When atoms come together to form ionic or covalent bonds, they form stable associations that change the properties of either atom alone. These new molecules exhibit unique emergent properties. A molecule exists when two or more atoms join together by forming chemical bonds. A single molecule can be as small as only two atoms, like the oxygen we breathe, or as large as 5 sextillion (5 with 21 zeroes after it!) carbon atoms, like a 1⁄2 carat diamond in an engagement ring.
When atoms of at least two different elements come together to form chemical bonds, these molecules can be called compounds. Sodium chloride (NaCl) is a classic example of an ionic compound, or compound formed by ionic bonds. Water (H2O) is often called a molecular compound, but is also known as a covalent compound because it is a compound formed by covalent bonds. Although less common, some biology textbooks refer to molecular compounds as molecules, and do not include ionic compounds in the term molecule. For clarity we will use the more common definition of molecule, which includes all types of compounds as well as molecules formed by atoms of a single element such as the examples of molecular oxygen and molecular carbon (diamonds).
All molecules can be written as a list of the atoms forming them. We write NaCl to represent the molecular formula for table salt, which is formed by an ionic bond between sodium (Na) and chlorine (Cl). Molecular formulas are also used for covalent molecules like water (H2O). When there is more than one atom of a particular element in a single molecule, we use a subscript number to the right of the atom’s symbol to indicate how many atoms of that element are present in each molecule. For example, the molecular formula for water (H2O) indicates there are two hydrogen atoms and one oxygen atom in each individual molecule. As this example also points out, the atoms in a molecular formula are not always written in order of their bonds. The structural formula for water is H-O-H, but when we write the molecular formula, we combine the hydrogen atoms into a single listing, H2O, for simplicity.
Molecule or Compound?
This activity tests your ability to determine if a molecular formula represents a molecule, an ionic compound, or a covalent compound.
Molecular and Structural Formulas
In this activity, you will match a molecule's molecular forumla to its corresponding structural formula.