Chapter 7: chemical formulas and compounds

Oxidation numbers

the charge that atom would have if the compound was composed of ions

Rule 1

The atoms in a pure element have an oxidation number of zero

Example:

Sodium by itself (just Na) = oxidation number of 0

Potassium by itself (just K) = oxidation number of 0

Rule 2

The more electronegative element in a binary molecular compound is assigned the number equal

to the negative charge it would have as an anion. The less electronegative atom is assigned the

number equal to the positive charge it would have as a cation

Example:

NaCl - Chlorine is more electronegative so it would have the negative charge. Sodium in less

electronegative so it would have the positive charge

Rule 3

Fluorine has an oxidation number of -1 in all of its compounds because it is the most electronegative

element

Rule 4

Oxygen has an oxidation number of -2 in almost all compounds.

Exceptions include

● when it is in peroxides such as H2O2, its oxidation number is -1

● when it is in compounds with fluorine, such as OF2, its oxidation number is +2

Rule 5

Hydrogen has an oxidation number of +1 in all compounds containing elements that are more

electronegative than it; it has an oxidation number of -1 in compounds with metals

Rule 6

The algebraic sum of the oxidation numbers of all atoms in a neutral compound is zero

Rule 7

The algebraic sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion

Rule 8

Although rules 1 through 7 apply to covalently bonded atoms, oxidation numbers can also be

assigned to atoms in ionic compounds

Rule 9

A monatomic ion has an oxidation number equal to the charge of the ion

Example:

Ions Na+, Ca2+, and Cl- have oxidation numbers of +1, +2, and -1

Chemical formula

indicates the relative number of atoms of each kind in a chemical compound

Monoatomic ions

Ions formed from a single atom

Monoatomic Cations

identified simply by the element’s name

● Na+ would just be Sodium cation

Monoatomic anions

drop the ending of the name and add -ide

● F- would be Fluoride anion

Binary compounds

compounds composed of two elements

● The total number of positive charges and negative charges must be equal

Stock system

a method used for naming ionic compounds where an element can form two or

more cations with different charges

● This applies to transition metals

○ D Block elements

● This system uses roman numerals after the first element in the compound

Covalent compounds

two nonmetals

● we use prefixes

○ Examples: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-

● CO2 - carbon dioxide

Polyatomic ions

an ion made of two or more atoms (with a positive or negative charge)

Oxyanions

polyatomic ions that contain oxygen

The name of the oxyanion depends on the number of oxygen atoms in the oxyanion

The name of the ion with the greater number of oxygen atoms ends in -ate and the ion with the

smaller number of oxygen atoms end in -ite

Hypo

is used when the oxyanion has one fewer oxygen atoms than the -ite anion

Per

is used when the oxyanion has one more oxygen atoms than the -ate anion

Ionic compunds w/ polyatomic ion

These compounds are named the same way binary ionic compounds are named.

Acids

A molecule or ion that donates protons in water and forms salts

Binary acids

Acids that consist of two elements, usually hydrogen and one of the halogens

(fluorine, chlorine, bromine, iodine).

oxyacids

Acids that contain hydrogen, oxygen, and a third element (usually a nonmetal).