When analyzing aluminum fluoride, or \(\text{AlF}_3\), the bond that forms between aluminum and fluorine is overwhelmingly classified as ionic. This classification is based on the significant difference in the electron-attracting power of the two elements, a measurement chemists use to define the nature of the chemical connection.
Defining Ionic and Covalent Bonds
The two primary types of chemical connections are ionic and covalent bonds, which represent two different ways atoms achieve stability. An ionic bond involves the complete transfer of one or more valence electrons from one atom to another. This transfer typically occurs between a metal atom, which tends to lose electrons, and a nonmetal atom, which tends to gain them.
The resulting ionic compound is held together by the strong electrostatic attraction between these oppositely charged ions, often forming a crystalline lattice structure. A covalent bond, conversely, involves the mutual sharing of valence electrons between two atoms.
Covalent bonds usually form between two nonmetal atoms. Instead of a complete transfer, the shared electrons orbit the nuclei of both atoms. This sharing can be equal (nonpolar covalent bond), or unequal (polar covalent bond) where electrons spend more time near one atom.
How Electronegativity Determines Bond Type
Chemists use the concept of electronegativity to quantitatively determine where a bond falls on the spectrum between purely covalent and purely ionic. Electronegativity is defined as an atom’s power to attract electrons to itself within a chemical bond. A numerical value is assigned to each element, with fluorine having the highest value.
The nature of the bond is determined by calculating the difference in electronegativity (\(\Delta \text{EN}\)) between the two bonded atoms. A small difference in values indicates that electrons are shared fairly, resulting in a covalent bond. A large difference, however, signifies that one atom is pulling the shared electrons much more strongly than the other.
Specific numerical cutoffs are generally used for classification. A difference of less than 0.4 typically points to a nonpolar covalent bond, whereas a difference between 0.4 and approximately 1.7 suggests a polar covalent bond. When the electronegativity difference is greater than 1.7, the bond character is considered predominantly ionic, reflecting the high likelihood of electron transfer rather than sharing.
Analyzing Aluminum Fluoride (\(\text{AlF}_3\))
Applying the electronegativity principle to aluminum fluoride confirms its overwhelmingly ionic nature. Aluminum (\(\text{Al}\)), a metal, has an electronegativity value of 1.61, while fluorine (\(\text{F}\)), the most electronegative nonmetal, has a value of 3.98. The difference between these two values is \(3.98 – 1.61 = 2.37\).
This calculated difference of 2.37 is significantly higher than the 1.7 threshold commonly used to define an ionic bond. The immense difference in electron attraction means that fluorine atoms essentially strip the valence electrons from the aluminum atom, forming \(\text{Al}^{3+}\) cations and \(\text{F}^{-}\) anions. This electron transfer establishes the powerful electrostatic forces characteristic of ionic compounds.
The physical properties of \(\text{AlF}_3\) strongly support this ionic classification. Aluminum fluoride is a white, crystalline solid that exists as a three-dimensional network structure, which is typical for ionic lattices. Furthermore, it has an extremely high sublimation point of 1290 °C, meaning it turns directly from a solid to a gas without melting under atmospheric pressure. The vast amount of energy required to break the strong electrostatic forces within its lattice confirms its ionic character.