Place the following in order of decreasing X-O bond length ("X" indicates the central atom
The Correct Answer and Explanation is:
The correct answer is SiO₃²⁻ > CO₃²⁻ > CO₂.
To determine the order of decreasing X-O bond length, we need to consider two main factors: bond order and the atomic radius of the central atom (X).
First, let’s analyze the bond order for each species. Bond order is the number of chemical bonds between a pair of atoms. A higher bond order results in a stronger attraction between the atoms, pulling them closer together and thus decreasing the bond length.
- CO₂ (Carbon Dioxide): The Lewis structure for CO₂ is O=C=O. It consists of two carbon-oxygen double bonds. Therefore, the C-O bond order is 2.
- CO₃²⁻ (Carbonate ion): The carbonate ion has three resonance structures. In each structure, there is one C=O double bond and two C-O single bonds. The actual structure is a resonance hybrid, where the electrons are delocalized across all three C-O bonds. The bond order is calculated as the average number of bonds, which is (2 + 1 + 1) / 3 = 4/3, or approximately 1.33.
- SiO₃²⁻ (Silicate ion): This ion is isoelectronic and isostructural with the carbonate ion. It also exhibits resonance, and its Si-O bond order is also 4/3, or approximately 1.33.
Based on bond order alone, CO₂ (bond order 2) has the shortest bond. Both CO₃²⁻ and SiO₃²⁻ have a lower bond order (1.33), so their bonds will be longer than the bond in CO₂.
Next, we compare CO₃²⁻ and SiO₃²⁻, which have the same bond order. Here, we must consider the size of the central atom. Carbon (C) is in Period 2 of the periodic table, while Silicon (Si) is directly below it in Period 3. Atomic radius increases as you move down a group. Therefore, the silicon atom is significantly larger than the carbon atom. A larger central atom leads to a longer bond length. Consequently, the Si-O bond in SiO₃²⁻ is longer than the C-O bond in CO₃²⁻.
Combining these points, we can establish the final order of decreasing bond length (from longest to shortest):
- SiO₃²⁻ (longest, due to the large Si atom)
- CO₃²⁻ (intermediate, smaller central atom than Si)
- CO₂ (shortest, due to the highest bond order)
This gives the order: SiO₃²⁻ > CO₃²⁻ > CO₂.
