Writing formulas and naming compounds

What is a chemical formula? It indicates the relative number of

atoms of each kind in an ionic compound.Ex Al2O3 has 2 atoms of Al and 3 atoms of

O.

It indicates the number of atoms of each element contained in a single molecule of a molecular compound.

Ex. C8H18 has 8 atoms of carbon and 18 atoms of H in a molecule of octane.

There are different rules for naming and writing formulas for ionic and covalent compounds.

The compound is ionic if it is composed of a cation and an anion, or a metal and a nonmetal. The elements will be located on opposite sides of the periodic table. They have a high electronegativity difference.

The compound is covalent (or molecular) if both elements are nonmetals.

Binary because composed of only 2 elements.

Ionic because composed of a metal and a nonmetal.

Example aluminum oxide.

1). Write the symbol and charge for the cation.2). Write the symbol and charge for the anion.3). Criss cross the oxidation numbers and drop

the charges. The oxidation number will become the subscript for the other element.

4). Reduce the subscripts if possible. If they are the same, they cancel completely.

5). If the subscript is 1, do not write anything.

Write the formula for calcium iodide.

Write symbols and charges.Ca+2 I-1

Criss-cross oxidation numbers, drop charges.Ca1I2

Cancel and take away ones.Answer: CaI2

Question: Write the formula for Magnesium Oxide.

1) Write the symbol and charge.Mg+2 O-2

2) Criss cross oxidation number and drop charges.Mg2O2

3). Cancel and don’t write 1’s.Answer: MgO

Complete page 4 and the bottom of page 5 of formula writing packet.

If a compound consists of a metal and a nonmetal, we use the name of the first element (always the cation) followed by the second element (always the anion). For the anion only, drop the last syllable and add –ide. Subscripts are not included in the name.

Examples: CaO: calcium oxide LiCl: lithium chloride K2S: potassium sulfide

P. 223 sample problem A P. 223 practice #1 a-e, and #2 a-f Complete packet page 1 and 8.

Some elements, such as iron, form 2 or more cations with different charges. To distinguish the ions formed by such elements, scientists use the Stock system of nomenclature. This system uses a Roman numeral to indicate the ion’s charge. The numeral is enclosed in parentheses and placed immediately after the metal name.

Fe+2 is iron (II) and Fe+3 is iron (III) Names of metals that commonly form only

one cation, do not include a Roman numeral.

CuCl2 Determine the charges of each element,

by writing ions side by side. Backwards criss-cross to determine

charge of cation. Name by writing name of cation , roman

numeral, name of anion with ide instead of last syllable.

Cu Cl-1 Cu+2 Cl-1 Copper(II) chloride

Example: Write the formula of lead(IV) oxide.

Write symbols with charge.Pb+4 and O-2

Criss cross and drop signs. Pb2O4

Reduce and remove 1’s. PbO2

Page 225 #1(a-f) and #2 (a-d) Complete packet page 3 and 5(top)

A covalently bonded group of atoms that has a positive or negative charge and act as a unit is a polyatomic ion.

The prefix poly means “many”.Most simple polyatomic ions are anions.

Ammonium (NH4) is the only polyatomic cation.

Sometimes there are parentheses in the formula, so you can tell its polyatomic.

If it contains a polyatomic ion, it is ionic, so it follows the rules for naming ionic compounds.

If anion is polyatomic, do not change the ending to ide. Keep it the same as is listed on the chart. Name cation then name anion.

If the cation is polyatomic and the anion is binary, name is just like before. The ide ending means the anion is not polyatomic.

Al2(SO4)3

Al+3 and SO4-2

Aluminum sulfate

Fe(OH)2

Fe+2 and OH-1

Iron(II)hydroxide

1. Write the symbols for the monatomic and polyatomic ions in the compound. 

2. Look up the oxidation numbers of the ions involved. If a single atom, use the periodic table. If roman numeral (variable oxidation number) or

polyatomic, use the chart. 3. Criss cross the oxidation numbers, and

drop the charges. 4. Put polyatomic ions in parentheses if

they have a subscript. 5. Reduce (simplify) and erase ones. 6. DO NOT ADD< SUBTRACT, MULTIPLY, OR

DIVIDE. Just criss cross and reduce if necessary!

Write the formula for Lead (II) hydroxide.

1. Write the symbol and oxidation number.Pb+2 OH-1

2. Criss cross charges and drop signs.Pb1 OH2

3. Use parentheses if needed.Pb1(OH)2

4. Reduce and drop 1’s.Pb(OH)2

Write the formula for ammonium sulfate. 1. Write the symbol and oxidation

number. NH4

+1 SO4-2

2. Criss cross charges and drop signs. Use parentheses if needed. (NH4)2 (SO4)1

3. Reduce and drop 1’s. (NH4)2 (SO4)

Molecular/covalent compounds are made of 2 nonmetals.

Generally, the most metallic element is written first. These elements are farther to the left in the periodic table. If both elements are in the same group, the more metallic element is closer to the bottom of the group.

The name of the second element is changed to -ide. Because molecular formulas can combine in

many combinations, prefixes are used to indicate the subscript. Do not criss cross and do not use oxidation numbers.

Don’t use mono on first element.

1- mono 2- di 3- tri 4- tetra 5- penta 6- hexa 7- hepta 8- octa 9- nona 10- deca

P2O5

Prefix of 1st element, name of 1st elementPrefix of 2nd element, name of 2nd element,

change ending to ide. Diphosphorus pentaoxide

COCarbon monoxide (Don’t write mono on first element.)

Write the symbols for the elements in the order that they appear in the name.

The prefixes indicate the number of atoms of each element in the molecule.

The prefixes appear as subscripts in the formulas.

If there is no prefix, there is only one atom. (1st element only)

Remember, do not criss cross. Example tetraphosphorus dichloride

P4Cl2

***1ST PUT I (ionic) OR C (covalent) BESIDE THE QUESTION

FRONT LEFT COLUMN:1—20, 25, 26

FRONT RIGHT COLUMN:1—18, 23, 25, 26

HCl This is binary, H and Cl. Follow rule #1. Hydrochloric acid

H2SO4- This is polyatomic. Hydrogen and sulfate (SO4)-2. Must find polyatomic anion and name. Ending is -ate so change to -ic. Sulfuric acid (notice no hydro, hydro is only

for binary acids).

HNO2- Polyatomic, H and nitrite. -ite becomes –ous. Nitrous acid.

Cation is always H+1. From the name of the acid, you can figure out the anion.

Write symbols and charges for cation and anion.

Criss cross oxidation numbers and drop signs.

Reduce and drop 1’s. Example

Phosphoric acid H+1 and phosphoric so phosphate, so H+1 and (PO4)-3

H3PO4