Problem 36 - Chemistry and Chemical Reactivity

Question

Compare the electron dot structures of the carbonate \(\left(\mathrm{CO}_{3}^{2-}\right)\) and borate \(\left(\mathrm{BO}_{3}^{3-}\right)\) ions. (a) Are these ions isoelectronic? (b) How many resonance structures does each ion have? (c) What are the formal charges of each atom in these ions? (d) If an \(\mathrm{H}^{+}\) ion attaches to \(\mathrm{CO}_{3}^{2-}\) to form the bicarbonate ion, \(\mathrm{HCO}_{3}^{-},\) does it attach to an \(\mathrm{O}\) atom or to the C atom?

Step-by-Step Solution

Answer

(a) Yes, isoelectronic. (b) Both have 3 resonance structures. (c) CO3^2-: O(-1), C(0); BO3^3-: O(-1), B(0). (d) Attaches to O.

Step 1: Determine valence electrons for CO3^2-

Calculate the total number of valence electrons for the carbonate ion \(\text{CO}_3^{2-}\). Carbon has 4 valence electrons, each oxygen has 6 valence electrons, and the 2 negative charges add 2 more electrons. Thus, \(4 + (3 \times 6) + 2 = 24\) valence electrons.

Step 2: Determine valence electrons for BO3^3-

Calculate the total number of valence electrons for the borate ion \(\text{BO}_3^{3-}\). Boron has 3 valence electrons, each oxygen has 6 valence electrons, and the 3 negative charges add 3 more electrons. Thus, \(3 + (3 \times 6) + 3 = 24\) valence electrons.

Step 3: Check Isoelectronic Nature

Both the \(\text{CO}_3^{2-}\) and \(\text{BO}_3^{3-}\) ions have 24 valence electrons. Therefore, they are isoelectronic.

Step 4: Determine Resonance Structures for CO3^2-

For \(\text{CO}_3^{2-}\), draw the Lewis structures considering each oxygen alternates forming a double bond with carbon. This results in 3 resonance structures.

Step 5: Determine Resonance Structures for BO3^3-

For \(\text{BO}_3^{3-}\), draw the Lewis structures considering each oxygen alternates forming a double bond with boron. This again results in 3 resonance structures.

Step 6: Calculate Formal Charges for CO3^2-

In each resonance structure, each singly bonded oxygen has a formal charge of \(-1\), and carbon has a formal charge of \(0\). The doubly bonded oxygen has a formal charge of \(0\).

Step 7: Calculate Formal Charges for BO3^3-

In each resonance structure, each singly bonded oxygen has a formal charge of \(-1\), and boron has a formal charge of \(0\) for the doubly bonded oxygens and \(+1\) for the single bonded oxygens.

Step 8: Determine Attachment of H+ to CO3^2-

The \(\text{H}^+\) ion attaches to an oxygen atom in the \(\text{CO}_3^{2-}\) ion. Oxygen atoms are more electronegative than carbon and carry negative charges, making them the attachment point for \(\text{H}^+\) ions.

Key Concepts

Isoelectronic Ions

When ions have the same number of electrons, they are referred to as isoelectronic. This concept plays an important role in predicting the chemical behaviors and properties of ions. For instance, the carbonate ion \( \text{CO}_3^{2-} \) and the borate ion \( \text{BO}_3^{3-} \) are isoelectronic because each has exactly 24 valence electrons. This similarity in electron count implies they can generally exhibit comparable bonding patterns. However, differences in atom types lead to variations in their chemical properties. Recognizing isoelectronic ions is particularly useful when thinking about ionic compounds and can help in understanding trends across the periodic table.

Resonance Structures

Resonance structures are multiple ways to draw Lewis structures for a molecule or ion. They illustrate different possible distributions of electrons in the molecule. The true electron configuration is a hybrid of these structures. For example, both \( \text{CO}_3^{2-} \) and \( \text{BO}_3^{3-} \) have 3 resonance structures. In these structures, double bonds can form alternately between a central atom, such as carbon or boron, and the surrounding oxygen atoms. These multiple structures help in understanding the molecule's stability and properties. By analyzing resonance patterns, chemists can predict bond lengths and molecule behavior under different conditions, allowing for a deeper understanding of the molecule's chemical nature.

Formal Charges

Formal charge is a handy concept used to determine the distribution of electrons among atoms in a molecule. It helps to keep track of the charge development within a molecule for more accurate depictions of chemical interactions. To calculate formal charges, the number of valence electrons in the isolated atom is adjusted based on its state in the compound. For instance, in \( \text{CO}_3^{2-} \), single-bonded oxygen atoms have a formal charge of \(-1\) while carbon remains at \(0\). In contrast, for \( \text{BO}_3^{3-} \), there's a mixture of \(0\) and \(+1\) depending on double or single bonds. Formal charges are crucial for evaluating the most stable resonance structure, thus providing insight into molecule reactivity and properties.

Bonding in Polyatomic Ions

Polyatomic ions, such as \( \text{CO}_3^{2-} \) and \( \text{BO}_3^{3-} \), consist of two or more atoms covalently bonded, with a positive or negative charge. Understanding the bonding in these ions helps to predict their chemical behaviors and how they interact in different environments. In many cases, such ions demonstrate how additional electrons affect bonding arrangements. For instance, with \( \text{CO}_3^{2-} \), when \( \text{H}^+ \) attaches, it prefers bonding with the oxygen rather than carbon due to oxygen's higher electronegativity and partial negative charge. This type of bonding affects the ion's geometry and response to surrounding charges, making the study of polyatomic ions crucial for grasping larger chemical phenomena like acid-base reactions and crystal lattice formations.