Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Counting Atoms!
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Chemical Vocabulary
Elements
e.g. H2, Pb, C, K
Compounds
e.g. NaCl, KOH, H2O2
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Chemical Vocabulary
Subscripts: Indicates how many atoms of a particular element
are present
Coefficients: Indicates the total number of
compounds/molecules present
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Counting Atoms
1) List the different elements in each compound
2) Count how many of that particular atom
there are.
3) Total how many atoms there are in the
compound.
e.g.
H2O
H (Hydrogen) 2 atoms
O (Oxygen) 1 atom
Total:
3 atoms
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
More Examples
3 H2O2
(3 molecules of hydrogen peroxide)
H (Hydrogen) (2 atoms
x 3 molecules)
O (Oxygen) (2 atom
x 3 molecules)
Total:
12 atoms
Pb(NO3)2
Pb (Lead)
1 atom
N (Nitrogen) (1 atom
O (Oxygen) (3 atom
Total:
x 2 molecules)
x 2 molecules)
9 atoms
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Counting Atoms
How many atoms are in these compounds?
KNO3
8 Cl2O
6 Ba(MnO4)2
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Balancing Chemical Equations
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Chemical Equation
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Balancing Equations
• GOAL: There must be the same number of
each atom on both sides of the equation.
REACTANTS = PRODUCTS
(LAW OF CONSERVATION OF MASS)
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Law of Conservation of Mass
• Atoms cannot be created or destroyed by
ordinary chemical means
• Reactants ALWAYS = Products
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Balancing Chemical Equations
http://www.wikihow.com/Balance-ChemicalEquations
Unit 3: Chemistry. Counting Atoms and Balancing Equations.
Tips when Balancing
• You can ONLY modify the coefficient #
• Balance H2 and O2 last
• If you have 3 hydrogen atoms on one side and 2 hydrogen atoms on
another, try finding a common number between the two.
E.g.
_H2 + _N2  _NH3
H: 2
H: 6
H:3
H:6
(Unbalanced)
(Balanced) - New Equation
3H2 + _N2 2 NH3
• Always double check your work by doing a final atoms count