Like elemental symbols, scientists use chemical formulae to provide information about compounds: Describe what elements are present Give the proportions of those elements Give the proportions of those elements Examples: 1.CaCl 2 - contains one atom of calcium and 2 atoms of chlorine 1.Mg(OH) 2 - contains one atom of magnesium, two atoms of oxygen, and two atoms of hydrogen

What do you notice about the number of positive charges and the number of negative charges based on its formula??? Ca 2+ N -3 Ca 2+

As we just noticed, in ionic compounds the positive charges balance out the negatives so that the compound is neutral As we just noticed, in ionic compounds the positive charges balance out the negatives so that the compound is neutral The ratio of positive : negative charges gives the proper formula The ratio of positive : negative charges gives the proper formula The ratio is always written in reduced form The ratio is always written in reduced form For example, what is the formula for magnesium phosphide? For example, what is the formula for magnesium phosphide?

 Magnesium is Mg 2+ Phosphorous is P 3–  Lowest common multiple of 2 and 3 is 6  3 Mg 2+ ions and 2 P 3– ions  Mg 3 P 2 Mg 2+ P -3 Mg 2+ Do you notice anything about the charge on the cation and the number of atoms of the anion? (and vice versa?)

Ca 2+ N 3- Ca 3 N 2

Ca 2+ O -2

Some transition metals are multivalent, meaning they have more than one ion form. In the name of the compound, Roman Numerals are used following the metal to indicate which ion was used In the name of the compound, Roman Numerals are used following the metal to indicate which ion was used

For example: What is the formula for Manganese (III) sulphide?  This manganese is Mn 3+ Sulphur is S 2–  Lowest common multiple of 3 and 2 is 6.  2 Mn 3+ ions and 3 S 2– ions  Or criss-cross! Mn 2 S 3

Ti 4+ Cl - Ti 3+  Titanium is Ti 4+ or Ti 3+ Chlorine is Cl –  Titanium is Ti 4+ or Ti 3+ Chlorine is Cl –  1 Ti 4+ ion and 4 Cl – ions  Titanium (IV) chloride Ti 4+ Cl - Reverse criss- cross??? TiCl 4 So it’s Ti 4 and Cl 1

Predict the compound formed from: Na + and OH - Magnesium and Nitrate Ammonium and Sulphate NaOH Mg(NO 3 ) 2 (NH 4 ) 2 SO 4

Predict the compound name: CaCO 3 KCN Ca 3 (PO 4 ) 2 Calcium carbonate Potassium cyanide Calcium phosphate

So this compound is: Iron (II) hydroxide 1 2 So Fe must have a 2+ charge and OH a 1- charge.