Lesson 3.05
Ionic Bonding and
Writing Formulas
3.05 Ionic Bonding and Writing Formulas
Watch the following video about ions:
Then, draw a line from each word to its correct definition.
An atom or a group of atoms that
form a charge by gaining or losing
one or more electrons.
Monoatomic Ion
An ion that has a positive charge
Two or more atoms bonded
together. Can have an overall
charge of positive or negative.
An ion that has a negative charge
An ion consisting of one element
Polyatomic Ion
Practice Determining Charge
Watch the video about common charges:
Download a periodic table with common charges here:
What is the common charge for:
a. Calcium: _____
b. Phosphorus: ____
c. Hydrogen: ____
d. Sulfur: _____
e. Aluminum: ____
f. Iodine: ______
Let’s Write That Formula
Ionic Compound
Cation &
(+ is always
How many
ions of each
charge of
you need is
unit is zero
its subscript
Ionic Compounds - Vocabulary
• Ionic Bonding – A chemical bond that is due to transfer of electrons
between a cation (metal or H) and an anion (nonmetal).
• Ionic Compounds - A compound composed of positive and negative ions
that are combined in a ratio where the positive and negative charges
cancel out. Overall net charge is zero. The cation is always written first.
• Formula Unit - The formula of an ionic compound that represents the
simplest whole number ratio of ions in the compound.
Writing Formula Units (Formulas) – The Basics
Example: Write the formula when calcium and phosphorus bond.
A. Write each ion
The positive ion is always
written first, then the
negative ion.
B. Determine how many of
each ion we need to make
the compound neutral. That
will be their subscripts.
Subscripts must be the lowest
whole number ratio.
C. Check your work
Make sure that the net charge
of the formula is zero.
Writing Formula Units Practice
1) Aluminum and
3) Bromine and Barium
2) Calcium and Lithium
4) Nitrogen and Fluorine
Writing Formula Units – Criss-Cross & Reduce Method
Write the formula that forms when the following pairs of elements form an ionic
1. Potassium and Nitrogen
3. Magnesium and sulfur
2. Hydrogen and oxygen
4. Strontium and selenium
3.05 Comprehension Check
1. What is the common charge of a magnesium ion?
2. What is the formula that results when chlorine bonds with barium?
3. Identify three atoms that would lose two electrons to empty its
valence energy level.
4. What subscripts are needed to complete the formula unit Al_N_?
5. Which of the following forms a cation?
a. Br
b. Li
C. N
6. Which of the following is a possible formula unit?
a. KN3
b. I3N
c. MgO
Thank You!
• It has been great working with you for Writing Ionic
Compounds! You will use this for the rest of module 3,
module 4 and most of segment 2.
• Please use our other recorded teaching sessions for the
other lessons. Don’t forget that you can use them to
review previous lessons too!

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