Chemical Formulas and Equations
Matter and Energy
Chemical Formulas and Equations
Part I: The Big Picture
Reminder: H is the element symbol for Hydrogen.
H2 is the chemical formula for hydrogen. The subscript 2 after the H
means that two atoms of hydrogen make up a single molecule of
hydrogen gas. Subscripts for a particular kind of molecule never
change because these numbers describe how many of an atom type
are present in a particular molecule.
Reminder: O is the element symbol for Oxygen.
O2 is the chemical formula for oxygen. The subscript 2 after the O means that two atoms of oxygen
make up a single molecule of oxygen gas. Remember that a subscript describes how many of an
atom type are in the molecule and does not change.
H2O is the chemical formula for water. The subscript 2 after the H means two hydrogen atoms and
one oxygen atom make up a single molecule of water. Remember that a subscript describes how
many of an atom type are in the molecule and does not change.
If an atom within a molecule is not followed by a subscript, then only one of that atom type is
present in the molecule.
For example:
A molecule of…
Is represented by the
chemical formula…
And is the same as…
Water
H 2O
H2O1
Carbon dioxide
CO2
C1O2
Continue with Part I on the next page.
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Chemical Formulas and Equations
Matter and Energy
Part I: The Big Picture, continued
Subscripts in a chemical formula do not change because they identify the substance. For
example, water is always H2O, never H3O. It doesn’t matter if water is a reactant or a product;
water is always identified as H2O to represent 2 atoms of hydrogen and 1 atom of oxygen in every
molecule of water.
However, the total number of water molecules present in a reaction can change. For example
2H2O means that two molecules of water are present, 3H2O means that three molecules of water
are present, 4H2O means that four molecules of water are present and so on. In these examples,
the numbers of 2, 3 and 4 before H2O is called a coefficient and means how many molecules are
present.
For example:
Is represented
by the
chemical
formula…
And when a
coefficient
appears before
the chemical
formula…
Water
H 2O
4H2O
Carbon dioxide
CO2
3CO2
A molecule
of…
Carbon dioxide
CO2
CO2
Then, that is the number of
molecules present.
If a coefficient does not appear
before the chemical formula, that
means just one molecule is
present.
4 water molecules
3 carbon dioxide molecules
1 carbon dioxide molecule
Means the same thing as 1CO2
Continue with Part I on the next page.
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Chemical Formulas and Equations
Matter and Energy
Part I: The Big Picture, continued
Now we know that the coefficient before the H2 and the H2O means there are 2 molecules of each
substance.
Let us break this chemical reaction down further and look at what happens.
2H2 + O2
2H2O
In this chemical reaction, the black dots represent hydrogen atoms and the white dots represent
oxygen atoms. The gray lines represent the atoms that are held together within the molecule. In
a chemical reaction, energy (represented by red Xs) pulls the atoms apart in the reactant
molecules. The atoms of the reactant molecules separate allowing them to re-combine to form the
product molecules.
Answer the questions for Part I in your Student Journal.
Part II: Chemical Formulas
Now that you know some of the basics, let us look at each part of the chemical reaction in further
detail. In a chemical reaction, substances react with each other to make new substances with
different properties. Each molecule is represented by a specific chemical formula.
A chemical formula tells you how many atoms of each element are found within each substance.
The numbers of atoms within a molecule are shown by the subscripts, which are found after each
elemental symbol. If there is no subscript after the atom, then there is only one atom of that
element in the substance.
Continue with Part II on the next page.
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Chemical Formulas and Equations
Matter and Energy
Part II: Chemical Formulas, continued
Glucose C6H12O6 Number of Carbon Atoms = 6 Number of Hydrogen Atoms = 12 Number of Oxygen Atoms = 6 You will work with your group to identify the type and number of atoms in a chemical formula and
make models to represent chemical formulas. Use different cube colors to represent each element
in the chemical formula as follows:
Red = Hydrogen
Orange = Copper
Light Brown = Oxygen
Dark Blue = Sodium
Green = Iodine
Yellow = Chlorine
Dark Brown = Zinc
Light Blue = Carbon
White = Potassium
Black = Magnesium
Procedure:
1. 
2. 
3. 
4. 
Identify the elements in the chemical formula.
Determine the number of atoms of each element in the chemical formula.
Use the color cubes to make a model of the chemical formula.
Draw colored boxes to represent the atoms in your chemical formula.
Complete the activity for Part II in your Student Journal.
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Chemical Formulas and Equations
Matter and Energy
Part III: Chemical Equations
A chemical equation uses chemical formulas and symbols to represent a chemical reaction.
Instead of saying: six molecules of carbon dioxide plus six molecules of water react to form one
molecule of glucose plus six molecules of oxygen, scientist use equations to represent the
reaction. Mathematicians use equations, too. For example, two plus two equals four is the same
as writing 2+2=4.
The following chemical equation represents the chemical reaction known as photosynthesis.
6CO2 + 6H2O ! C6H12O6 + 6O2
coefficient
reactants
coefficientproducts
coefficient
The chemical formulas in a chemical equation represent the reactants and products. Reactants are
the substances that take part in and undergo change during a chemical reaction. The products are
the substances produced during a chemical reaction. The coefficient is a number placed in front of
a chemical symbol or chemical formula to represent the number of molecules of each substance.
Work with your group to identify the number and types of atoms in the reactant and products of
several different chemical reactions. You will use the color cubes to model the chemical reaction
represented by each chemical equation.
Procedure
1.  List the elements present on the reactant and product sides of the equation.
2.  Determine the number of atoms of each element on the reactant and product sides of the
equation.
3.  Use the color cubes to make a model of the substances on the reactant side of the equation.
Then draw a picture of your model in the chart in your Student Journal.
4.  Place your reactant models in the plastic bag. Use your hand to break apart the models. This
action represents the energy required to break the bonds between the atoms in a substance.
5.  Use the color cubes in the bag to build models for the products. Then draw a picture of your
model in the chart in your Student Journal.
Complete the activity in Part III of your Student Journal.
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Chemical Formulas and Equations
Matter and Energy
Part IV: The Balancing Act
Before we begin this section, turn to Part IV in your Student Journal and complete the table
following the same procedures you used in Part III for the following equation.
Zn + HCl ! ZnCl2 + H2
How is this chemical equation different from the equations in Part III? Did you notice that there
were not enough cubes to build the reactants?
All of the equations that you used in Part III were balanced equations. The coefficients were
included so that both sides of the reaction contained an equivalent number of atoms of each
element. The equation above is an unbalanced equation, because it does not contain an equal
number of atoms of each element on both sides of the equation.
The Law of Conservation of Mass states that matter is neither created nor destroyed, but can be
rearranged. Thus, the mass of all the reactants must equal the mass of all the products. In a
chemical reaction, the number and types of atoms present in the reactants will be equal to the
number and types of atoms in the products. A balanced chemical equation follows the Law of
Conservation of Mass.
Complete Part IV in your Student Journal and answer the Reflection and Conclusion questions.
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