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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
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
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.
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.
P2O5
Prefix of 1st element, name of 1st element
Prefix of 2nd element, name of 2nd element,
change ending to ide.
Diphosphorus pentaoxide
CO
Carbon 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
Acids always start with the element
hydrogen.
1. Binary acids are Hydrogen and an anion.
Hydro______________ic acid. (Complete for
the root of the anion.)
2. Polyatomic ions with the ending ate is
changed to __________ic acid.
3. Polyatomic ions with the ending ite are
changed to ______________ous acid.
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.