Question

Handbooks and lists of chemicals do not contain entries under the formulas \(\mathrm{Al}\left(\mathrm{HCO}_{3}\right)_{3}\) and \(\mathrm{Al}_{2}\left(\mathrm{CO}_{3}\right)_{2} .\) Explain why these compounds do not exist.

Short answer

The compounds Al(HCO3)3 and Al2(CO3)2 do not exist because Aluminum has a +3 valency which does not align with the charges of the bicarbonate and carbonate ions, and fractional stoichiometry is not allowed in chemistry.

Step-by-step solution

Understanding the Valency of Aluminum

Firstly, Aluminum (Al) has a valency of +3, which means it needs to lose 3 electrons to achieve a stable electronic configuration.

Evaluating Bicarbonate Ion

Secondly, for the supposed compound \(\mathrm{Al}\left(\mathrm{HCO}_{3}\right)_{3}\), the bicarbonate ion (HCO3) has a -1 charge. Therefore, three bicarbonate ions will together provide an overall charge balance to an aluminum ion. However, bicarbonates of metals exist only for metals forming +1 cations like Na, K. Hence, Al(HCO3)3 does not exist.

Evaluating Carbonate Ion

Next, for the supposed compound \(\mathrm{Al}_{2}\left(\mathrm{CO}_{3}\right)_{2}\), the carbonate ion (CO3) has a -2 charge. Thus, an Aluminum ion would need one and a half carbonate ions to balance its +3 charge, and this fractional stoichiometry is not allowed in chemistry. Therefore, the compound Al2(CO3)2 cannot exist.

Key concepts

Valency of Aluminum

The concept of valency is crucial to understanding how elements interact to form compounds. Valency refers to the combining power of an element, specifically the number of electrons an atom can lose, gain, or share. In the case of aluminum (Al), its valency is +3. This means that an aluminum atom can lose three electrons to achieve a stable electronic configuration, which resembles that of the nearest noble gas, neon.
From the perspective of chemical stability, an aluminum atom seeks to achieve a noble gas electron configuration by bonding with other atoms that can accept the electrons it donates. Since each aluminum atom can provide three electrons, it tends to form compounds with other elements or groups that can altogether receive those electrons. When forming ionic compounds, aluminum is typically found in a trivalent state, meaning it forms Al3+ ions.
In basic terms, whenever aluminum engages in chemical reactions to create a stable compound, it must be able to successfully distribute its three valence electrons. This requirement dictates which ions aluminum can successfully bond with and why certain compounds like Al(HCO3)3 are not commonly found.

Bicarbonate Ion

The bicarbonate ion (HCO3−) is an intermediate form in the deprotonation of carbonic acid. It has a single negative charge and is often present in the chemistry of carbonates in biological systems as well as in certain salts. Given that the bicarbonate ion carries a -1 charge, it readily forms compounds with positively charged metal ions, particularly those with a +1 charge.

Challenges with Aluminum

Despite its prevalence, when trying to pair the bicarbonate ion with aluminum, which has a valency of +3, difficulties arise. Metals that form +1 cations, such as sodium (Na) and potassium (K), can easily bind with bicarbonate to create compounds like sodium bicarbonate (NaHCO3). However, for aluminum with a +3 charge, the -1 charge of the bicarbonate ion creates an imbalance. You would require three bicarbonate ions to balance one aluminum ion (Al3+), but this typically does not happen because aluminum forms stronger bonds with other anions and bicarbonates are not stable enough to support the valency of aluminum in a compound. This instability is why we do not commonly see compounds such as Al(HCO3)3.

Carbonate Ion

Understanding the carbonate ion (CO32−) is essential to grasp the behavior of many minerals and the chemistry behind carbon cycles in nature. The carbonate ion, with a charge of -2, is a polyatomic ion that can bind to various metal cations to form carbonates, which are an important component of rocks and sediments.

Why Aluminum Doesn't Form Al2(CO3)2

Let's consider attempting to pair aluminum with the carbonate ion to form a compound. Aluminum's +3 valency and the carbonate ion's -2 charge mean that you can't have an even exchange; an aluminum ion can't evenly partner with a whole number of carbonate ions to form a stable ionic compound. To balance the charges completely, you would need one and a half (1.5) carbonate ions for each aluminum ion, which is impossible due to the law of definite proportions that dictates compounds must have whole number ratios. Therefore, the hypothetical compound Al2(CO3)2 can't exist because it violates this fundamental rule in chemistry. Instead, aluminum readily combines with other ions that can perfectly balance its +3 charge, helping it achieve the necessary stable electronic structure.