Unit 3.3 Chemical Formulas and Molecular Models
15 minutes – Types of chemical formulas
The quickest and easiest way to represent a compound is with a chemical formula. Chemical formulas indicate what
elements are present in a compound and how many of each type of element there are. A good example of a chemical
formula is the chemical formula for water
Chemical symbol for
Hydrogen
H2O
Chemical symbol for
Oxygen
Subscripts indicate how many of each type of atom are
present. In this case the 2 represent two hydrogen atoms. If
there is no subscript, then it is understood as one atom. So
there is only one oxygen.
Chemical formulas usually list the more metallic and or the most positive ions first. In the case of H2O neither element is
a metal, but hydrogen forms a (+) cation and oxygen forms a (-) anion, that is why Hydrogen is listed first. In the
chemical compound NaCl, sodium is a metal and forms a cation, chlorine is a nonmetal that forms an anion, so sodium is
listed first. There are various types of chemical formulas. Empirical formulas provides the relative number of atoms of
each element in a compound. Molecular formulas provide the actual number of atoms of each of the elements in a
molecule or compound. For example the empirical formula for hydrogen peroxide is HO. This means that for every one
Hydrogen there is one oxygen, the molecular formula for hydrogen peroxide is H2O2 and shows the actual numbers of
each element in one molecule of hydrogen peroxide. The molecular formula is always a whole number ratio of the
empirical formula. For some compounds the molecular formula and the empirical formula are the same, an example
would be water H2O.
Structural formulas use lines to represent covalent bonds and shows how atoms in a molecule are connected. The
structural formula for hydrogen peroxide is H-O-O-H .The number of lines between each element indicate the number of
shared pairs of electrons between the two. All bonds in hydrogen peroxide are single lines and indicate one shared pair
of electrons. It is possible to also have a double bond (O=C=O) and a triple bond (NΞN). We will discuss the reason
behind these types of bonds in more depth a little later in the course, for now you just need to know what they
represent.
1. List and explain the different ways to represent compounds.
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2. What is the difference between an empirical formula and a molecular formula?
3. Write the correct molecular formula for a compound that contains one oxygen atom and two lithium atoms.
4. Determine the number of each type of atom in each formula
a. BaCl2
______________________
_____________________
b. Fe(NO2)2
_____________________
______________________
_____________________
c. 2Ca(OH)2
____________________
______________________
______________________
5. Write the empirical formulas for the compounds represented by the following molecular formulas.
a. C4H8 ____________________
b. B2H6 _______________________________
c. CCl4
____________________
d. C5H12 ____________________
e. Hg2Cl2 ____________________
f. C2H4O2 ____________________
6. Write the empirical formula and molecular formula for the following molecule.
H-CΞC-H
Empirical formula
Molecular Formula
______________
______________
for class discussion
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