1. Understanding Chemical Formulas A chemical formula indicates the relative number of atoms of each kind of element in a chemical compound

2. Chemical Formulas CO 2 Indicates that there is only 1 carbon atom. “Ones” are “understood”, they do not need to be written. Indicates that there are 2 oxygen atoms

3. Chemical Formulas Al 2 (SO 4 ) 3 Indicates there are 2 aluminum atoms Indicates there are 4 oxygen atoms The subscript 3 refers to everything inside the parentheses. There are 3 sulfate atoms. We multiply the subscripts on the outside by the subscripts on the inside. This gives a total of 3 sulfur atoms and 12 oxygen atoms.

4. Charged Atoms Ions are charged atoms. –Ions can be either positive or negative Na + Cl - Mg +2 O -2 Indicates a positive 1 charge. Remember, the “one” is understood. Indicates a positive 2 charge Indicates a negative 1 charge Indicates a negative 2 charge

5. Atoms to Ions? An atom becomes an Ion by gaining or losing an electron. –Gaining an electron means there is an extra negative charge Extra negative charge gives us a negative ion (Anion) –Losing an electron means there is an extra positive charge Extra positive charge gives us a positive ion (Cation)

6. What’s the Charge???? Just like everything else in chemistry, the charge can be determined by the periodic table. (sometimes there are exceptions, but we will get to this later )

7. Predicting Ionic Charges Group 1: Lose 1 electron to form +1 ions

8. Predicting Ionic Charges Group 2: Lose 2 electrons to form +2 ions

9. Predicting Ionic Charges Group 13: Lose 3 electrons to form +3 ions

10. Predicting Ionic Charges Group 14: Can gain or lose 4 electrons to form +4 or - 4 ions

11. Predicting Ionic Charges Group 15: Gain 3 electrons to form - 3 ions

12. Predicting Ionic Charges Group 16: Gain 2 electrons to form - 2 ions

13. Predicting Ionic Charges Group 17: Gain 1 electron to form - 1 ions

14. Predicting Ionic Charges Group 18: Stable Noble Gases do not gain or lose any electrons, therefore do not form ions.

15. Predicting Ionic Charges Groups 3-12: Have more than one possible ion. (we will discuss this later)

16. Polyatomic Ions Poly – “many” Atomic – “atom” Polyatomic ion – ion of many atoms OH - Entire compound has a negative one charge!!! NH 4 + Entire compound has a positive one charge!!!