2. Writing Chemical Formulas

3. When you combine atoms, you use the oxidation numbers to help you figure out the ratio. 1+ 2+ 3+4+4+3-2-1- 0 Oxidation Number Varies

4. Show the elements & the ratio of the elements in a compound. C0 2 carbon, oxygen 1:2C NaCl sodium, chlorine 1:1I Formula ElementsRatio Type

5. Writing Chemical Formulas Use the Criss Cross Method! 1. Write the cation first (+) with its oxidation number as a superscript. 2. Write the anion second (-) with its oxidation number as a superscript. 3.“Criss-Cross” the numbers (move the superscripts down to subscripts, and delete the charges) 4. Write the new formula (without the + and – signs)

6. Writing Formulas Continued Remember: Do NOT write a 1 as a subscript; it is understood to be thereDo NOT write a 1 as a subscript; it is understood to be there Always reduce the subscripts to the lowest numbersAlways reduce the subscripts to the lowest numbers If the two subscripts are equal (ex: both are 2), don’t write them in the final formula.If the two subscripts are equal (ex: both are 2), don’t write them in the final formula.

7. Mg +2 Cl -1 MgCl 2 1 2 Mg +2 O -2 MgO 22 Fe +3 O -2 Fe 2 O 3 23

8. Na +1 O 2- Sn 4+ S 2- K 1+ Cl 1- Mg 2+ N 3-

9. Polyatomic Compounds are a group of atoms that act as a single ion (+ or -) Copy this table into your notes

10. Mg +2 PO 4 -3 Mg 3 (PO 4 ) 2 3 2 Li +1 SO 4 -2 Li 2 SO 4 21 Fe +3 NO 4 -2 Fe 2 (NO 4 ) 3 23 **Note the ( ) on multiple polyatomic ions

11. Naming Chemical Formulas

12. Rules for naming Binary Ionic compounds 1. Write the name of the metal first. Example: KBr Potassium 2. Write the name of the nonmetal changing the ending to “ide.” Bromine Bromide

13. If the polyatomic Ion is first….. 1. Use your chart to write the name of the polyatomic ion. Example: NH 4 Cl Ammonium 2. Write the name of the nonmetal changing the ending to “ide.” Chloride Rules for naming Polyatomic Ionic compounds

14. 1. Write the name of Cation first. Example: K 2 SO 4 Potassium 2. Refer to your chart and write the name of the polyatomic ion second. Sulfate If the polyatomic Ion is second…..