Chapter 7 cont.
What are polyatomic ions?
Review

How does an atom form a negative ion, a
positive ion?

In a formula, the subscript tells you what kind
of information?

In a formula, a superscript tells you what kind
of information?
Many atoms can form one ion.

A simple ion is called monoatomic, meaning
“one atom”.

The term polyatomic ion means a charged
group or two or more bonded atoms that
functions as a single ion.

Atoms in a polyatomic ion are covalently
bonded , but the ion as a whole forms ionic
bonds with other ions in the same way that
simple ions do.

Most polyatomic ions consist of atoms of
several elements.
Compounds with monoatomic ions.

Calcium bromide
CaBr2

Sodium oxide
Na2O

Barium iodide
BaI2
Compounds with polyatomic ions

Sodium nitrate
Na2SO4

Calcium chlorate
Ca[ClO3]2

Ammonium nitrate
NH4NO3
* Each polyaotmic ion is treated as a single unit and is kept together
within a pair of brackets.
Naming a polyatomic ion

The names of polyatomic ions can be
complicated.

There is no logical way of naming these types
of compounds, but..

There are rules that help determine the
names of polyatomic ions.

Many polyatomic ions contain oxygen.
The endings –ite and –ate show the
presence of oxygen.

Such ions are called oxyanions.

The prefix oxy stands for
the presence of oxygen.

The presence of hydrogen in a polyatomic
anion is often indicated by the ion’s name
starting with hydrogen. The prefix mono- and
di- are also used.

For example:
HPO4 is called monohydrogen phosphate.
and H2PO4 is called dihydrogen phosphate.
 So
how do you name
polyatomic compounds????
First,

In order to understand naming, you must be
able to identify the cations and anions in the
formula.

NaClO3

K2SO3

(NH4)2CrO4
Na+ , ClO3-
Let’s Practice

Write the name of the compound with the
formula K2CO3 ?
Step 1: Name the cation (positive charge)
K2
Potassium

Step 2: Name the compound
Potassium carbonate

If you look at Table 5-7 on page 178, you will
2see that CO3 ion is called the carbonate ion.
Writing the formula

Step 3: Name the anion (negative charge)
[K]2[CO3]3
K+
K+
CO 23
You must balance the charges: total of 2 positive
charges and 2 negative charges.
More practice


Write the formula for the compound
containing a polyatomic ion.
calcium dihydrogen phosphate.

Step 1: Use the table to determine the
formula and charge for the polyatomic anion.
-
dihydrogen phosphate is H2PO4.
Step 2: Determine the formula and charge for
the cation.

If you look at your chart from your notes, you
will be able to find the charge of the anion.

Calcium is

Because calcium has two positive charges,
dihydrogen phosphate needs two negative
charges.
Ca2+

Therefore you write the balanced equation as
follows:
Ca[H2PO4]2
Ca2+
H2PO4-
-
H2PO4
*there must be two dihydrogen phosphates.
Oxidation Numbers

Where do you write the charge of an ion ?

The charges listed for ions are used to show a
transfer of electrons in the forming of an ionic
bond.

Atoms within polyatomic ions are bonded
covalently.

In the next chapter you will learn that in covalent
bonds electrons are shared between atoms.

Unlike transferred electrons in ionic bonds,
shared electrons in covalent bonds are not
considered part of either of the atoms.

For example, the bonds between sulfur
and oxygen atoms in SO42-, contain
shared electrons.

Neither Sulfur or Oxygen is an ion in SO4



Chemists have come up with a way for
polyatomic ions and molecular compounds to
describe the distribution of electrons among
bonded atoms.
This convention is called oxidation number.
Oxidation number- a number assigned to
an atom in a polyatomic ion or molecular
compound based on the assumption of
complete transfer of electrons.
Some Guidelines for Assigning Oxidation
Numbers

Figure 5-13 shows a list of guidelines that
can be used in assigning oxidation numbers.
How to use the guidelines

Assign oxidation numbers to all of the atoms
in the compound:
KOH
1st- determine the cation and the anion
2nd- determine whether they are monoatomic or
polyatomic.
3rd- Read the rules to assign oxidation
numbers.
How To:

Assign Oxidation numbers to all atoms in
S2O72-this is a single polyatomic anion, there is no
cation bonded to it. So different rules apply
to polyatomic ions.
Step 1 - If you know an elements
oxidation number, start by placing
those oxidation numbers above the
elements (shown in red).
-
From rule 6, you know that oxygen has an
oxidation number of -2.
-2
S2O72-
Step 2- If more than one atom of an
element is present, calculate the total
charge by those atoms by multiplying the
individual atom by it’s subscript in the
formula.

For oxygen the total charge is calculated as
follows
O: (-2) 7 = -14
-2
S2O72-14
Step 3- Determine the charge needed to
reach a net -2 for the ion.
2S + O7 = -2
2S + ( -14) = -2
2S = +12
Step 4- Divide the charge by the number
of atoms to determine the oxidation
number for each atom.
S = 12/2
S = +6
In S2O72-, the oxidation number assigned to
S is +6 and O is -2.