Chemical Formulas and Equations

​What is a Chemical Formula?

1. Definition:
   A chemical formula shows the types of atoms and the number of each type in a compound.

2. Water: H₂O

   • How many hydrogen atoms? ______

   • How many oxygen atoms? ______

II. What is a Subscript?

1. Definition:
   A small number written to the lower right of an atom’s symbol that tells you how many of that atom are present.

2. Example:
   H₂O

   • What does the “₂” next to hydrogen (H) mean? ___________

3. If there is no subscript:
   It means there is only ______ atom of that element.

III. What is a Coefficient?

1. Definition:
   A large number written in front of a chemical formula that multiplies the entire formula.

2. Example:
   2H₂O

   • What does the “2” in front of H₂O mean? ________

   • How many hydrogen atoms in total? __________

   • How many oxygen atoms in total? __________

IV. Counting Atoms in a Chemical Formula

Step-by-Step Process:

1. Identify the atoms:
   Write down the symbols of all the elements in the formula.

2. Look at subscripts:
   Write down how many of each atom you have (if there is no subscript, assume it is 1).

Look at coefficients:
Multiply the number of atoms of each element by the coefficient.

V. Practice Example 1: CO₂

• Step 1: Identify the elements: C, O

• Step 2: Subscripts:

  • Carbon (C): ______

  • Oxygen (O): ______

• Step 3: Coefficient (assume 1 if none): ______

• Total number of atoms:

  • Carbon (C): ______

  • Oxygen (O): ______

VI. Practice Example 2: 3NaCl

• Step 1: Identify the elements: Na, Cl

• Step 2: Subscripts:

  • Sodium (Na): ______

  • Chlorine (Cl): ______

• Step 3: Coefficient: 3

• Total number of atoms:

  • Sodium (Na): ______

  • Chlorine (Cl): ______

VII. Complex Formula Example: 2Ca(OH)₂

• Step 1: Identify the elements: Ca, O, H

• Step 2: Subscripts:

  • Calcium (Ca): ______

  • Oxygen (O): ______

  • Hydrogen (H): ______

• Step 3: Coefficient: 2

• Total number of atoms:

  • Calcium (Ca): ______

  • Oxygen (O): ______

  • Hydrogen (H): ______

VIII. Practice Problem Set: Counting Atoms

1. CH₄

   • Carbon (C): ______

   • Hydrogen (H): ______

2. 2H₂SO₄

   • Hydrogen (H): ______

   • Sulfur (S): ______

   • Oxygen (O): ______

3. 4Fe₂O₃

   • Iron (Fe): ______

   • Oxygen (O): ______

IX. Chemical Equations

1. Definition:
   A chemical equation represents a chemical reaction, showing reactants (what you start with) and products (what you end with).

General Form:
Reactants → Products

X. Balancing Chemical Equations

1. Why Balance?
   The number of atoms of each element must be the same on both sides of the equation to follow the Law of Conservation of Mass.

2. Example:
   H₂ + O₂ → H₂O (Is this equation balanced? ______ Why or why not? ______)