Lesson 5 Ionic Compounds - Naming and Formulas: Formulas of Binary Ionic Compounds | MoodleHUB 🎓

Did you know that sapphires, emeralds, and rubies are all crystal lattice structures of the same binary ionic compound?

B5.3 Three rings: one with a sapphire, one with an emerald, and one with a ruby

Sapphires, emeralds, and rubies are composed of aluminium oxide. The characteristic colours come from trace amounts of transition metals imbedded in the structures.

The formula of aluminum oxide is Al₂O₃, which seems to imply that the structure is a simple molecule of two Al atoms and three O atoms. How can large gem stones be created from this? In reality, the formula shows the ratio of cations to anions that exist in the giant crystal lattice structure.

Subscripts indicate the ratio of cations to anions needed to create a neutral compound.

The guidelines for writing formulas for binary ionic compounds are as follows:

Identify the type of compound—a metal element with a non-metal element is ionic.
Write the ion symbols.
Determine the number of the ions of each element that will result in a neutral compound. Do this by determining the lowest common multiple.
Determine the factor (number) that each ion needs to be multiplied by to create the lowest common multiple.
Write these as subscripts. Do not write the subscript 1.

Examples

Each example has a video to go with it. To play the video, click on the play icon next to the example.

magnesium chloride

Identify the type of compound.

Check your work.

magnesium—metal
chloride—non-metal
This is an ionic compound.

Write the ion symbols.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Mg«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«msup»«mi»Cl«/mi»«mo»§#8211;«/mo»«/msup»«/math»

Determine the lowest common multiple of the two charges.

Check your work.

The lowest common multiple between 2 and 1 is 2.

Determine the factor to multiple each ion by.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mtable columnspacing=¨0px¨ columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi»Mg«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»1«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»+«/mo»«/mtd»«/mtr»«mtr»«mtd»«msup»«mi»Cl«/mi»«mo»§#8211;«/mo»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#8211;«/mo»«/mtd»«/mtr»«/mtable»«/math»
This will create a neutral compound.
The ratio of «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Mg«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«/math» to Cl^– is 1:2.

Write the formula with subscripts.

Check your work.

MgCl₂

Short cut: There is a short cut that is known as swap and drop (or criss-cross the charges) to help you determine the subscripts. If you use this method, you need to remember to reduce the subscripts to lowest terms.

1. Write the formulas of the ions.	«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Mg«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«msup»«mi»Cl«/mi»«mo»§#8211;«/mo»«/msup»«/math»
2. Swap and drop the charges (just the numbers, not the signs).
3. Reduce subscripts to lowest terms.	(cannot be reduced)
4. Write the formula.	MgCl₂

Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/aj2i67vem6

Identify the type of compound.

Check your work.

zinc—metal
nitride—non-metal
This is an ionic compound.

Write the ion symbols.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Zn«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«msup»«mi mathvariant=¨normal¨»N«/mi»«msup»«mn»3«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«/math»

Determine the lowest common multiple of the two charges.

Check your work.

The lowest common multiple between 2 and 3 is 6.

Determine the factor to multiple each ion by.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mtable columnspacing=¨0px¨ columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi»Zn«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»3«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»6«/mn»«mo»+«/mo»«/mtd»«/mtr»«mtr»«mtd»«msup»«mi mathvariant=¨normal¨»N«/mi»«msup»«mn»3«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»6«/mn»«mo»§#8211;«/mo»«/mtd»«/mtr»«/mtable»«/math»
This will create a neutral compound.
The ratio of «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Zn«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mi»to«/mi»«mo»§#160;«/mo»«msup»«mi mathvariant=¨normal¨»N«/mi»«msup»«mn»3«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«/math» is 3:2.

Write the formula with subscripts.

Check your work.

Zn₃N₂

Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/oy12333a3k

Identify the type of compound.

Check your work.

calcium—metal
oxide—non-metal
This is an ionic compound.

Write the ion symbols.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Ca«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«msup»«mi mathvariant=¨normal¨»O«/mi»«msup»«mn»2«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«/math»

This can be reduced to CaO.

Determine the lowest common multiple of the two charges.

Check your work.

The lowest common multiple between 2 and 2 is 2.

Determine the factor to multiple each ion by.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mtable columnspacing=¨0px¨ columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi»Ca«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»1«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»+«/mo»«/mtd»«/mtr»«mtr»«mtd»«msup»«mi mathvariant=¨normal¨»O«/mi»«msup»«mn»2«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»1«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#8211;«/mo»«/mtd»«/mtr»«/mtable»«/math»
This will create a neutral compound.
The ratio of «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Ca«/mi»«msup»«mn»2«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mi»to«/mi»«mo»§#160;«/mo»«msup»«mi mathvariant=¨normal¨»O«/mi»«msup»«mn»2«/mn»«mo»§#8211;«/mo»«/msup»«/msup»«/math» is 1:1.

Write the formula with subscripts.

Check your work.

CaO

Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/kfqs0m6914

aluminium fluoride

Identify the type of compound.

Check your work.

aluminium—metal
fluoride—non-metal
This is an ionic compound.

Write the ion symbols.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Al«/mi»«msup»«mn»3«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«msup»«mi mathvariant=¨normal¨»F«/mi»«mo»§#8211;«/mo»«/msup»«/math»

Determine the lowest common multiple of the two charges.

Check your work.

The lowest common multiple between 3 and 1 is 3.

Determine the factor to multiple each ion by.

Check your work.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mtable columnspacing=¨0px¨ columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi»Al«/mi»«msup»«mn»3«/mn»«mo»+«/mo»«/msup»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»1«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»3«/mn»«mo»+«/mo»«/mtd»«/mtr»«mtr»«mtd»«msup»«mi mathvariant=¨normal¨»F«/mi»«mo»§#8211;«/mo»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»3«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»3«/mn»«mo»§#8211;«/mo»«/mtd»«/mtr»«/mtable»«/math»
This will create a neutral compound.
The ratio of «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msup»«mi»Al«/mi»«msup»«mn»3«/mn»«mo»+«/mo»«/msup»«/msup»«/math» to F^– is 1:3.

Write the formula with subscripts.

Check your work.

AlF₃

Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/sn7r92mttf

Creating Compounds Manipulative

To help you visualize how ions combine in certain ratios to make ionic compounds, work through the following examples. You can use this manipulative to help you write formulas for ionic compounds in the practice question section and on your assignment.

For cations, each extension represents an electron that needs to be lost. For anions, each divot represents an electron that needs to be gained. The goal is to combine the correct ratio of cations to anions to create neutral compounds.

Click on the procedure 1 tab to continue.

Click on the play icon to make a copy of the creating compound manipulative. Print students can access the document in the Online Resources for Print Students section of their online course.
Predict the formula for magnesium chloride by dragging a magnesium ion to the work surface.

How many bonds will magnesium form?

Check your work.

2
Drag a chloride ion onto the work surface.

How many bonds will chloride form?

Check your work.

1
Drag enough chloride ions to create a stable compound.

How many magnesium ions are needed? Chloride ions?

Check your work.

magnesium ions = 1
chloride ions = 2
Clear the work surface by returning all cations and anions.
Click on the procedure 2 tab to continue.

Predict the formula for zinc nitride by dragging a zinc ion to the work surface.

How many bonds will zinc form?

Check your work.

2
Drag a nitride ion to the work surface.

How many bonds will nitride form?

Check your work.

3
Drag enough zinc ions and nitride ions to create a stable compound.

How many zinc ions are needed? Nitride ions?

Check your work.

zinc ions = 3
Nitride ions = 2
Clear the work surface by returning all cations and anions.
Click on the procedure 3 tab to continue.

Predict the formula for calcium oxide by dragging a calcium ion to the work surface.

How many bonds will calcium form?

Check your work.

2
Drag an oxide ion onto the work surface.

How many bonds will oxide form?

Check your work.

2
Drag enough oxide ions to create a stable compound.

How many calcium ions are needed? Oxide ions?

Check your work.

calcium ions = 1
oxide ions = 1
Clear the work surface by returning all cations and anions.
Click on the procedure 4 tab to continue.

Predict the formula for aluminium fluoride by dragging an aluminium ion to the work surface.

How many bonds will aluminium form?

Check your work.

3
Drag a fluoride ion onto the work surface.

How many bonds will fluoride form?

Check your work.

1
Drag enough fluoride ions to create a stable compound.

How many aluminum ions are needed? Fluoride ions?

Check your work.

aluminum ions = 1
fluoride ions = 3
Clear the work surface by returning all cations and anions.

Digging Deeper

© Wikimedia Commons
B5.4 Star sapphire

The Star of Adam is the largest star sapphire in the world. It weighs 1 404 carats (281 g) and has been valued at over $300 million. It was discovered in Sri Lanka, which is a well-known location for gem mining.

Please read pages 42 to 43 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on how to write formulas for binary ionic compounds. Remember, if you have any questions or you do not understand something, ask your teacher!

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

Provide the correct IUPAC name for each of the following compounds.

zinc bromide
potassium nitride
lithium iodide
barium phosphide
aluminium sulfide
magnesium oxide
sodium arsenide

Check your work.

zinc bromide	ZnBr₂
potassium nitride	K₃N
lithium iodide	LiI
barium phosphide	Ba₃P₂
aluminium sulfide	Al₂S₃
magnesium oxide	MgO
sodium arsenide	Na₃As