Which one of the following alkali metal elements does not form a stable solid hydrogen carbonate (bicarbonate) salt under normal conditions?

Introduction / Context:
Hydrogen carbonates, also called bicarbonates, of alkali metals show interesting trends in stability and physical state across the group. Most alkali metals form stable solid hydrogen carbonates that can be isolated as crystalline salts. This question asks which alkali metal is an exception and does not form a stable solid hydrogen carbonate, highlighting periodic trends in properties.

Given Data / Assumptions:

• We consider alkali metals in Group 1: lithium, sodium, potassium, rubidium, and caesium.
• Hydrogen carbonates have the general formula MHCO3 for an alkali metal M.
• We focus on whether stable solid hydrogen carbonates exist for each element.
• Standard conditions of temperature and pressure are assumed.

Concept / Approach:
As we move down Group 1, the stability of solid hydrogen carbonates generally increases. Solid sodium hydrogen carbonate (baking soda) and potassium hydrogen carbonate are common. However, lithium hydrogen carbonate is not stable as a solid at ordinary conditions and exists only in solution. This is due to the small size and high polarising power of the lithium ion, which destabilises the hydrogen carbonate ion in the solid state. Therefore, lithium is the element that does not form a stable solid hydrogen carbonate, distinguishing it from other alkali metals.

Step-by-Step Solution:

Verification / Alternative check:
Periodic trends often show that lithium behaves somewhat differently from other alkali metals, a phenomenon called diagonal relationship, where lithium resembles magnesium in some properties more than its own group. Magnesium also does not form a stable solid hydrogen carbonate. Textbooks on inorganic chemistry emphasise that LiHCO3 cannot be isolated as a solid, whereas NaHCO3 is easily crystallised. This pattern confirms that lithium is the exception in the group.

Why Other Options Are Wrong:

• Sodium: Forms stable solid sodium hydrogen carbonate, widely used in baking and fire extinguishers.
• Potassium: Forms stable solid potassium hydrogen carbonate.
• Caesium: Its hydrogen carbonate is more stable and can exist as a solid, though it is less common in school examples.
• Rubidium: Similar to caesium, it can form solid hydrogen carbonate under appropriate conditions.

Common Pitfalls:
Students sometimes generalise that all alkali metals behave identically and form similar salts. However, lithium often shows anomalous behaviour due to its small ionic size and high charge density. Another pitfall is to assume that because sodium and potassium hydrogen carbonates are common, lithium must behave the same way. Remember that LiHCO3 is only stable in solution and decomposes on drying, which is why it is not isolated as a solid.

Final Answer:
The alkali metal that does not form a stable solid hydrogen carbonate is Lithium.