Formula
Formulas of !
Ionic Compounds
A formula indicates the type(s) of atoms/ions
and the number of atoms/ions that are
bound together in a substance.!
NaCl,
FeF3,
Ca(NO3)3,
H 2,
C6H12O6!
In the formulas above, note that the
subscripted numbers indicate how many of
the preceding atoms/ions are present
Ionic Compounds
Def.: Compounds made up of ions held together
by the attraction between opposite charges.!
Writing Formulas for
Ionic Compounds
1.
Opposite charges (+,-) attract. Like charges (+,+ or -,-) repel.!
When cations (positive ions) and anions
(negative ions) combine to form a neutral
compound, the total number of positives must
equal the total number of negatives.
Use charges to determine the smallest number of
ions needed for neutral formula (the subscripts)!
A. Total Number of + = Total Number of -
(or)!
B. Criss-cross and Reduce if necessary!
2.
Write formula with cation first and anion second and
any necessary subscripts. Charges are not written as
part of the formula, since the compound is neutral.
Examples
Na1+
Cl1-!
(Charge Balance)
Al3+
!
!
!
NH41+
!
!
!
SO32-!
!
!
!
Fe3+
S2-!
Cu2+
PO43-!
!
!
!
F1-!
Sn4+ O2-!
!
!
!
Writing Formulas for
Ionic Compounds
1.
Use charges to determine the smallest number of
ions needed for neutral formula (the subscripts)!
A. Total Number of + = Total Number of -
(or)!
B. Criss-cross and Reduce if necessary!
2.
Write formula with cation first and anion second and
any necessary subscripts. Charges are not written as
part of the formula, since the compound is neutral.
Examples
(Charge Balance Answers)
Na1+
Cl1-!
(1+)=(1-)!
NaCl!
!
1+
NH4
SO32-!
2(1+)=1(2-)!
(NH4)2SO3!
!
Fe3+ F1-!
(3+)=3(1-)!
FeF3!
!
Examples
Na1+
Cl1-!
!
NaCl!
!
NH41+ SO32-!
!
(NH4)2SO3!
!
3+
Fe
F1-!
!
FeF3!
!
Al3+
S2-!
2(3+)=3(2-)!
Al2S3!
!
2+
Cu
PO43-!
3(2+)=2(3-)!
Cu3(PO4)2!
!
Sn4+ O2-!
(4+)=2(2-)!
SnO2
(Criss-Cross)
Al3+
S2-!
!
Al2S3!
!
Cu2+ PO43-!
!
Cu3(PO4)2!
!
4+
Sn
O2-!
!
SnO2
Examples
Na1+
Cl1-!
!
NaCl!
!
NH41+ SO32-!
!
(NH4)2SO3!
!
3+
Fe
F1-!
!
FeF3!
!
(Criss-Cross)
Al3+
S2-!
!
Al2S3!
!
Cu2+ PO43-!
!
Cu3(PO4)2!
!
4+
Sn
O2-!
!
SnO2
Examples
Na1+
Cl1-!
!
NaCl!
!
NH41+ SO32-!
!
(NH4)2SO3!
!
3+
Fe
F1-!
!
FeF3!
!
(Criss-Cross/Reduce)
Al3+
S2-!
!
Al2S3!
!
Cu2+ PO43-!
!
Cu3(PO4)2!
!
4+
Sn
O2-!
Sn2O4!
SnO2