1. Chemical Formulas Now that we know that elements can bond together to make compounds…

2. Why is water H 2 O? (and not H 2 O 2 or anything else?) How most elements come together (bond) is based on the number of VALENCE ELECTRONS. (and where it is on the periodic table)

3. Periodic Table 1 2 14 15 16 13 17 18 12 3 4 5 6 7 8 9 10 11 12345678 VALENCE ELECTRONS

4. But, why is water H 2 O? Since elements want to ACHIEVE THE GOAL (of 8 valence electrons), they will either GIVE electrons away OR they will TAKE electrons from another atom.

5. But, why is water H 2 O? GROUPVALENCE ELECTRONSION CHARGE 1 2 13 14 15 16 17 18

6. GROUPVALENCE ELECTRONSION CHARGE 11 22 133 144 155 166 177 188 neutral But, why is water H 2 O? +1 +2 +3 neutral -3 -2

7. But, why is water H 2 O? If SODIUM AND CHLORINE are going to bond, what will happen? Na is in Group 1 so it has a charge of +1. (That means that it will GIVE UP ONE ELECTRON.) Cl is in Group 17 so it has a charge of -1. (That means that it will TAKE ONE ELECTRON.) They will bond together and form NaCl. Let’s talk about salt (sodium chloride), first.

8. Finally, why is water H 2 O? H is in Group 1 so it has a charge of +1. O is in Group 16 so it has a charge of -2. Since all compounds have to be NEUTRAL, there would have to be 2 Hydrogen atoms for every 1 Oxygen atom. That is why water’s chemical formula is H 2 O!

9. Quick Summary How do you write a chemical formula? Step 1 write down each symbol & charge (what group = charge!) Step 2 crisscross the charges to make subscripts (little numbers down below ) What is the chemical formula for aluminum sulfide? 3 Step 1 3 Al +3 (in group 13) S -2 (in group 16) Step 2 3 Al 2 S 3

10. What do chemical formulas tell you? 1 what elements are in the compound 2 how many elements are in the compound 3 and, it might tell you what TYPE of compound it is

11. What type of compound…? 1. if a formula starts with Carbon… it is an organic compound. For example; CH 4, C 3 H 8, C 12 H 22 O 11

12. What type of compound…? 2. if a formula starts with Hydrogen… it is an acid For example; H 2 SO 4, H 3 PO 4, HCl

13. What type of compound…? 3. and, if a formula ends with –OH… it is a base. For example; NaOH, Ba(OH) 2, KOH