Writing Ionic Formulas

Ionic Compounds Things you should know:

Ionic = metal-nonmetal combo of elements Metal: loses e- to become stable; positive

ions Nonmetal: gains e- to become stable;

negative ions The charge value (1, 2, or 3) depends on

how many e- were lost or gained. The # lost or gained was the # needed to

be “full”.

Predicting Charges on IonsPredicting Charges on IonsKNOW THESE !!!!KNOW THESE !!!!

+1 +2 +3 -3 -2 -1 0

Method One: electron dot diagrams

The # of dots shown = # of valence e-.

Column # = # of valence e-.e- are gained or lost in the #

needed for each atom/ion to be stable.

e- dot diagrams for some common elements

Use e- dot diagrams to show the transfer of e- to make the metal and nonmetal atoms stable.

e- dot diagram for Potassium (K) e- dot diagram for oxygen (O)

Transfer of e- to form potassium oxide

Writing the Formula

If the ratio is 1:1, no subscript is needed. Ex: sodium chloride = NaCl

If the ratio is NOT 1:1 use a subscript for each element which has more than 1 ion involved in the transfer. Ex: potassium oxide = K2O

Your Turn!

What is the formula of barium iodide? Show the e- dot diagram of each element. Show the transfer of e-. Write the formula based on the ratio of the

ions.

Barium Iodide

e- dot diagram for Barium (Ba) e- dot diagram for iodine (I)

Transfer of e- to form barium iodide

Formula

BaI2

Method 2: using charges of the ions

Ion charge for Aluminum (Al) Ion charge for chlorine (Cl)

“Adding” the ions to get a neutral compound

The formula

AlCI3

Your Turn!

What is the formula of lithium sulfide? Determine the ion charge of each element. Determine the # of each ion needed to

form a neutral compound. Write the formula based on the ratio of the

ions.

The Formula

Li2S

Transition Metals

Transition metals = B column elements Charge can vary. It may be: +1, +2, +3,

or +4. A number in parentheses following the

name of the metal gives the ion charge. Ex: Iron (II) = Fe+2

Iron (III) = Fe+3

Teacher Example

What is the formula of chromium (III) sulfide?

Ion charge for Chromium (Cr) Ion charge for sulfide

“Adding” the ions to get a neutral compound

The formula

Cr2S3

Your Turn!

What is the formula for silver (I) chloride?

The formula

AgCI

Polyatomic Ions Not all ionic compounds are composed

of metal and nonmetal elements. What?!?! All ionic compounds ARE composed of

positive and negative ions. Some ions are composed of 2 or more

elements. Poly = “many” Atomic = refers to

atoms

Polyatomic Ions-continued= a group of atoms covalently

bonded together that has a net charge.

Examples:NH4

+ Ammonium C2H3O2

- AcetateCN- CyanideO2

2- PeroxideNO3

- Nitrate NO2

- NitriteSO4

-2 SulfateSO3

-2 Sulfite

Together, the group has a charge. Like a molecule, the O and H atoms stay bonded together and act as one particle.

1–

HO

OClO

1–

One chlorine atom and two oxygen atomscovalently bonded, together carrying a 1- charge

ClO2-

Writing a formula that contains a polyatomic ion

Use the charge method. “Adding” the charges of the ions, the compound must be neutral.

Compound: Magnesium hydroxide

Magnesium hydroxide

Ion charge for Magnesium (Mg) Ion charge for hydroxide (OH-)

“Adding” the ions to get a neutral compound

The formula

Mg(OH)2

Your Turn!

Write the formula for lithium phosphate.

The formula

LiPO4

Let’s Put It All TogetherTransitional metal and Polyatomic Ion What is the formula of chromium (III)

sulfate?

Ion charge for Chromium (Cr) Ion charge for sulfate

“Adding” the ions to get a neutral compound