A major challenge in implementing the 'hydrogen economy' is finding a safe, lightweight, and compact way of storing hydrogen for use as a fuel. The hydrides of light metals are attractive for hydrogen storage because they can store a high weight percentage of hydrogen in a small volume. For example, NaAlH₄ can release 5.6% of its mass as H2 upon decomposing to NaH(s), Al(s), and H₂(g). NaAlH₄ possesses both covalent bonds, which hold polyatomic anions together, and ionic bonds. (d) What is the formal charge on hydrogen in the polyatomic ion?

Verified step by step guidance

1. Identify the polyatomic ion in NaAlH_4, which is the AlH_4^- ion.
2. Recall that the formal charge formula is: \( \text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{1}{2} \times \text{Bonding Electrons} \).
3. Determine the number of valence electrons for hydrogen, which is 1.
4. In the AlH_4^- ion, each hydrogen is bonded to aluminum, contributing 1 bonding electron per bond. Since hydrogen forms one bond, it has 0 non-bonding electrons.
5. Apply the formal charge formula for hydrogen: \( \text{Formal Charge} = 1 - 0 - \frac{1}{2} \times 2 = 0 \).

Key Concepts

Formal Charge

Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons, the number of bonds, and the number of non-bonding electrons. It helps in determining the most stable structure of a molecule by indicating how electrons are distributed among atoms. The formula for calculating formal charge is: Formal Charge = Valence Electrons - (Non-bonding Electrons + 1/2 Bonding Electrons).

Covalent and Ionic Bonds

Covalent bonds are formed when two atoms share electrons, typically between nonmetals, resulting in the formation of molecules. In contrast, ionic bonds occur when electrons are transferred from one atom to another, leading to the formation of charged ions that attract each other, usually between metals and nonmetals. Understanding these bonding types is crucial for analyzing the structure and properties of compounds like NaAlH4.

Hydrides

Hydrides are compounds formed between hydrogen and other elements, often metals, where hydrogen can exist in various bonding states. In the context of metal hydrides, such as NaAlH4, hydrogen is typically present as a hydride ion (H-), which can influence the compound's stability and its ability to release hydrogen gas. This property is significant for applications in hydrogen storage technologies.