Unit 3:
Molecules and Compounds
 Chemical

Formula
How to determine the charge of an ion:

If an atom gains electrons it will become
negatively charged (an anion) because it is
gaining negative charges. If an atom loses
electrons it is losing negative charges so it
will be positive (a cation).

For each of the atoms listed below, determine how many
valence electrons they have, how many valence electrons they
would need to gain or lose to fill their valence shell, and what
the charge would be on the ion with a full valence shell.
Element
Sulfur
Hydrogen
Sodium
Oxygen
# of Valence Electrons
# of e- to gain or lose
Charge


How to Determine Chemical Formula for Ionic
Compounds:
In order to determine the chemical formula
for a compound we need to look at the
charges that the atoms would have with a full
valence shell. For the transition metals we
will always be given its charge.


# of Atoms:
First we need to be able to read how many atoms
are in a molecule:
◦ O=
◦ O2=
◦ Na2O=
◦ 3Na2O=
◦ Mg(OH)2=
◦ (NH4)2SO4=


Chemical Formula for Binary Compounds:
To figure out how many of each atom goes
into a molecule we use the criss-cross
method:

Sr and Cl
Look on the common ion sheet to find the
charges:

Drop the Charge (leave the number):

Criss Cross the numbers:

Ones do not need to be written:


Ba and O

Cu and F


Chemical Formula for Ionic Compounds that
Contain Polyatomic Ions
For polyatomic ions, we look on the common
ion sheet to find their charge. We use this
charge in our criss cross method.


Ammonium NH4+1 and Ferrocyanide Fe(CN)6 4Polyatomic ions need to have brackets around them
(if there is more than one)

Magnesium and Carbonate

Cobalt (III) and Sulfate

(See Determining Number of Atoms and Criss
Cross Assign Pg. 37)
 How
to determine the Lewis dot
structure for ionic compounds

Example: Determine the lewis dot structure
for the ionic compound that will form
between calcium and chlorine.



Determine the number of atoms of each element there will be (Criss
Cross).
Draw out the lewis dot structure for each of the atoms. Circle the
electrons that are going to leave and create the cation and draw arrows
to the anion they are going to.
Draw out the lewis dot for each of the ions created and put them in
brackets with the charge in the upper right corner outside of the
brackets.

Example: Aluminum and Oxygen
 (See
Ionic Bonding Lewis
Dot Assign Pg. 38)