Chemical Formulas and Reactions
Chemical formulas are an easy way to know the components of a compound. They are
basically shorthand compound names. Just like scientists use e- instead of writing out
the word “electron”, scientists use chemical formulas instead of writing out the name of
the compound.
Ex: sodium chloride is the name of table salt, but the chemical formula is NaCl.

Knowing the above example, how do you think scientists write chemical
formulas?
The chemical (atomic) symbols
o In compound names, the metal is always written first and the non-metals written
last.
o When writing compound names, the last 3 letters (usually –ine) are dropped and
replaced with –ide. Example: Chlorine becomes Chloride, oxygen become oxide
o Sometimes, the last 2 letters of the non-metal are replaced with –ate. These are
usually non-metals that end in –ur or -on like sulfur. Sulfur becomes sulfate
o Remember: mono=1, di=2, tri=3, tetra=4, penta=5, hexa=6, hepta=7, octo=8
Practice:
Write the chemical formulas for the following compounds. (*hint: don’t forget about
valence electrons!)
1. Sodium chloride
NaCl
6. Potassium hydroxide
KOH
2. Potassium bromide
KBr
7. Sodium hydroxide
NaOH
3. Carbon dioxide
CO2
8. Calcium iodide
CaI2
4. Carbon monoxide
CO
9. Potassium sulfate
K2S
5. Hydrogen chloride HCl
10. Magnesium carbonide Mg2C
Part 2: We can actually figure out the elements and number of atoms in the compound
from the chemical formula.


How do we know which elements are present? What tells you this information?
The chemical (atomic)symbols
How do we know the number of atoms present in the compound? What tells you
this information? The subscripts and coefficients
Example: Sodium chloride has 2 atoms present, one of sodium and one of chlorine.
Looking at the chemical formula, NaCl, we see that there is only 1 Na (sodium) written
and 1 Cl (chlorine) written. Therefore, there are 2 total atoms that make up this
compound and 1 is sodium, 1 is chlorine.
o Remember that subscripts are small and below the element and are only
attached to the element directly in front of the subscript
o Remember that co-efficient are applied to the whole compound using the
distributive property
o Remember that parenthesis mean you have to use the distributive property
and the subscript applies to everything within the parenthesis
Practice:
List the total number of atoms present in the compound and how many of each type of
atom the compound contains.
1. LiCl
Li - 1
Cl - 1
2. CaCO3
Ca – 1 C - 1
O-3
3. Ca(OH)2
Ca - 1
O–2 H-2
4. Na2CO3
Na – 2 O - 3
C-1
5. CaSO4
Ca – 1 O - 4
S-1
6. (NH4)2SO4
N–2
H–8
7. 2H2O
H-4
O-2
8. 3NH3
N-3
H-9
9. 4Cl2
Cl - 8
10. KNO3
K–1
N–1
O-3
S-1
O-4
Part 3: Chemical reactions
There are 2 parts to a chemical reaction: the reactants and the products.

Label the reactants side and the products side of the equation below.
Reactants
Products
HCl + NaOH
H2O + NaCl
The reactants are the things you are putting together, or reacting. They are your starting
materials. The products are the things that you end up with.

What does the law of conservation of mass state?
Mass or matter cannot be created or destroyed, only transformed or changed

Looking at the above reaction, how many atoms are on the reactants side?
2 hydrogen, 1 Chlorine, 1 Sodium, 1 Oxygen, = 5 total

Looking at the above reaction, how many atoms are on the products side?
2 Hydrogen, 1 Oxygen, 1 Sodium, 1 Chlorine = 5 total

Are there the same number of each atom on both sides of the reaction?
yes
The above reaction is called a chemical equation. The arrow is like the equal sign. Both
sides of the equation have to be equal, just like in math (Please try and suppress your
groan )
Since the law of conservation says that we can’t create or destroy matter, then we just
have to rearrange them. This is why both sides of the chemical equation have to be the
same.
Part 4: Rules for balancing equations
1. Rewrite the equation with enough room for you to change the coefficients (the big
number before the molecule).
2. Draw a line down from the arrow dividing the equation into 2 sides.
3. List all the elements on the reactants side down below the first element with the total
number of that element for the side.
4. List the elements and the totals on the products side IN THE SAME ORDER AS THE
REACTANTS.
5. Balance all the elements. Balance Oxygen (O) last and Hydrogen (H) second to
last!
Example:
Fe + K2S
K + Fe2S3
Fe: 1
Fe: 2
K: 2
K: 1
S: 1
S: 3
2Fe + 3K2S
6 K + Fe2S3
Fe: 12
Fe: 2
K: 2 6
K: 1 6
S: 1 3
S: 3