Warmup 10/14/15
What does the little 2 in CO2 mean again?
Objective
Tonight’s Homework
To learn how to count atoms
in a molecule
practice on counting atoms
Notes on Formula Units
We’re almost ready to start doing chemical
equations, but first we need to be able to count
the number of atoms in molecules.
We can do this with a few simple rules, but
before that, we want to illustrate what this looks
like.
Notes on Formula Units
3NaNO3
Looking at this, we can see sodium atoms,
nitrogen atoms, and oxygen atoms. But how
many do we have of each? Let’s draw this
compound step-by-step to find out.
Notes on Formula Units
3NaNO3
First we have a sodium. There’s no little number
after the “Na”, so we only have 1 sodium atom.
Na
Notes on Formula Units
3NaNO3
Next, we see NO3. This looks familiar. It’s one of
our polyatomic ions. So we know this whole
thing is together.
NO3
Na
Notes on Formula Units
3NaNO3
But there’s more. We need to break that NO3 up
into pieces. In pieces, we see 1 “N” atom, and 3
“O” atoms (because there’s a 3 after the O).
N
OOO
Na
Notes on Formula Units
3NaNO3
There’s one last step, though. The 3 in front
tells us how many of this compound – this
“sodium nitrate” we have. We make copies.
In the end, we have:
3 nitrogen
9 oxygen
N
OOO
Na
N
3 sodium
OOO
Na
N
OOO
Na
Notes on Formula Units
Let’s do another example:
4Cu3(PO4)2
This is about as complicated as it can get.
copper(II) phosphate
Here we have a new thing added. We put
parenthesis around the “PO4” and put a little
number after. This means we have multiple
“PO4” groups.
In this case, we would count everything in the
parenthesis twice.
Notes on Formula Units
Instead of writing out all the pieces, this time
we’ll count it up like you would in math.
4Cu3(PO4)2
PO4
(PO4)2
Means 1 P and 4 O.
The little 2 means multiply
everything inside by 2. that gives us
2 P and 8 O total now.
Cu3
Means 3 Cu
4Cu3(PO4)2 The 4 in front means multiply
everything by 4. So our final result
would be:
12 Cu, 32 O, 8P
Practice with Formula Units
Name and count how many of each element are in the
compounds below:
3NaCl
2H2S
Na2SO4
2Mg(NO3)2
4Al2(B4O7)3
Exit Question
How many O atoms are there in 4Al2(B4O7)3?
a) 4
b) 7
c) 21
d) 36
e) 84
f) None of the above