The correct answer is option 2: Potassium.

Solid hydrogen carbonates, also known as bicarbonates, are compounds that contain the bicarbonate ion (HCO3-) combined with a metal ion. This combination forms a salt that is solid at room temperature.

Sodium hydrogen carbonate (NaHCO3), also known as baking soda, is a common example of a solid hydrogen carbonate. Caesium hydrogen carbonate (CsHCO3) and lithium hydrogen carbonate (LiHCO3) are also examples of solid hydrogen carbonates.

However, potassium (K) does not form a solid hydrogen carbonate. Potassium bicarbonate (KHCO3) does exist, but it is not a solid at room temperature. Instead, it is a white crystalline powder that can be easily dissolved in water.

Therefore, the correct option is 2: Potassium does not form a solid hydrogen carbonate.