Counting Atoms in a Chemical
Formula
How many atoms?
• The symbol for an atom is either one capital
letter, or a capital letter followed by a lower case
letter. For example:
• F is the symbol for Fluorine
• Br is the symbol for Bromine
• If you write 2 capital letters together, it means
they are a compound. CO is carbon monoxide,
but Co is cobalt.
Subscripts
• Subscript-small number that is written below
and after the element. It indicates how many
atoms of that element are present in a
substance. For example, CO2 tells you there is
one atom of carbon and 2 atoms of oxygen in
this molecule of carbon dioxide.
• In CO2, since there is only 1 atom of carbon
present, the subscript is 1 but is not written.
Coefficients
• Coefficient-regular sized number in front of a
chemical formula. It represents the number of
molecules in that formula.
• 2CO tells you there are 2 carbon atoms and 2
oxygen atoms in this molecule of carbon
monoxide. You must multiply the 2 by BOTH
atoms.
Coefficients with Subscripts
When you have both a coefficient and a
subscript as part of a chemical formula, you
must follow the order of operations as follows:
Example 2H2SO4
Step 1- Find each atom and its number
H-number of subscripts = 2 x coefficient of 2 = 4
hydrogen total
Coefficients with Subscripts
Example: 2H2SO4
• Step 2- starting on the left side, determine the
number of atoms per subscripts for the first
element.
H2 shows 2 hydrogen atoms.
• Step 3 –is there a coefficient?
If no, then that is the number of atoms in that molecule
If yes, then multiply the coefficient by the subscript
number.
2H2 = 4 hydrogen atoms
Example: 2H2SO4
How many sulfur atoms are there?
S = 1 (subscript) X 2 (coefficient) for a total of 2
sulfur atoms
How many oxygen atoms are there?
O = 4 (subscript) X 2 (coefficient) for a total of 8
oxygen atoms
Practice
2CaCO3
Ca = 1 (subscript) X 2 (Coefficient)
Ca = 2 atoms
C= 1 (subscript) X 2 (Coefficient)
C = 2 atoms
O= 3 (subscript) X 2 (Coefficient)
O = 6 atoms