Understanding Alkali Metal Reactivity with Water

The question asks us to identify the element among Lithium, Sodium, Potassium, and Cesium that is least reactive with water. These elements belong to Group 1 of the periodic table, also known as the alkali metals.

Alkali metals are known for being highly reactive, especially with water. When alkali metals react with water, they produce a metal hydroxide and hydrogen gas. The general reaction is:

\(\text{2M(s) + 2H}_2\text{O(l) \(\rightarrow\) 2MOH(aq) + H}_2\text{(g)}\)

Where M represents an alkali metal.

Reactivity Trend of Alkali Metals

The reactivity of alkali metals increases as you move down Group 1 of the periodic table. The elements given in the options are Lithium (Li), Sodium (Na), Potassium (K), and Cesium (Cs). Their order in Group 1 from top to bottom is Lithium, Sodium, Potassium, and Cesium.

Here's why the reactivity increases down the group:

• Atomic Size: The atomic size of alkali metals increases down the group because new electron shells are added.
• Ionization Energy: The ionization energy (energy required to remove the outermost electron) decreases down the group because the valence electron is further away from the nucleus and is shielded by more inner electrons.
• Electron Removal: Since it is easier to remove the outermost electron as you go down the group, the metals become more willing to react and lose that electron to form a positive ion (M⁺).

Therefore, Cesium, being at the bottom of this list (and Group 1), is the most reactive with water among the given options, while Lithium, being at the top, is the least reactive.

Comparing Reactivity with Water

Let's briefly look at how they react:

• Lithium: Reacts steadily with water, producing hydrogen gas. The reaction can be slow and is often less vigorous than with other alkali metals.
• Sodium: Reacts rapidly with water, melting into a sphere that skitters across the surface. The hydrogen gas produced often ignites due to the heat generated.
• Potassium: Reacts very vigorously with water, often leading to an explosion as the hydrogen gas ignites immediately.
• Cesium: Reacts extremely violently and explosively with water, even more so than Potassium.

Based on the trend and observations, Lithium shows the least vigorous reaction with water among the options provided.

Conclusion

Considering the trend of increasing reactivity down Group 1, Lithium (Li) is the element that is least reactive with water among the given choices: Lithium, Sodium, Potassium, and Cesium.

Revision Table: Alkali Metal Properties

Additional Information: Alkali Metal Reactions

Besides reacting with water, alkali metals exhibit other characteristic reactions that highlight their high reactivity.

• Reaction with Air: Alkali metals tarnish rapidly in air due to reaction with oxygen, moisture, and carbon dioxide. They are typically stored under oil to prevent this.
• Reaction with Halogens: They react vigorously with halogens (like chlorine or bromine) to form ionic halides (e.g., NaCl, KCl). The reactivity with halogens also increases down the group.
• Reaction with Hydrogen: Alkali metals react with hydrogen gas at high temperatures to form solid metal hydrides (e.g., LiH, NaH).

The high reactivity of alkali metals is primarily due to their electronic configuration, having a single valence electron that is easily lost to achieve a stable noble gas configuration.