Question

On reaction with dihydrogen the alkali metals (a) form hydrides which are ionic solids with high melting points (b) form hydrides which are molecular solids with low melting points (c) form hydrides which are ionic solids with low melting points (d) form hydrides which are non-stoichiometric.

Short answer

(a) Alkali metals react with dihydrogen to form hydrides which are ionic solids with high melting points.

Step-by-step solution

Understanding the Reactivity of Alkali Metals with Dihydrogen

Alkali metals are highly reactive and readily react with dihydrogen to form metal hydrides. These metals are located in the first group of the periodic table and include elements like lithium, sodium, potassium, etc.

Characterizing Alkali Metal Hydrides

Alkali metal hydrides are ionic compounds. The metal donates its outer electron to hydrogen, forming a cation (M+) and an anion (H-). The resulting compound is an ionic solid due to this electron transfer.

Assessing the Melting Points of Alkali Metal Hydrides

Since alkali metal hydrides are ionic compounds, they have strong electrostatic forces of attraction between the cations and anions. These strong ionic bonds give alkali metal hydrides high melting points.

Determining the Correct Option

Based on the properties of ionic bonding and the reactivity of alkali metals with dihydrogen, it can be concluded that the hydrides formed are ionic solids with high melting points.

Key concepts

Reactivity of Alkali Metals

When considering the reactivity of alkali metals, it is essential to look at their placement within the periodic table. Occupying Group 1, these metals, including lithium, sodium, potassium, etc., display a characteristic eagerness to lose their single valence electron and form cations. This eagerness, also known as reactivity, increases as we move down the group from lithium to cesium.

The high reactivity is partly due to the low ionization energy required to remove the outermost electron. Given this propensity to react, when alkali metals come into contact with dihydrogen, they rapidly form metal hydrides. These compounds are notable for their high melting points, which can be attributed to the strength of the ionic bonds formed between the respective metal cations and hydride anions. By understanding this fundamental behavior, learners can predict and explain the reactivity patterns of alkali metals.

Ionic Compounds

Diving deeper into the nature of ionic compounds, such as those formed by the reaction of alkali metals with dihydrogen, we find that their distinct properties are all tied to the ionic bonds that hold them together. An ionic bond is the electrostatic force of attraction between positively charged ions (cations) and negatively charged ions (anions).

In the context of metal hydrides, the alkali metal atom donates an electron to become a cation, while the hydrogen atom accepts an electron to become an anion (H-). This exchange results in a crystalline lattice structure that is characteristically solid with a high melting point due to the substantial forces holding the ions in place. Strength and rigidity define these compounds, making them significantly different from molecular solids that rely on weaker forces and generally possess low melting points. Understanding the structure and bonding of ionic compounds aids in predicting properties such as conductivity, melting and boiling points, and solubility.