Presentation on theme: "Writing Formulas for Ionic Compounds. Anatomy of a Chemical Formula Chemical formulas express which elements have bonded to form a compound. The subscripts."— Presentation transcript:
Anatomy of a Chemical Formula Chemical formulas express which elements have bonded to form a compound. The subscripts express the elements' numerical relationships to one another.
Formula Unit for an Ionic Compound §Since ionic compounds do not exist as single units, the formula of an ionic compounds represents the simplest ratio of ions. It is called a FORMULA UNIT. §Example: MgCl 2 is a formula unit of magnesium chloride. §Cations are written first in the formula unit; anions second.
Formula Unit for an Ionic Compound §The charges of the ions that make up an ionic compound must cancel out - they must add up to zero! §Example: MgCl 2 is made up of one Mg +2 ion and 2 Cl - ions. The charges cancel (add uo to zero): +2 + 2(-1) = 0
An easy way to write a chemical formula in which the charges of the cations and anions cancel each other out is to use the criss- cross method.
Criss-cross method §1. Determine the ions (their symbols and their charges) that are present in the compound §2. Take the NUMBER ONLY of the charge for the cation and write it as a subscript after the anion §3. Take the NUMBER ONLY of the charge for the anion and write it as a subscript after the cation
Example: Magnesium chloride Mg +2 Cl - 1 becomes the 2 becomes the subscript of Mg subscript of Cl MgCl 2
Exceptions §Ca 2+ and O 2- would form Ca 2 O 2 if you used the criss-cross method §However, in reality, Ca 2+ and O 2- form CaO. §For ionic compounds, you must simplify (reduce) the subscripts when necessary.
Practice Problems §Write the formula for the compounds listed below. l potassium oxide l lithium nitride l calcium phosphide l magnesium sulfide l aluminum oxide l aluminum iodide Remember: the name of the anion is the root of the element’s name with the suffix –ide!
Writing Formulas for Compounds with Transition Metals §The charge of transition metal ions and Group 4A metal ions can vary. Example: Iron can form a +2 or +3 ion (Fe +2 or Fe +3 ); Lead can form a +2 or +4 ion (Pb +2 or Pb +4 ) §In most cases, a roman numeral written in parentheses after the metal in the compound’s name will tell you the charge of the metal. §When this information is NOT given to you, it means the transition metal has only one possible charge. To find out what that is, you must look on your “ion sheet”.
Writing Formulas for Compounds with Transition Metals Examples iron (II) oxide contains Fe +2 and the compound’s formula is FeO iron (III) oxide contains Fe +3 and the compound’s formula is Fe 2 O 3 Zinc oxide contains Zn +2 and the compound’s formula is ZnO.
Practice Problems §Write the formulas for the following compounds: l cobalt (III) sulfide l mercury (I) chloride l chromium (VI) oxide l tin (IV) nitride l cobalt (II) sulfide
Writing the Formula Unit for an Ionic Compound §Binary ionic compounds are made up of 2 kinds of elements. §Polyatomic ionic compounds are made up of more than 2 kinds of elements. (Ex. NaOH) l One or both of the ions in such a compound must be a polyatomic ion (an ion made up of more than 1 kind of atom). In our example, Na + is the cation and OH - is the polyatomic anion. OH - contains O and H!
Polyatomic Ions §The charge of the polyatomic ion applies to the entire ion (all of the atoms as a group) §A polyatomic ion acts as if it were an ion with one kind of atom. The different atoms act together, as a group.
Polyatomic ions §Example: NH 4 + is a polyatomic cation made up of one N atom and 4 H atoms bonded together. The overall charge on this bonded group is +1. §Example: OH - is a polyatomic anion made up of 1 O atom and 1 H atom bonded together. The bonded atoms have an overall charge of -1.
Writing Formulas for Compounds with Polyatomic Ions §Determine the ions (and their charges) that are present in the compound. If one is a polyatomic ion, use your “ion sheet” to find the formula of the ion and its charge.
Writing Formulas for Compounds with Polyatomic Ions §Follow the criss-cross rules with 1 exception: l If more than one polyatomic ion is required in the compound’s formula, the formula of the polyatomic ion MUST BE set off by parentheses. l The subscript will be written to the right of the parentheses to tell how many units of the polyatomic ions are present.
Writing Formulas for Ionic Compounds §Example: magnesium nitrate l Mg 2+ is the cation; NO 3 - is the anion.
NEVER EVER change the subscripts of the atoms within a polyatomic ion.
Magnesium chlorate §Mg 2+ is the cation; ClO 3 - is the anion; Mg(ClO 3 ) 2 is the formula of the compound. NOT l Mg2ClO 3 l MgCl 2 O 6 l Mg 3 ClO 2 l Mg 2 ClO 3
Practice Problems §Write the formulas for the following compounds: l calcium hydroxide l tin (IV) sulfate l aluminum carbonate l nickel (II) nitrate l ammonium phosphate
Rules for Naming Ionic Compounds §Binary ionic compounds l Name the cation first and the anion second. l Monatomic cations use the element name. l Monatomic anions take their name from the root of the element name plus the suffix –ide. Li 3 P lithium phosphide
Rules for Naming Ionic Compounds §Ionic compounds with transition metals l The charge of the transition metal ion must be determined. l You must work back from the formula to determine the charge of the transition metal ion. l Once it is determined, use parentheses after the metal ion name to indicate this charge.
Co 2 S 3 Co 3+ S 2- The name is cobalt (III) sulfide The subscript on the S indicates the charge of Co is 3+ The subscript on the Co indicates the charge of S is 2-
Rules for Naming Ionic Compounds §If the compound contains any polyatomic ion, name the ion to name the compound. NaHCO 3 Sodium hydrogen carbonate or sodium bicarbonate
Practice Problems 1.CaSO 4 2.CuO 3.AlBr 3 4.Mg(OH) 2 5.FePO 4 6.NH 4 NO 3