Questions: Which of these ionic salts would you expect to have the highest melting point based on Coulombic interactions? Ca3 P2 Mgg P2 Be3 P2 Srg P2

Solution Steps

Coulombic interactions, or electrostatic forces, between ions in an ionic compound are determined by the charges of the ions and the distance between them. The formula for the force of attraction is given by:

\[ F = \frac{k \cdot |q_1 \cdot q_2|}{r^2} \]

where \( F \) is the force of attraction, \( k \) is Coulomb's constant, \( q_1 \) and \( q_2 \) are the charges of the ions, and \( r \) is the distance between the centers of the ions.

For each compound, we need to consider the charges of the cations and anions:

• \(\mathrm{Ca}_3\mathrm{P}_2\): Calcium (\(\mathrm{Ca}^{2+}\)) and Phosphide (\(\mathrm{P}^{3-}\))
• \(\mathrm{Mg}_3\mathrm{P}_2\): Magnesium (\(\mathrm{Mg}^{2+}\)) and Phosphide (\(\mathrm{P}^{3-}\))
• \(\mathrm{Be}_3\mathrm{P}_2\): Beryllium (\(\mathrm{Be}^{2+}\)) and Phosphide (\(\mathrm{P}^{3-}\))
• \(\mathrm{Sr}_3\mathrm{P}_2\): Strontium (\(\mathrm{Sr}^{2+}\)) and Phosphide (\(\mathrm{P}^{3-}\))

All these compounds have the same anion (\(\mathrm{P}^{3-}\)) and cation charge (\(\mathrm{M}^{2+}\)), so the charge product \(|q_1 \cdot q_2|\) is the same for all.

The melting point is also influenced by the distance between ions, which is related to the ionic radii. Smaller ions will have a shorter distance between them, leading to stronger Coulombic interactions and thus a higher melting point.

The ionic radii generally increase down a group in the periodic table:

• \(\mathrm{Be}^{2+}\) has the smallest ionic radius.
• \(\mathrm{Mg}^{2+}\) is larger than \(\mathrm{Be}^{2+}\).
• \(\mathrm{Ca}^{2+}\) is larger than \(\mathrm{Mg}^{2+}\).
• \(\mathrm{Sr}^{2+}\) is the largest.

Final Answer