Ionic Formulas

Ionic Formulas

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A chemical formula shows the
kinds and numbers of atoms in
the smallest representative unit of
a substance.

A formula unit is the lowest
whole-number ratio of ions in an
ionic compound.

Every ionic compound has a 3D
array of positive and negative ions.

Formula Units

Predicting an Ionic Formula

By examining the electronegativities of two elements it can be
determined if they will form an ionic compound.

Then, if they will form an ionic compound, its formula can be
predicted by examining the atoms' valence electrons.

Let's use potassium (K) and oxygen (O) as an example.

We will use the electronegativity table on the next slide to answer
the first question.

Example 1: K and O

Potassium's electronegativity is 0.8 and oxygen's is 3.5.

That difference of 2.7 is well more than 1.7, so they will form an
ionic compound.

Now, let's find its formula.

The answers to these questions will determine the formula.

·How many additional valence electrons does oxygen need?

·How many valence electrons does potassium have to offer?

·How many potassium atoms will it take to give oxygen the electrons
it needs?

(answers on next page)

Example 1: K and O

·How many additional valence electrons does oxygen need? 2

·How many valence electrons does potassium have to offer? 1

·How many potassium atoms will it take to give oxygen the electrons
it needs? 2

So, we will need 2 K for each O.

The formula is written with the metal first, so the formula unit for the
ionic compound formed by K and O is: K2O

K

O

K

K

O

K

Example 1: K and O

Note that O captured the electron: it is not shared with K. K is nearby due
to its electrical attraction to O.

Example 2: Mg and N

Will magnesium (Mg) and nitrogen (N) form an ionic compound?

If so, what will its formula be?

Try it first on your own, before looking at the answers on the
following slides.

Mg's electronegativity is 1.2 and N's is 3.0.

That difference of 1.8 is more than 1.7, so they will form an ionic
compound.

Now, let's find its formula.

Example 2: Mg and N

Once again, these are the key questions:

·How many additional valence electrons does nitrogen need?

·How many valence electrons does magnesium have to offer?

·How many magnesium atoms will it take to give nitrogen the
electrons it needs?

(answers on next page)

Example 2: Mg and N

·How many additional valence electrons does nitrogen need? 3

·How many valence electrons does magnesium have to offer? 2

·How many magnesium atoms will it take to give nitrogen the
electrons it needs? 1.5

Since atoms are indivisible, we need to find the lowest ratio of whole
numbers to give us 1.5 Mg for each N. The answer: 3 Mg for every 2 N.

The formula unit for the ionic compound formed by Mg and N is: Mg3N2.

Example 2: Mg and N

N

Mg

Mg

Mg

N

N

Mg

Mg

N

Mg

This is an alternative method to finding the formula once you
know the ionic charges:

1. Write down the ions side by side along with their charge,
always writing the metal first.

Mg2+ N3-

2. "Criss-cross" the numerical values of the charges.

Mg2+N3-

3. Write the formula unit.

Mg3N2

4. Reduce to lowest terms if necessary. It's not necessary here.

Alternate Method

Write the formula for the ionic compound formed by Ca and S.

1. Write down the ions side by side along with their charge,
always writing the metal first.

Ca2+ S2-

2. "Criss-cross" the numerical values of the charges.

Ca2+ S2-

3. Write the formula unit.

Ca2S2

4. Reduce to lowest terms if necessary.

CaS

Example 3: Ca and S

Naming Binary Ionic Compounds

The formula for a binary ionic compound begins with the cation
(metal) and is followed by the anion (non-metal).

For example, CaS is the ionic formula formed by calcium and
sulfur...the metal's symbol comes first.

Similarly, the name of an ionic compound starts with the name of
the cation (metal) followed by the name of the anion (non-metal).

The name of the cation is usually the same as that of the neutral
atom, but the name of the anion is slightly different than that of its
neutral atom.

Names of Cations

+1

+2

+3

H+
Li+
K+
Cs+
Ag+

Mg2+
Ca2+
Ba2+

Cd2+

Al3+

Hydrogen ion
Lithium ion
Potassium ion
Cesium ion
Silver ion

Magnesium ion
Calcium ion
Barium ion
Cadmium ion

Aluminum ion

Charge

Formula

Name

Many cations, including all of these, have the same name as their
original, neutral atom.

The names of monoatomic anions are almost the same as that of
their original, neutral atom, but end in "-ide".

The ions produced from Group 17 elements are called halide ions.

Nitride

N3-

Phosphide P3-

Oxide

O2-

Sulfide

S2-

Fluoride

F-

Chloride

Cl-

Bromide

Br-

Iodide

I-

Group 15

Group 16

Group 17

Naming Anions

Binary (two-element) compounds are named by writing the
name of the cation followed by the name of the anion.

The name of the cation is the same as the metal name.

The name of the anion is the name of the non-metal with
the suffix changed to -ide.

Binary ionic compounds end in "-ide."

Naming Binary Ionic Compounds

Examples:

NaCl = sodium chloride
KI =

potassium iodide
Li2S =

lithium sulfide

24 The formula for barium sulfide is Ba2 S2 .

A True

B False

C

I need help

Answer

https://njctl.org/video/?v=8jbv8YP_ghU

24 The formula for barium sulfide is Ba2 S2 .

A True

B False

C

I need help

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Answer

B

https://njctl.org/video/?v=8jbv8YP_ghU

25 The formula for the ionic compound between Mg
and S is:

A

Mg2S2

B

MgS

C

MgS2

D

SMg

E

I need help

Answer

https://njctl.org/video/?v=aZoDXdZQTBw

25 The formula for the ionic compound between Mg
and S is:

A

Mg2S2

B

MgS

C

MgS2

D

SMg

E

I need help

[This object is a pull tab]

Answer

B

https://njctl.org/video/?v=aZoDXdZQTBw

26 The formula for the ionic compound between Cs
and O is:

A

CsO2

B

OCs2

C

Cs2O

D

OCs2

E

I need help

Answer

https://njctl.org/video/?v=CCDfFh-P2d8

26 The formula for the ionic compound between Cs
and O is:

A

CsO2

B

OCs2

C

Cs2O

D

OCs2

E

I need help

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Answer

C

https://njctl.org/video/?v=CCDfFh-P2d8

27 The ionic compound formed between Ca and N is:

A

CaN

B

Ca2N2

C

Ca3N2

D

Ca2N3

E

I need help

Answer

https://njctl.org/video/?v=vsi8DjIEKVI

27 The ionic compound formed between Ca and N is:

A

CaN

B

Ca2N2

C

Ca3N2

D

Ca2N3

E

I need help

[This object is a pull tab]

Answer

C

https://njctl.org/video/?v=vsi8DjIEKVI

28 The ionic compound formed between Al and O is:

A

Al3 O2

B

Al2 O3

C

AlO

D

Al2 O2

E

I need help

Answer

https://njctl.org/video/?v=t13K8qr7org

28 The ionic compound formed between Al and O is:

A

Al3 O2

B

Al2 O3

C

AlO

D

Al2 O2

E

I need help

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Answer

B

https://njctl.org/video/?v=t13K8qr7org

29 What is the ionic compound formed between
Ca and Al?

A CaAl

B Ca3Al2

C Al2Ca3

D None of the above

E I need help

Answer

https://njctl.org/video/?v=kXHO6NqVpm8

29 What is the ionic compound formed between
Ca and Al?

A CaAl

B Ca3Al2

C Al2Ca3

D None of the above

E I need help
[This object is a pull tab]

Answer

D

https://njctl.org/video/?v=kXHO6NqVpm8

30 What is the ionic compound formed between
P and Br?

A

P3 Br

B

BrP

C

This compound is not considered ionic

D

(BrP)2

E

I need help

Answer

https://njctl.org/video/?v=TLBSOVDv_ZA

30 What is the ionic compound formed between
P and Br?

A

P3 Br

B

BrP

C

This compound is not considered ionic

D

(BrP)2

E

I need help

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Answer

C

https://njctl.org/video/?v=TLBSOVDv_ZA

31 What is the formula for sodium phosphide?

A

SP3

B

NaP

C

Na3P

D

NaP3

E

I need help

Answer

https://njctl.org/video/?v=4ILsXwB_Jsk

31 What is the formula for sodium phosphide?

A

SP3

B

NaP

C

Na3P

D

NaP3

E

I need help

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Answer

C

https://njctl.org/video/?v=4ILsXwB_Jsk

32 What is the formula for strontium bromide?

A

SrBr

B

SrBr2

C

Sr2 Br

D

BrSr2

E

I need help

Answer

https://njctl.org/video/?v=UAKy0VxsEjk

32 What is the formula for strontium bromide?

A

SrBr

B

SrBr2

C

Sr2 Br

D

BrSr2

E

I need help

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Answer

B

https://njctl.org/video/?v=UAKy0VxsEjk

33 The correct name for Na2 S is _____.

A

Sodium sulfate

B

Sodium sulfide

C

Di-sodium sulfide

D

Sulfur nitride

E

I need help

Answer

https://njctl.org/video/?v=z_n5gYwSlAE

33 The correct name for Na2 S is _____.

A

Sodium sulfate

B

Sodium sulfide

C

Di-sodium sulfide

D

Sulfur nitride

E

I need help

[This object is a pull tab]

Answer

B

https://njctl.org/video/?v=z_n5gYwSlAE

34 The correct name for SrO is _____.

A

strontium oxide

B

strontium hydroxide

C

strontium peroxide

D

strontium monoxide

E

I need help

Answer

https://njctl.org/video/?v=Gj1if_qNRnU

34 The correct name for SrO is _____.

A

strontium oxide

B

strontium hydroxide

C

strontium peroxide

D

strontium monoxide

E

I need help

[This object is a pull tab]

Answer

A

https://njctl.org/video/?v=Gj1if_qNRnU

35 The correct name for Al2 O3 is _____.

A

aluminum trioxide

B

dialuminum oxide

C

dialuminum trioxide

D

aluminum oxide

E

I need help

Answer

https://njctl.org/video/?v=Ai7HxvsABy0

35 The correct name for Al2 O3 is _____.

A

aluminum trioxide

B

dialuminum oxide

C

dialuminum trioxide

D

aluminum oxide

E

I need help

[This object is a pull tab]

Answer

D

https://njctl.org/video/?v=Ai7HxvsABy0