Counting Atoms
Molecules and Subscripts
Atoms can stick together (chemically bond)
When 2 atoms of the same element bond, we call this a molecule
H, O, F, Br, I, N, Cl are all elements that exist naturally as molecules
To show this in symbols, we write the number AFTER the element symbol,
dropped lower, called a SUBSCRIPT
Example: H2 means that there are 2 atoms of hydrogen bonded to each
other
H
H
When there is no subscript, that just means there is one atom.
Example: He means that there is one helium atom
He
Coefficients
Atoms can “hang out” together, but not be chemically bonded.
If there were 2 neon atoms floating around the room, we would write: 2 Ne
The large 2 in front of the symbol is called a COEFFICIENT
Ne
Let’s say 2
be written like
Ne
Hydrogen molecules (H2) “hang out”, it would
this: 2 H2
H
H
H
H
There are 4 ATOMS of hydrogen in total here (2 molecules).
Compounds
Atoms of different elements can also bond together, and we call these
COMPOUNDS
Like water, H2O
O
HH
2
atoms of hydrogen bonded to one atom of oxygen.
Remember, the subscript 2 refers to the element that it is BEHIND (so the 2
is talking about the H, NOT the O)
If there were 5 compounds of water in a glass, we would write: 5 H2O
There are: 10 H and 5 O
A total of 15 atoms
Brackets
If you see a formula with brackets: Ca(OH)2
The atoms in the brackets act as a unit.
The subscript tells you how many of these units there are.
So in this case there are 2 OH’s (2 O and 2 H)
And of there is also 1 Ca
So in total here there are 5 atoms (2 + 2 + 1)
Candy Models
Let’s build some compounds and molecules with candy...no eating yet!
Decide on your candy code before you begin!
You can use different colours or different types of candies to represent
different atoms (example: you might say “yellow smarties are Cl and blue
smarties are O”, or you could say “Rockets are H and round jujubes are
C”...it’s up to you, but be consistent when you build your models).
Candy Atom Code
N=
Na =
C=
H=
Cl =
Fe =
O=
S=
Cu =
Now let’s start building! Make sure your teacher checks each candy model
before you disassemble it or move on to the next one!
1. Chemical formula: CO
Name: Carbon monoxide
Use or description: poisonous gas released from car exhaust
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
2. Chemical formula: NH3
Name: ammonia
Use or description: found in some floor cleaners
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
3. Chemical formula: O2
Name: Oxygen
Use or description: large percent of the air we breathe
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
4. Chemical formula: 3 NaCl
Name: Sodium Chloride
Use or description: table salt
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
5. Chemical formula: 2 H2O
Name: dihydrogen monoxide
Use or description: water
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
6. Chemical formula: NaOH
Name: sodium hydroxide
Use or description: common bathroom cleaner
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
7. Chemical formula: CH4
Name: Methane gas
Use or description: natural gas used to heat homes
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
8. Chemical formula: H2SO4
Name: sulfuric acid
Use or description: a very corrosive acid, a very dilute form is in acid rain
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
9. Chemical formula: Fe(CO)5
Name: Iron pentacarbonyl
Use or description: a chemical used as a reactant in chemistry
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present:
10. Chemical formula: Cu(NO3)2
Name: Copper (II) nitrate
Use or description: a bright blue crystal used in ceramic dyes
Compound or molecule?:
Name and Number of each atom present:
Total number of atoms present: