Download Chemical Formulas

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
VALENCE ELECTRONS
3 4 5 6 7 8
13
1 2 3 4 5 6 7 8 9
10
11 12
14
15
16
17
18
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
1
2
13
14
15
16
17
18
VALENCE ELECTRONS
ION CHARGE
But, why is water H2O?
GROUP
VALENCE ELECTRONS
ION CHARGE
1
1
2
2
13
3
+1
+2
+3
14
4
15
5
16
6
17
7
18
8
neutral
-3
-2
-1
neutral
But, why is water H2O?
Let’s talk about salt (sodium chloride), first.
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.
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 Summary
How do you write a chemical formula?
What is the chemical formula for
Step 1 write down each symbol & charge
aluminum sulfide? 3
(what group = charge!)
Step 1
Step 2 crisscross the charges to make
+3 (in group 13)
3
Al
subscripts (little numbers down below)
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