Question

Consider the following statements: 1\. Cs ion is more highly hydrated than other alkali metal ions 2\. Among the alkali metals \(\mathrm{Li}, \mathrm{Na}, \mathrm{K}\) and \(\mathrm{Rb}\), \(\mathrm{Li}\) has the higher melting point 3\. Among the alkali metals, only Li form a stable nitride by direct combination (a) 1 and 3 are correct (b) 2 and 3 are correct (c) 1,2 and 3 are correct (d) 1 and 2 are correct

Short answer

(b) 2 and 3 are correct

Step-by-step solution

Evaluate Statement 1

Statement 1 claims that the Cs ion is more highly hydrated than other alkali metal ions. In general, the degree of hydration of an ion is inversely proportional to its ionic size; smaller ions get more highly hydrated. Since Cs has the largest ionic size among the alkali metals, this statement is incorrect.

Evaluate Statement 2

Statement 2 claims that among the alkali metals Li, Na, K, and Rb, Li has the higher melting point. Melting point trends in alkali metals decrease down the group due to increasing atomic size. Li, being at the top of the list (smallest atom), indeed, has a higher melting point than Na, K, and Rb. Therefore, Statement 2 is correct.

Evaluate Statement 3

Statement 3 claims that among the alkali metals, only Li forms a stable nitride by direct combination. Lithium can react with nitrogen to form lithium nitride (Li₃N), while other alkali metals do not typically form stable nitrides in the same way. Thus, this statement is correct.

Determine the Correct Pair of Statements

Since Statement 2 and Statement 3 are correct, the option that includes both correct statements is (b) 2 and 3 are correct.

Key concepts

Hydration of Ions

Hydration is a concept related to how water molecules surround and interact with ions. When an ion dissolves in water, it attracts water molecules around it. This process is called hydration. It is crucial because it influences how ions will behave in solutions. The degree of hydration depends on the size and charge of the ion. Smaller ions generally attract more water molecules due to their higher charge density. This means they become more hydrated. Among alkali metals, lithium ions ( Li^ + ) are the smallest and thus, the most highly hydrated compared to their larger counterparts like cesium ions ( Cs^ + ). Therefore, the statement that cesium ions are more hydrated is incorrect because cesium is the largest ion among alkali metals. Understanding ion hydration helps us predict solubility and the physical behavior of ionic compounds in water.

Melting Point Trends

The melting point of a substance indicates the temperature at which it transitions from a solid to a liquid state. For alkali metals, these points are important for practical uses where temperature is a factor. Alkali metals display a trend where their melting points decrease as we move down the group in the periodic table. This means lithium, being the smallest and at the top of Group 1, has the highest melting point among its peers like sodium (Na), potassium (K), and rubidium (Rb). The atomic and ionic sizes increase as you move downward, and the attraction between atoms in the metal lattice becomes weaker, resulting in lower melting points. Therefore, it's important to note that the structure and atomic interactions play essential roles in these melting trends. This is especially crucial in industries that utilize these metals for their thermal properties.

Stability of Nitrides

Nitrides are compounds composed of nitrogen with a less electronegative element. Nitrides can be quite stable under certain conditions, especially depending on what they are bonded with. In the case of alkali metals, lithium stands out as it uniquely forms a stable nitride ( Li₃N ) through a direct reaction with nitrogen. Other alkali metals, like sodium or potassium, do not readily form stable nitrides under normal conditions. This difference can be attributed to lithium's unique size and its ability to stabilize the nitride structure. Lithium's unique capability affects its applications in areas such as battery technology and chemistry, where stable compounds are essential for long-term energy storage and release. Understanding which metals form stable nitrides informs chemists and engineers on how to design materials with specific desirable properties.