22.3 – Writing formulas for Ionic Compounds

Part A

Focus on what is in PINK

Objectives

Write formulas for ionic compounds

Writing Ionic Formulas

Watch the Tyler DeWitt Video and complete the viewing guide, you will add it to your notes on this chapter

Youtube video

Binary Ionic Compounds

• A compound composed of 2 elements• To write a chemical formula for a

binary compound, you need to know:• which elements are involved• how many electrons they lose or gain to

become stable

Ionic Compounds are NEUTRAL

• Although the individual ions in a compound carry charges, the compound itself is neutral

• A formula must have the right number of positive ions and the right number of negative ions so the charges balance

Oxidation Number

The number of electrons an atom must gain, lose or share to become stable Cations = lose electrons Anions = gain electrons

Oxidation Number

Determined by which group the element is in and how many valence electrons it has

Valence electrons = The BONDING electrons found in the outermost energy level

Oxidation Number

Group 1 = 1 valence electron Loses 1 e- to become stable Ion = +1 charge

Group 2 = 2 valence electron Loses 2 e- to become stable Ion = +2 charge

Group 3 = 3 valence electron Loses 3 e- to become stable Ion = +3 charge

Oxidation Number

Groups 3-12 = Transition metals = variable # of electrons, unpredictable charges

Group 13 = 3 valence electron Loses 3 e- to become stable Ion = +3 charge

Oxidation Number

Group 14 = 4 valence electron Can gain or lose 4e- to become stable Ion = ±4 charge

Group 15 = 5 valence electron Gains 3e- to become stable Ion = -3 charge

Group 16 = 6 valence electron Gains 2e- to become stable Ion = -2 charge

Oxidation Number

Group 17 = 7 valence electron Gains 1e- to become stable Ion = -1 charge

Group 18 = *8 valence electron STABLE No charge, no ions

Oxidation #/Charges of Ions

Write Formulas for these ions

Potassium ion

Magnesium ion

Oxide (oxygen) ion

K+

Mg 2+

O 2-

Transition Elements

• Can have multiple oxidation numbers

• Express using Roman numerals

• For example, the oxidation number of iron in

iron (III) oxide is 3+.

Write Formulas for these ions

Copper (I) ion

Chromium (VI) ion

Mercury (II) ion

Cu 1+

Cr 6+

Hg 2+

Step By Step – Writing Formulas

1. Write the symbol of the element with the positive oxidation number or charge

2. Write the symbol of the element with the negative oxidation number or charge

3. The charge (without the sign) of one ion becomes the subscript of the other ion. • Reduce the subscripts to the smallest whole

numbers that retain the ratio of ions.

Criss-Cross Method

Writing Formulas

Write the formula for calcium chloride. Calcium is Ca+2 Chloride is Cl-1

Ca+2 Cl-1 Cl-1

CaCl2

In-Class Assignment/Homework

PS 22.3 A WKT 1 - Criss Cross practice