THE FORMULA UNIT
Editor’s Note: In this piece, Genesis Andrade (‘19) explains the often confusing differences between molecules, atoms, and the formula unit.
Molecule, atom, and formula unit are terms that are often mistaken for one another in classrooms. The least popular of the three is the formula unit, which is used empirically to describe the smallest unit of an ionic or covalent compound. The formula unit can be thought of as an “ionic molecule.” However, unlike an actual molecule, a formula unit describes the smallest whole number ratio composition within a network of cations and anions stuck together. For example, in a compound such as NaCl, sodium and chlorine atoms are ionically bonded in order to form a lattice structure. This means that sodium, the cation, must lose an electron so that chlorine can have a full valence shell. The formula unit is set at a 1:1 ratio and so there would be chlorine anions surrounding sodium cations on all sides in the lattice structure.
Molecules only apply to covalent compounds and describe two or more elements covalently bonded, meaning that they share electrons from their valence shells to fill each other. In this way, all of the elements in the molecule will be able to stabilize each other’s valence shells. The difference between a molecule and a formula unit is important to note because of how subtle yet related they are. They both describe the smallest unit of a kind of compound, but molecules can be more easily separated from each other than formula units due to the properties of different kinds of bonding. Because ionically bonded compounds are very structured and close together, shifting the formula units in a solid ionic compound would cause the solid to break into separate pieces because the components would no longer be in the lattice structure and thus, it would break into smaller solids made up of these same lattice structured formula units. Molecules can shift around more easily than formula units since the structure of the composition is not as strictly defined.
The main requirement of a covalent compound is that the atoms in the molecules remain together because of the electrons shared among the elements. We recognize the term molecule more easily since we’ve studied water which is composed of H2O molecules, and we recognize the term atom since we’ve been taught from earlier on that all matter is composed of atoms. Although the terms molecule and atom have been heard and seen more often in the world of chemistry among students, formula units play just as big of a role, and knowing the meaning of the term itself helps to avoid confusion and certain mistakes involving different kinds of compounds and bonding.
Molecules only apply to covalent compounds and describe two or more elements covalently bonded, meaning that they share electrons from their valence shells to fill each other. In this way, all of the elements in the molecule will be able to stabilize each other’s valence shells. The difference between a molecule and a formula unit is important to note because of how subtle yet related they are. They both describe the smallest unit of a kind of compound, but molecules can be more easily separated from each other than formula units due to the properties of different kinds of bonding. Because ionically bonded compounds are very structured and close together, shifting the formula units in a solid ionic compound would cause the solid to break into separate pieces because the components would no longer be in the lattice structure and thus, it would break into smaller solids made up of these same lattice structured formula units. Molecules can shift around more easily than formula units since the structure of the composition is not as strictly defined.
The main requirement of a covalent compound is that the atoms in the molecules remain together because of the electrons shared among the elements. We recognize the term molecule more easily since we’ve studied water which is composed of H2O molecules, and we recognize the term atom since we’ve been taught from earlier on that all matter is composed of atoms. Although the terms molecule and atom have been heard and seen more often in the world of chemistry among students, formula units play just as big of a role, and knowing the meaning of the term itself helps to avoid confusion and certain mistakes involving different kinds of compounds and bonding.
