5.5 Molecular Formula and Structural Formula

Molecular Formula:

• The molecular or chemical formula represents the types and numbers of atoms in a molecule of an element or compound. It uses chemical symbols to show what elements are present and how many atoms of each element exist.

• For example, the molecular formula for propane (a compound) is C3H8. This tells us that propane consists of three carbon (C) atoms and eight hydrogen (H) atoms.

Structural Formula:

• The structural formula expresses the organization of atoms in a molecule and the bonds between them. It shows how atoms are connected by bonds, giving insight into the molecule’s structure.

• For propane, the structural formula is:

H   H   H

|   |   |

H-C-C-C-H

|   |   |

H   H   H

• In this example, C represents carbon and H represents hydrogen. Each line between carbon atoms or between carbon and hydrogen atoms represents a covalent bond (which you will learn about in this chapter). The carbon atoms form a chain, and hydrogen atoms fill in to complete the valency of carbon (which is 4).

Example of Other Molecules:

1. Water (H2O):

• Molecular formula: H2O.

• Structural formula:

H-O-H

• This shows two hydrogen atoms are bonded to one oxygen atom.

2. Methane (CH4):

• Molecular formula: CH4.

• Structural formula:

H

|

C

|

H

|

H

|

H

• In methane, one carbon atom is bonded to four hydrogen atoms.

Key Points:

• The molecular formula gives the number and types of atoms, but the structural formula shows how these atoms are arranged and bonded.

• The structural formula provides more detail than the molecular formula, helping us understand the shape and bonding in the molecule.