Question

The halide having the highest melting point is: (a) \(\mathrm{NaF}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{NaBr}\) (d) \(\mathrm{NaI}\)

Short answer

The halide with the highest melting point is \( \text{NaF} \).

Step-by-step solution

Understanding Melting Points

To solve this, we need to recall that the melting point of ionic compounds depends on the strength of the ionic bonds, which are affected by the size and charge of the ions. Stronger ionic bonds typically result in higher melting points.

Ion Sizes

In this case, we are comparing halides of sodium. Larger ions generally result in weaker ionic bonds due to increased distance between the ions. The size order from smallest to largest for the halide ions is \ \( \text{F}^- < \text{Cl}^- < \text{Br}^- < \text{I}^- \).

Determine Melting Point Trend

Considering that smaller ions form stronger ionic bonds, the compound with the smallest anion should have the highest melting point. Therefore, \( \text{NaF} \), which uses \( \text{F}^- \), should have the highest melting point.

Comparison with Other Compounds

To confirm, observe that as the ionic size increases from \( \text{F}^-\) to \( \text{I}^- \), the melting points of the corresponding compounds decrease: \ \( \text{NaF} > \text{NaCl} > \text{NaBr} > \text{NaI} \).

Key concepts

Ionic Bond Strength and Its Influence on Melting Points

Ionic bond strength is a critical factor affecting the melting points of ionic compounds. These bonds are electrical attractions between positively and negatively charged ions. The strength of these attractions can determine how much energy is needed to break the bonds and change the compound into a liquid state. The primary factors influencing ionic bond strength include:

• Charge of the Ions: Higher charged ions have stronger electrostatic interactions, leading to stronger ionic bonds.
• Distance Between Ions: Smaller ions allow for the ions to be closer together, which results in stronger attractions.

Sodium halides like NaF, NaCl, NaBr, and NaI exhibit decreasing melting points as the size of the negative halide ion increases. This decrease is due to the weakening of the ionic bonds as a result of increased distance between the ions.

The Relationship Between Halide Ion Size and Bond Strength

The size of halide ions directly affects the strength of the ionic bonds they form. As ion size increases, the ability for ions in an ionic compound to closely approach each other decreases, resulting in weaker bonds. This happens because:

• Smaller ions can pack more closely, leading to stronger electrostatic attractive forces.
• Larger ions increase the distance between charges, which weakens the overall bond strength.

In the context of the problem, we compare halides: fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻). The size order is: F⁻ < Cl⁻ < Br⁻ < I⁻. This order reveals that fluoride ion, being the smallest, allows for a stronger ionic bond when paired with sodium, resulting in a higher melting point.

Trends in Melting Points of Halides

Observing trends in the melting points of halides can deepen our understanding of ionic compound behavior. For sodium halides, the melting point trend is observed as: NaF > NaCl > NaBr > NaI. This sequence highlights that NaF has the highest melting point among the four, aligning with the concept of stronger ionic bonds due to smaller ion size. The trend occurs because:

• As halide ions increase in size, the ionic bond weakens, lowering the melting point.
• The energy needed to break these bonds, and thus achieve melting, decreases with weaker bonds.

Understanding these trends can help predict the properties of other ionic compounds and their behavior under various conditions.