The Rearranging of Atoms
Rearranging Atoms
 When two or more atoms
combine and bond to form a
new substance, a chemical
change occurs.
Evidences for a
Chemical Change
A change in color
A solid (precipitate) forms
Gas is produced
A change in Energy
The Rearrangement of Atoms
 Chemical reactions always involve the
rearrangement of the way atoms are
grouped.
 For example: When methane, CH4,
combines with oxygen, O2, in the air and
burns, carbon dioxide, CO2, and water,
H2O, are produced.
Chemical Reaction Between
O2 & CH4
 We represent this chemical reaction by
writing a chemical equation.
 methane + oxygen  carbon dioxide + water
 CH4
+
O2

CO2
+
H2O
Chemical Equations
 The chemicals represented before the reaction
(to the left of the arrow), are called reactants. The
chemicals produced by the reaction (to the right
of the arrow), are called products. The arrow
indicates the direction of the change and is read as
“yields” or “produces”.
reactants
 CH4
+ O2
produce

products
CO2 + H2O
Reactants & Products
 CH4
+
O2

CO2
+ H2O
 Notice, there are the same KINDS of
atoms on both sides of the chemical
equation. We begin with carbon, hydrogen
and oxygen, (C, O & H), and end with
carbon, hydrogen and oxygen.
The Law of Conservation
of Mass
 In chemical reactions, atoms are neither created nor
destroyed. ALL atoms present in the reactants must
be accounted for in the products. The mass of the
reactants equal the mass of the products.
 The same number of each type of atom on the reactant
side must appear on the product side.
 Obeying the law of conservation of mass in chemical
equations is called balancing the chemical
equation.
Balancing Chemical Equations
 The equation for the reaction between methane and
oxygen, is not balanced.




CH4
+ O2

one carbon atom
four hydrogen atoms
two oxygen atoms
CO2
+ H2O
one carbon atom
two hydrogen atoms
three oxygen atoms
 As written, this equation states that one oxygen atom
was created and two hydrogen atoms were destroyed.
Balancing Chemical Equations
 CH4
+ O2

CO2
+ H2O
 We can fix this imbalance by involving another O2
molecule on the left (reactants) and displaying the
production of more H2O molecules on the right
(products).





CH4 + O2 + O2  CO2 + H2O + H2O
one carbon atom
one carbon atom
four hydrogen atoms
four hydrogen atoms
four oxygen atoms
four oxygen atoms
This chemical equation is now balanced!
A Balanced Chemical Equation
 methane + oxygen  carbon dioxide
+ water
 The rearrangement below leaves no leftovers.
Coefficients
 CH4 + O2 + O2

CO2 + H2O + H2O
 When we write balanced chemical equations we group
like molecules together. The finished equation is below.
 CH4
+
2O2

CO2
+
2H2O
 The numbers in front of the O2 and the H2O molecules
are called coefficients. Coefficients are written in front
of atoms and molecules. They multiply all atoms to their
right and tell us how many of that particular molecule or
atom is needed to balance the chemical equation.
Subscripts vs. Coefficients
 A subscript is a small number written to the
right of an atom in a formula indicating how
many atoms of that element are present in
the formula.
 A coefficient is a number written in front of
a reactant or product that indicates the
smallest number of particles of the
substance that are involved in the reaction.
Balancing Chemical Equations
 CH4
+
2O2

CO2
+
2H2O
 Notice we did not change any of the actual atoms.
 We did not change any subscripts.
 We did not take any molecules away.
 When balancing chemical equations, you can only
add more of the same molecules or atoms.
Another Example
 Oxygen combines with hydrogen to produce water.
 O2
+
H2

H2O
 First count the number of atoms on each side of the
chemical equation.
 O2
+
H2
___ oxygen atoms
___ hydrogen atoms

H2O
___ oxygen atom
___ hydrogen atoms
 We need more _________ atoms on the __________ side.
Water
 Oxygen is only present in H2O on the product side. The
only way to add more oxygen atoms to the product side
of the equation is to add more H2O molecules.
 O2
+
H2

2H2O
 We add the coefficient 2 in front of water and count
again. Coefficients multiply everything to their right.
____ oxygen atoms
____ hydrogen atoms
____ oxygen atoms
____ hydrogen atoms
Water
 O2
+
H2
2H2O

 This equation is not yet balanced.
 We need more _______ atoms on the __________side.
 We add the coefficient ___ in front of the ________
molecule on the __________ side and count again.
 O2
+
__H2

2H2O
 The chemical equation is now balanced!
2H2 + O2  2H2O
Diatomics
 There are seven elements that are not found
in nature as single atoms. The molecules
formed are more stable than the individual
atoms. If they are not in a chemical
compound with other atoms, they pair up
with another atom of their own kind.
 Br2 I2 N2 Cl2 H2 O2 F2
 Notice how the diatomics were used in the
chemical equations we looked at earlier.