Atoms and Bonding
Interpreting and Writing
Chemical
Formulas
Chemical Formulas represent compounds.
Interpreting Chemical
Formulas
• Coefficient
• Number that is placed in front of a
chemical formula or a symbol in a
chemical equation that indicates the
number of moles or molecules of this
substance involved in the reaction
• Subscript
• Number placed to the lower right of a
chemical symbol to indicate the number
of atoms of the element in the
compound.
Interpreting Chemical
Formulas
• 2Na2O
• 2 = coefficient= 2 moles of this
element
• Number of atoms for each
element coefficient * subscript
• If you do not see a subscript, it is
understood to be 1
• Na = 2*2 = 4
• O = 2*1 = 2
• Total # of atoms = 4+2 = 6
Interpreting Chemical
Formulas
• 3Ba3(PO4)2
•
•
•
•
Ba = 3*3 = 9
P = 3* (2*1) = 3*2 = 6
O = 3* (4*2) = 3*8 = 24
Total # of atoms = 9+6+24 = 39
• When you have parenthesis, you
must multiply the subscript on the
outside of the parenthesis by every
subscript on the inside of the
parenthesis before you multiply by
the coefficient
Writing Chemical
Formulas
• You need to know the chemical
symbol for the element
• You need to know the element’s
oxidation number
• Oxidation number is the number of
valence electrons an element gains,
loses, or shares during bonding
• For metals, the oxidation number is positive
• For nonmetals, the oxidation number is
negative
• Column 14 is considered an exception; its
oxidation number is +/-4 depending on what it
is bonding with
Writing Chemical
Formulas
• Metals
• Oxidation number = # of valence
electrons it gives away when bonding (in
short = number of valence electrons
• Nonmetals
• Oxidation number = # of valence
electrons – 8
• Family 14
• Oxidation number is +/-4
• If 1st element in compound will be +4; if
second element in compound will be -4
• Family 18
• Oxidation number is 0
Understanding Chemical
Formulas
• Chemical
formulas are
composed of a
positive half and
a negative half.
• Water is a
compound you
know to have a
formula of H2O.
Understanding Chemical
Formulas
• The element with the positive
oxidation number is always
written first.
• The element with the negative
oxidation number is always
written second.
+1
•H
-2
O
Understanding Chemical
Formulas
• The total of the oxidation
numbers in a compound must
equal zero.
Add subscripts after a chemical symbol, when
needed, to make the oxidation numbers total
zero.
H2O
The easiest way to think of writing chemical
formulas is to use the oxidation number (without
the + or -) of one element as the subscript of the
other element.
+2
-1
Writing and Understanding
Chemical Formulas
Cross over the oxidation numbers
without the charges!!!
+2
-1
Writing and Understanding
Chemical Formulas
2
Remember, DO NOT write a subscript of 1.
Examples of Writing
Chemical Formulas
Examples of Writing
Chemical Formulas