Ionic character refers to the degree of electron sharing in a chemical bond. A purely ionic bond involves complete transfer of electrons, while a purely covalent bond involves equal sharing. Most bonds fall somewhere in between, exhibiting partial ionic and covalent character. Determining How To Compare Ionic Character can be complex, as demonstrated by the conflicting information surrounding alkali metal hydrides (LiH, NaH, KH, RbH, CsH).
Factors Influencing Ionic Character
Several factors influence the ionic character of a bond. Electronegativity difference is a key factor, with larger differences leading to greater ionic character. Fajans’ rules, which consider cation size and anion polarizability, also play a role. Generally, larger cations and smaller, less polarizable anions favor ionic character. Lattice energy, the energy required to separate ions in a crystal lattice, is another important consideration. Higher lattice energy indicates stronger ionic interactions.
Conflicting Views on Alkali Metal Hydrides
The ionic character of alkali metal hydrides presents a fascinating case study. Fajan’s rule suggests that ionic character should increase down the group (LiH < NaH < KH < RbH < CsH) due to increasing cation size. However, conflicting explanations exist:
Argument for Increasing Ionic Character Down the Group
One perspective emphasizes the decreasing ionization enthalpy of alkali metals down the group. As ionization enthalpy decreases, the tendency to form cations increases, suggesting a corresponding increase in the ionic character of the hydrides. This aligns with the general principle that lower ionization energy favors ionic bond formation.
Argument for Decreasing Ionic Character Down the Group
An alternative perspective focuses on lattice energy. Because the hydride anion (H-) is small, the size difference between the cation and anion becomes more significant as you move down the group. This increasing size disparity weakens the electrostatic attraction between ions, leading to lower lattice energy and, consequently, reduced ionic character. In this view, the small size of the hydride anion overrides the effect of decreasing ionization enthalpy.
Resolving the Discrepancy: Considering Charge Density
The key to understanding the ionic character trend in alkali metal hydrides lies in considering charge density. Lithium, with its small size, has a high charge density. This results in a strong electrostatic attraction to the hydride anion, forming a strong ionic bond with high lattice energy. As cation size increases down the group, charge density decreases, weakening the electrostatic attraction and leading to lower lattice energy. Thus, while ionization enthalpy decreases down the group, the decreasing charge density and resulting weaker lattice energy dominate, ultimately leading to a decrease in ionic character.
Conclusion: Ionic Character Decreases Down the Group
Considering charge density and lattice energy provides a more accurate understanding of the ionic character trend in alkali metal hydrides. While Fajan’s rule generally holds true, the unusually small size of the hydride anion necessitates a nuanced approach. Ultimately, the ionic character of alkali metal hydrides decreases down the group due to the decreasing charge density of the cations and the corresponding decrease in lattice energy. Therefore, the correct order of decreasing ionic character is: LiH > NaH > KH > RbH > CsH.
