Molecular Elements
There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is composed of more than one atom.
Table \(\PageIndex{1}\): Elements That Exist as Diatomic Molecules
Hydrogen, H |
Oxygen |
Nitrogen |
Fluorine |
Chlorine |
Bromine |
Iodine |
Some elements exist naturally as molecules. For example, hydrogen and oxygen exist as two-atom molecules. Other elements also exist naturally as diatomic molecules—a molecule with only two atoms (Table \(\PageIndex{1}\)). As with any molecule, these elements are labeled with a molecular formula, a formal listing of what and how many atoms are in a molecule. (Sometimes only the word formula is used, and its meaning is inferred from the context.) For example, the molecular formula for elemental hydrogen is H2, with H being the symbol for hydrogen and the subscript 2 implying that there are two atoms of this element in the molecule. Other diatomic elements have similar formulas: O2, N2, and so forth. Other elements exist as molecules—for example, sulfur normally exists as an eight-atom molecule, S8, while phosphorus exists as a four-atom molecule, P4 (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Molecular Art of S
8 and P
4 Molecules. If each green ball represents a sulfur atom, then the diagram on the left represents an S
8 molecule. The molecule on the right shows that one form of elemental phosphorus exists, as a four-atom molecule.
Figure \(\PageIndex{1}\) shows two examples of how molecules will be represented in this text. An atom is represented by a small ball or sphere, which generally indicates where the nucleus is in the molecule. A cylindrical line connecting the balls represents the connection between the atoms that make this collection of atoms a molecule. This connection is called a chemical bond.
Ionic Compounds
The elements in the periodic table are divided into specific groupings; the metals, the non-metals, the semi-metals, and so on. These groupings are largely based on physical properties and on the tendency of the various elements to bond with other elements by forming either an ionic or a covalent bond. As a general rule of thumb, compounds that involve a metal binding with either a non-metal or a semi-metal will display ionic bonding. Thus, the compound formed from sodium and chlorine will be ionic (a metal and a non-metal). The basic unit of ionic compounds is the formula unit.
Molecular Compounds
Compounds that are composed of only non-metals or semi-metals with non-metals will display covalent bonding and will be classified as molecular compounds. Nitrogen monoxide (NO) will be a covalently bound molecule (two non-metals) and silicon dioxide (SiO2) will also be a covalently bound molecule (a semi-metal and a non-metal). The basic unit of molecular compounds is the molecule.
Example \(\PageIndex{1}\)
Provide the classification (i.e. atomic element, molecular element, molecular compound, or ionic compound) of each substance.
- Fe
- PCl3
- LiBr
- P4
- oxygen gas
Solution
- Fe (iron) is an element that is represented with no subscript, so it is an atomic element.
- PCl3 is made up of two nonmetals, so it is a molecular compound.
- LiBr is made up of lithium, a metal, and bromine, a nonmetal, so it is an ionic compound.
- P4 is a substance that is made up of four atoms of the same element, so it is a molecular element.
- The formula for oxygen gas is O2 so it is a molecular element.
Exercise \(\PageIndex{1}\)
Provide the classification (i.e. atomic element, molecular element, molecular compound, or ionic compound) of each substance.
- I2
- He
- H2O
- Al
- CuCl
- Answer a:
- molecular element
- Answer b:
- atomic element
- Answer c:
- molecular compound
- Answer d:
- atomic element
- Answer e:
- ionic compound