Question

The melting points of \(\mathrm{NaCl}, \mathrm{KCl}\), and \(\mathrm{RbCl}\) are \(1074 \mathrm{~K}, 1043 \mathrm{~K},\) and \(991 \mathrm{~K},\) respectively. Based on this, which compound exhibits the strongest ionic bonds, and which exhibits the weakest? How is this trend related to the relative size of the cations?

Short answer

NaCl has the strongest bonds; RbCl has the weakest. Smaller cations strengthen bonds.

Step-by-step solution

Understanding Ionic Bonds and Melting Points

Ionic bonds are formed between a metal and a non-metal, where electrons are transferred from the metal to the non-metal. The strength of these ionic bonds affects the compound's melting point. A higher melting point indicates stronger ionic bonds.

List Melting Points of Compounds

First, we list the given melting points: \(\mathrm{NaCl} = 1074 \mathrm{~K}, \mathrm{KCl} = 1043 \mathrm{~K}, \mathrm{RbCl} = 991 \mathrm{~K}\). This provides a clear comparison of the melting points of the three compounds.

Identify the Compound with the Strongest Ionic Bonds

NaCl has the highest melting point at 1074 K. According to the principle that higher melting points indicate stronger ionic bonds, we can infer that NaCl has the strongest ionic bonds among the three compounds.

Identify the Compound with the Weakest Ionic Bonds

RbCl has the lowest melting point at 991 K. Consequently, RbCl has the weakest ionic bonds when compared to KCl and NaCl.

Relate Melting Points to Cation Size

The size of the cations increases in the order \(\text{Na}^+ < \text{K}^+ < \text{Rb}^+\). Smaller cations lead to stronger electrostatic attractions to anions due to the reduced distance between ions, thereby resulting in stronger ionic bonds and higher melting points. Therefore, NaCl, with the smallest cation, exhibits the strongest ionic bonds.

Key concepts

Melting Points

Melting points are an important property when analyzing ionic compounds. They provide insight into the strength of the ionic bonds within the compound. When you hear about a substance's melting point, it tells us the temperature at which it transitions from a solid to a liquid. With ionic compounds, the melting point is directly related to how tightly the ions are held together by ionic bonds.
Generally:

• Higher melting points mean stronger ionic bonds.
• Lower melting points indicate weaker ionic bonds.

In our example,

• a high melting point, like that of NaCl at 1074 K, suggests strong ionic bonds.
• a lower melting point, like RbCl at 991 K, points to weaker bonds.

The melting point order (NaCl > KCl > RbCl) correlates with the strength of the ionic sticks holding the ions together.

Cation Size

The size of a cation plays a vital role in the strength of ionic bonds in compounds. Cations are positively charged ions that result from a metal losing electrons.
The size of these cations affects the overall bond strength:

• Smaller cations are able to get closer to anions.
• As a result, they create stronger attractions due to the closer spacing.

In the example given, sizes of cations increase gradually from Na⁺, through K⁺, to Rb⁺.

• Na⁺ is smaller compared to K⁺ and Rb⁺, thus it forms stronger ionic bonds in NaCl.
• As Rb⁺ is the largest, it forms the weakest bonds in RbCl.

So, understanding cation size helps explain why NaCl has the highest melting point, due to the strong attraction between its small cation and related anion.

Ionic Compounds

Ionic compounds are characterized by the ionic bonds they contain. These bonds are formed when electrons are transferred from a metal atom to a non-metal atom. This process creates positively charged cations and negatively charged anions, which are attracted to each other.
Some essential aspects of ionic compounds include:

• Each bond in an ionic compound results from the attraction between oppositely charged ions.
• The strength of these bonds is influenced by factors like the size of ions and their charge.

In compounds like NaCl, KCl, and RbCl, the bond strength varies because of the differences in cation size. NaCl, with its relatively small cation, exhibits stronger ionic interactions and hence, has a high melting point. Conversely, RbCl, with larger cations, shows weaker interactions which lead to a lower melting point.
Understanding these nuances allows us to predict the behavior of various ionic compounds effectively.