Transcript Ionic Bonding - Midland Park School District

Chapter 7
Sec. 7.3: Names and Formulas for
Ionic Compounds
Objectives

Relate a formula unit of an ionic compound
to its composition.
 Write formulas for ionic compounds and
oxyanions.
 Apply naming conventions to ionic
compounds and oxyanions.
Anatomy of a Chemical
Formula

Chemical formulas express which elements
have bonded to form a compound. The
subscripts express the elements' numerical
relationships to one another.
Formulas for Ionic Compounds

Since ionic compounds do not exist as single
units, the formula of an ionic compounds
represents the simplest ratio of ions. It is called
a FORMULA UNIT.
For example: MgCl2 is a formula unit of magnesium
chloride.
 Remember that a formula unit of an ionic
compound has no charge!

 The symbol for the cation is always written
first, followed by the symbol of the anion.
Formulas for Ionic Compounds

Monatomic ions are one-atom ions.
 Polyatomic ions are a group of covalentlybonded atoms that together have a net charge.


Since polyatomic ions exist as a unit, the
subscripts of the atoms within the ion is NEVER
changed. When more than one polyatomic ion is
needed, parentheses MUST be placed around the
ion and the appropriate subscript is written
outside the parentheses.
Oxidation number, or oxidation state, is the
charge of a monatomic or polyatomic ion.
Writing Formulas for Binary
Ionic Compounds

Binary: made up of 2 elements
A monatomic metal ion and a monatomic
nonmetal ion
 The numbers of each ion (their ratios) in
the formula are determined by the charges Remember: the # of electrons lost must
equal the # gained

Binary Ionic Compounds Can
Be Type I or Type II
Determining the Type of Metal

A representative (A group) metal is
called a Type I metal. Its charge (or
oxidation number) can be found from the
location of the metal in the periodic table.
Binary Type I
Ionic Compounds

Example:
aluminum oxide
Shortcut to Writing Formulas
 Criss-cross method
– 1. Determine the ions (and their charges)
that are present in the compound.
– 2. Take the NUMBER ONLY of the charge
for the cation and write it as a subscript
after the anion.
– 3. Take the NUMBER ONLY of the charge
for the anion and write it as a subscript
after the cation.
Shortcut Example
Calcium Nitride
1.Calcium is Ca+2 and nitride is nitrogen: N-3
2. 2 of calcium becomes the subscript on the
N.
3. 3 of nitrogen becomes the subscript on the
Ca.
Ca3N2
Exceptions
Ca2+ and O2- would form Ca2O2 if you used
the criss-cross method
 However, in reality, Ca2+ and O2- form CaO.
 For ionic compounds, you must simplify
(reduce) the subscripts when necessary.

Practice Problems
Determine the formula for the following
binary ionic compounds:
1. potassium oxide
2. aluminum sulfide
3. Iron nitride (use Fe2+)
4. Copper phosphide (use Cu+)
5. aluminum bromide
Determining the Type of Metal

A transition (B group) metal is called a
Type II metal. Its charge (or oxidation
number) can vary. A roman numeral written
in parentheses after the name of the metal
will tell you what the charge of the type II
metal is.
 If there is no roman numeral, it means the ion
has only one charge (so you must look it up).
Binary Type II Ionic
Compounds

Examples
• iron (II) oxide: Fe2+ + O2- --> FeO
• iron (III) oxide: Fe3+ + O2• zinc oxide: Zn2+
Fe2O3
+ O2-  ZnO
Writing Formulas for Compounds
with Polyatomic Ions

Ionic Compounds with Polyatomic ions
(ex. Na2SO4)
Contains 1 or more polyatomic ion (an ion
made up of more than 1 atom, ex. SO4-2)
 The charge of the polyatomic ion applies to
the entire ion (all the atoms as a group)
 A polyatomic ion acts as an individual ion

Writing Formulas for Compounds
with Polyatomic Ions

Example: magnesium phosphate
– The ions are Mg2+ and PO43– Remember: If more than one polyatomic ion is
required in a formula, it MUST BE set off by
parentheses
– The subscript will be written to the right of the
parentheses to tell how many units of the
polyatomic ions are present.
Magnesium phosphate
Mg+2
PO43-
3 becomes the
2 becomes the
subscript of Mg
subscript of PO4
Mg3(PO4)2
NEVER
EVER
change the subscripts of the
atoms witihin a polyatomic ion.
The formulas for the
polyatomic ions are written in
stone!
Magnesium chlorate

Mg2+ ClO3- (- is understood to be -1)
Mg(ClO3)2
NOT
– Mg2ClO3
– MgClO6
– MgCl2O6
Practice Problems
Determine the formula for the following
compounds with polyatomic ions:
1. Calcium phosphate
2. Sodium nitrate
3. Aluminum carbonate
4. Magnesium sulfate
5. Ammonium chromate
Rules for Naming Binary Ionic
Compounds

Binary type I ionic compounds: name the
cation first and the anion second.
–
–
Monatomic cations use the element name.
Monatomic anions take their name from the
root of the element name plus the suffix –ide.
Li3P
lithium phosphide
Rules for Naming Binary Ionic
Compounds

Binary type II ionic compounds: The
charge of the transition metal ion must
be determined.
–
–
You must work back from the formula to
determine the charge of the transition metal
ion.
Once it is determined, use parentheses after
the metal ion name to indicate this charge as a
Roman numeral.
The subscript on
The subscript on the Co
the S indicates
indicates the charge of S
the charge of Co
is 2-
Co2S3
3+
2Co
S
cobalt (III) sulfide
is 3+
Sometimes you have to do a little bit more
thinking!
Subscript of 2
on O indicates
that the charge
of Sn is +2.
SnO2
No subscript on Sn
indicates that the
charge of the oxygen
anions is -1.
You have to stop, however, and say to yourself that
that isn’t right. You KNOW each O has a charge
of -2!! The charge of Sn has to balance that and,
so, MUST BE +4. The compound is tin (IV)
oxide.
Rules for Naming Compounds
with Polyatomic Ions

You must first determine the name of the
polyatomic ion.
 Once that is known, use it to name the
compound.
 Example: NaHCO3
– the polyatomic ion, HCO3-, has 2 possible
names
– the compound is named sodium hydrogen
carbonate or sodium bicarbonate
Practice Problems
Name the following compounds.
1. NaBr
2. KOH
3. CuCl2
4. FeCrO4
Naming Polyatomic Ions

Most polyatomic ions are oxyanions. An
oxyanion is a polyatomic ion composed of
an element, usually a nonmetal, bonded to
one or more oxygen atoms.
 More than one oxyanion exists for some
nonmetals such as nitrogen and sulfur.
Naming Oxyanions
of Sulfur and Nitrogen

The ion with more oxygen atoms is named
using the root of the nonmetal plus the
suffix –ate. Ex. SO42- is sulfate.
 The ion with fewer oxygen atoms is named
using the root of the nonmetal plus the
suffix –ite. Ex. SO32- is sulfite.
Naming Oxyanions of Chlorine
1. The oxyanion with the greatest number of oxygens is
named using the prefix per-, the root of the nonmetal,
and the suffix -ate
Ex. ClO4- is perchlorate
2. The oxyanion with 1 less O is named with the root of the
nonmetal and the suffix -ate
Ex. ClO3- is chlorate.
3. The oxyanion with 2 less O’s is named with the root of
the nonmetal and the suffix -ite Ex. ClO2- is chlorite.
4. The oxyanion with 3 less O’s is named using the prefix
hypo-, the root of the nonmetal and the suffix -ite
Ex. ClO- is hypochlorite.