Which one of the following elements forms compounds with pronounced covalent character?
1. The cation is small.
2. The cation has a high charge (not applicable here as all form +1 ions).
3. The anion is large (not directly comparable here as the anion is not specified, but assume it’s a common one like a halide).
4. The cation has a pseudo noble gas configuration (not applicable here as alkali metals form noble gas configuration ions).
The given elements are alkali metals: Lithium (Li), Sodium (Na), Potassium (K), and Rubidium (Rb). When they form compounds, they form +1 ions: Li⁺, Na⁺, K⁺, Rb⁺.
Their ionic radii increase down the group: Li⁺ < Na⁺ < K⁺ < Rb⁺. According to Fajans' Rules, smaller cations have higher polarizing power (the ability to distort the electron cloud of the anion), which leads to increased covalent character in the bond. Li⁺ is the smallest cation among Li⁺, Na⁺, K⁺, and Rb⁺. Therefore, Li⁺ has the highest polarizing power and forms compounds with anions that exhibit the most pronounced covalent character compared to the compounds of Na, K, and Rb with the same anion. For example, LiCl has significantly more covalent character than NaCl, KCl, or RbCl.