5.3: A Closer Look at Elements and Compounds
- Page ID
- 289374
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- Classify substances as atomic elements, molecular elements, molecular compounds, or ionic compounds.
Atomic Elements
Most elements exist with individual atoms as their basic unit. It is assumed that there is only one atom in a formula if there is no numerical subscript to the right of a chemical symbol.
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 retains the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is comprised of two or more atoms.
Some elements exist naturally as molecules. For example, hydrogen and oxygen exist as two-atom molecules, called diatomic molecules. A complete list of elements that exist naturally as diatomic molecules are shown 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 single molecule.
| Hydrogen | Oxygen | Nitrogen | Chlorine | Bromine | Iodine | Fluorine |
| H2 | O2 | N2 | Cl2 | Br2 | I2 | F2 |
It would be helpful to memorize the elements that exist naturally as diatomic molecules. One way to do this is to use a mnemonic. The elements in Table \(\PageIndex{1}\) were purposefully arranged to provide such a mnemonic. When arranged in this order, the elements H, O, N, Cl, Br, I, F may be pronounced phonetically as HONK-uhl-briff.
Other elements exist as polyatomic molecules. For example, sulfur naturally exists as an eight-atom molecule, S8, while phosphorus exists as a four-atom molecule, P4 , as shown in Figure \(\PageIndex{1}\) below.
Ionic Compounds
The elements in the periodic table are divided into specific groupings: the metals, the nonmetals, the metalloids, 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 bonding with either a nonmetal or a metalloid will display ionic bonding.
Figure \(\PageIndex{2}\) shows an atomic level view of sodium chloride. This compound would be expected to be ionic since a metal, Na, is bonded to a nonmetal, Cl. Since the ratio of sodium ions to chloride ions is 1:1, the formula for sodium chloride is NaCl. The basic unit of an ionic compound that retains the identity of the compound is called a formula unit. It is incorrect to refer to the basic unit of an ionic compound as a molecule (refer to the section on Molecular Compounds below). One formula unit of NaCl contains one sodium ion and one chloride ion.
Molecular Compounds
Compounds that are composed of only nonmetals and/or metalloids with non-metals will display covalent bonding and will be classified as molecular compounds. Examples of molecular compounds include nitrogen monoxide (NO), consisting of two nonmetals covalently bonded together, and silicon dioxide (SiO2), consisting of a metalloid covalently bonded to a nonmetal. 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
This page is shared under a CK-12 license and was authored, remixed, and/or curated by Melissa Alviar-Agnew, Henry Agnew, and Lance S. Lund (Anoka-Ramsey Community College). Original source: https://www.ck12.org/c/chemistry/.
