.00 amu= 16.00 amu

Average mass of H2O molecule = 18.02 amu

Step 3: Add the atomic mass units to find the average mass

Molar MassFinding the molar mass for H2O

Step 1: Find the molar mass for hydrogen and oxygen

Step 2: Use the conversion table to change moles to grams.

Molar mass of H: 1.01 g/mol

Molar mass of O: 16.00 g/mol

2 mol H

1 mol

1.01 g= 2.02 g

1 mol O

1 mol

16.00 g= 16.00 g

Average mass of H2O molecule = 18.02 g

Step 3: Add the masses to find the total mass