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I. Formulas
a) Chemical Formula: represents the type and number of element(s) in a
chemical compound.
•
The type of element is represented by its chemical symbol.
•
The number of atoms of the element in the compound is represented by its
subscript.
NOTE:
Li3PO4
Lithium = Li
3 atoms
Phosphorus = P
1 atoms
Oxygen = O
4 atoms
Subscripts are the
numbers found to the
lower right of a given
chemical symbol.
If a subscript is not
present assume that it is
a number one.
Types of Formulas
1. Molecular Formulas
• Represents the type and number of elements in a covalent
compound.
• The subscripts do not need to be reduced in a molecular formula.
Ex:
C2H4
C6H12O6
Ethene
Glucose
2. Empirical Formulas
• Represents the lowest whole number ratio of atoms in a compound.
• For ionic
compounds, the formula is always empirical.
This is due to the fact that one must reduce the subscripts in an
ionic compound to the lowest whole number ratio.
Ex:
Calcium Sulfide
Ca S
Ca+2 S-2
Crisscross Method
Ca2 S2
Disregard the signs.
Ca S
Reduce
• For covalent compounds (molecules), reducing its
molecular formula to an empirical formula only shows the ratio
of elements within the compound.
Ex:
C6H12O6
C1H2O1
Ratio: C:H:O = 1:2:1
Molecular Formula
Empirical Formula
H2O
Molecular and Empirical
Formula
3. Structural Formulas
• Structural formulas indicate the type,
number, and
arrangement of atoms within a compound.
O
H
O=C=O
H
+
[Na]
[Cl]
Glucose
II. Formula Writing
a) Ionic compounds:
Given the name of the compound:
1. Determine if the compound is binary or ternary.
2. If the compound is binary, write out the symbols for the two
elements present: metal then non-metal.
* Elements with lower electronegativity values are usually listed
first in a chemical formula.
3. Find and write out the oxidation number for the metal and non-metal.
4. Perform the crisscross method to determine the subscripts.
* Eliminate the signs from the subscripts and if necessary reduce
the subscripts to the lower whole number.
5. If the compound is ternary, refers to Reference Table E.
6. Write out the symbol for the metal and the polyatomic ion.
7. Place parenthesis around the polyatomic ion.
8. Write in the oxidation number for the metal and the ionic charge of
the polyatomic ion.
9. Perform the crisscross method to determine the subscripts.
* Eliminate the signs from the subscripts and if necessary reduce
the subscripts to the lower whole number.
Naming Ionic Compounds with a Transition Metals
Elements that can have more than one possible charge MUST have a Roman
Numeral to indicate the charge on the individual ion.
Example #1 : Copper (II) Sulfate
CuSO4
Copper’s oxidation number is +2.
Sulfate’s charge is -2.
The Roman Numeral “II” indicates the oxidation number of the transition metal.
Example #2: Iron (III) Oxide
Fe2O3
Iron’s oxidation number is +3.
Oxygen’s oxidation number is -2.
The Roman Numeral
“III” indicates the oxidation number of the transition metal.
Examples of Older Names of Cations formed from Transition Metals
Complete the names of the following binary
compounds with variable metal ions:
FeBr2
iron (_____) bromide
CuCl
copper (_____) chloride
SnO2
___(_____ ) ______________
Fe2O3
________________________
Hg2S
________________________
Complete the formulas of the following ternary
compounds with variable metal ions:
1. copper(II) nitrate
2. Iron (III) hydroxide
3. Tin(IV) hydroxide
b) Covalent Compounds (molecular substances)
Prefix System (binary compounds)
1. Less electronegative non-metal comes first.
2. Add prefixes to indicate # of atoms. Omit mono- prefix on
the FIRST element. Mono- is OPTIONAL on the
SECOND element.
3. Change the ending of the second element to -ide.
Greek Numerical Prefixes
1 mono
2 di
3 tri
4 tetra
5 penta
6 hexa
7 hepta
8 octa
9 nona
10 deca
Given the names of the following molecules, determine
their formulas.
Dinitrogen monoxide
Potassium sulfide
Dichlorine heptoxide
Iodine monochloride
Given the formulas of the following molecules,
determine their names.
BaI2
P4S3
I2O5
CS2
B2Cl4