Writing Formulas for Ionic

Compounds

Oxidation Numbers for the Group A Elements

•When you write ions, use the element symbol and the oxidation number written as a superscript.

bromine

sulfur

cesium

Br-1

S-2

Cs+1

called bromide in a compound

called sulfide in a compound

Anions change end of name to –ide.

• If you need the oxidation number for a transition or inner transition metal, it will be given to you because these elements typically have more than one oxidation number.

• Ox. # for transition/inner transition metals will be in the form of a Roman Numeral!– Exceptions:

• Pb and Sn use Roman numerals even though they are NOT transition metals

• Ag and Zn do NOT use Roman Numerals even though they ARE transition metals

example:

•nickel (II) chloride (NiCl2)

The (II) belongs to nickelwhich means nickel has a +2 charge while chlorine has a -1 charge!

What is the oxidation number for the metal in each

compound?

• Lead (IV) oxide

• Scandium (II) nitride

Pb+4

Sc+2

Monatomic Ions

•A monatomic ion is an ion made up of one element.

•examples: H+1 O-2 F-1 Ba+2

•Monatomic ions bond to make a binary compound.–Binary compounds are made of a monatomic cation and a monatomic anion.

To Write Formulas with Monatomic Ions, use

the criss-cross method!Write the formula for barium bromide.

• First: Write each element symbol and oxidation number.

***Always write the positive ion first!!!

• Next: Criss-cross the oxidation numbers. Swap the ox. #’s and write them as SUBSCRIPTS (lose the + and – signs)

Ba+2 Br-1

Ba+2 Br-1

Ba1Br21 =BaBr2

bromine: name changed to bromide in a compound

Write the formulas for:

• Iron (III) chloride

• Sodium oxide

• Magnesium nitride

Polyatomic Ions

•A polyatomic ion is an ion made up of more than one element.

•These are on the back of your periodic table!

•The entire polyatomic ion has an oxidation number (NOT the individual elements).

Polyatomic Ions

Example: PO4-3 is the polyatomic

ion phosphate with an oxidation number of -3.

Example: SO3-2 is the polyatomic

ion sulfite with an oxidation number of -2.

•Most polyatomic ions end in the suffix –ate or –ite.

To write formulas with polyatomic ions:

• First: Write each element and polyatomic ion symbol and oxidation number.

***Always write the positive ion first!!!

• Next: Place polyatomic ion in PARANTHESES

• Finally: criss cross the oxidation numbers

• NEVER, NEVER, NEVER, EVER CHANGE THE SUBSCRIPT WITHIN THE POLYATOMIC ION ITSELF!

Write formulas for:

• Calcium Phosphate

• Magnesium Nitrate:

• ONE LAST NOTE: Chemical formulas must be in the lowest, reduced form!!!

Write formulas for:

• lead (II) oxide

• calcium sulfide

Let’s Try More!

Nomenclature of Ionic Compounds

• Nomenclature means naming.

Naming Ionic Compounds with Monatomic Ions

• First: Name the cation (positive ion) – DO NOT CHANGE THE CATION’S NAME IN ANY WAY and write it down.

• Note: If the cation is a transition or inner transition metal, you must indicate its charge (the oxidation number) using Roman numerals in parentheses after the metal’s name.

• Ex: iron (II) chloride; its chemical formula is:FeCl2

• Ex: What would these anions be named in a compound?

•Oxygen ______________

•Sulfur ______________

•Nitrogen ______________

• Iodine ______________

•Phosphorus ______________

•Chlorine ______________

Name These:

Ex: NaCl ____________________Pb3P2 ___________________

Naming Ionic Compounds with Polyatomic Ions

• First: Name the cation ion first (NEVER CHANGE THE CATION’S NAME).

• Note: If the cation is a transition or inner transition metal, you must indicate its charge using Roman Numerals in parentheses after the metal’s name. Ex: iron (II) hydroxide (its chemical formula is Fe(OH)2)

• Next: Name the polyatomic ion – ON THE BACK OF THE PT – (NEVER CHANGE THE POLYATOMIC’S NAME).– HINT: Polyatomic ions usually end in –ate

or –ite!

Try These:

• Li2SO4

• (NH4)2O

I will do the rest with you on the board!