Chemical Formulas

Now that we know that elements can bond together to make

compounds…

Why is water H2O? (and not H2O2 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)

Periodic Table

1 2

14

15

16

13

17

18

12

3

4

5

6

7

8

9

10

11

1 2 3 4 5 6 7 8VALENCE ELECTRONS

But, why is water H2O?

 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.

But, why is water H2O?

GROUP VALENCE ELECTRONS

ION CHARGE

1

2

13

14

15

16

17

18

GROUP VALENCE ELECTRONS

ION CHARGE

1 1

2 2

13 3

14 4

15 5

16 6

17 7

18 8 neutral

But, why is water H2O?

+1

+2

+3

neutral

-3

-2

-1

But, why is water H2O?

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.

Finally, why is water H2O?

 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 H2O!

Quick SummaryHow 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

Al2S3

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

What type of compound…?

1. if a formula starts with Carbon… it is an organic compound.

For example; CH4, C3H8, C12H22O11

What type of compound…?

2. if a formula starts with Hydrogen… it is an acid

For example; H2SO4, H3PO4, HCl

What type of compound…?

3. and, if a formula ends with –OH… it is a base.

For example; NaOH, Ba(OH)2, KOH