Question

Amongst the alkali metal hydrides, the most stable one is (1) \(\mathrm{LiH}\) (2) \(\mathrm{NaH}\) (3) \(\mathrm{KH}\) (4) \(\mathrm{RbH}\)

Short answer

LiH

Step-by-step solution

- Understanding Alkali Metal Hydrides

Alkali metal hydrides are formed when alkali metals react with hydrogen. These hydrides are generally ionic compounds.

- Stability Trend in Alkali Metal Hydrides

The stability of alkali metal hydrides decreases as we move down the group in the periodic table. This is because as the atomic size increases down the group, the bond between the metal ion and the hydrogen ion becomes weaker.

- Analyzing the Options

Lithium is at the top of the alkali metal group, followed by sodium, potassium, and rubidium. Therefore, the hydride formed with lithium (LiH) is the most stable.

- Conclusion

Based on the trend of decreasing stability from top to bottom in the alkali group, \(\text{LiH}\) is the most stable hydride.

Key concepts

Stability Trend

The stability of alkali metal hydrides decreases as you move down the periodic table. But why? As we move from lithium (Li) down to rubidium (Rb), the size of the metal atoms increases.
This means the distance between the metal ion and the hydrogen ion in the hydride also gets bigger. A bigger distance weakens the bond between the ions, lowering the stability of the compound.
Therefore, lithium hydride (LiH) is the most stable, followed by sodium hydride (NaH), potassium hydride (KH), and rubidium hydride (RbH).

Periodic Table

The periodic table is a chart that organizes elements by increasing atomic numbers and groups elements with similar properties together.
Alkali metals are found in Group 1 of the periodic table and include lithium (Li), sodium (Na), potassium (K), and rubidium (Rb).
As you go down Group 1 from Li to Rb, the atomic size and reactivity of the elements increase, but the stability of their hydrides decreases. Understanding this arrangement helps predict the properties and behaviors of elements and their compounds.

Ionic Compounds

Alkali metal hydrides are examples of ionic compounds, which are formed by the transfer of electrons from one atom to another.
In these compounds, alkali metals (which are in Group 1 of the periodic table) lose an electron to hydrogen, forming positively charged metal ions (cations) and negatively charged hydrogen ions (anions).
This transfer creates a strong electrostatic attraction between the oppositely charged ions, but as we've discussed, the strength of this attraction decreases down the group in the periodic table, affecting stability.

Alkali Metals

Alkali metals are a group of elements in Group 1 of the periodic table, known for being highly reactive, especially with water.
They include lithium (Li), sodium (Na), potassium (K), and rubidium (Rb). These metals have one electron in their outermost shell, which they readily lose to form positive ions.
When they react with hydrogen, they form alkali metal hydrides (e.g., LiH, NaH, KH, RbH).
The reactivity of these metals increases as you go down the group, but the hydrides they form become less stable due to the increasing atomic size.