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.

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.

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.