Question

Draw two resonance structures for the bromate ion \(\left(\mathrm{BrO}_{3}^{-}\right),\) one that obeys the octet rule and one in which the formal charge on the central atom is zero.

Short answer

One structure obeys the octet rule, Br central with single/double bonds. Another gives Br 0 formal charge, having double bonds with two O atoms.

Step-by-step solution

Structure with Octet Rule

Start by drawing a Lewis structure for the bromate ion \( \text{BrO}_3^- \) that follows the octet rule. Bromine (Br) will be the central atom, and you’ll surround it with three oxygen atoms. Count the valence electrons: Br has 7, each O has 6, and there's an extra electron due to the negative charge, giving a total of \( 7 + 3(6) + 1 = 26 \) electrons. Arrange these to give each atom an octet: make two of the Br-O bonds single bonds, and the third a double bond. Place lone pairs on each oxygen as needed to satisfy their octet.

Assign Formal Charges for Octet Structure

Calculate the formal charge for each atom in the structure from Step 1. For each single-bonded oxygen, the formal charge is \(-1\) because it has 6 valence electrons and is using 7 (sharing 1 from Br plus the 6 in lone pairs it holds). The double-bonded oxygen has 0 formal charge. Bromine has a formal charge of \(+1\) because it has 7 valence electrons and uses 6 (sharing 3 bonds). Overall sum is \(-1\), coinciding with the charge of the ion.

Structure with Zero Formal Charge on Bromine

To comply with stable chemical structures, attempts should be made to reduce the formal charge on the central atom. Draw a resonance structure with double bonds between Br and 2 O atoms and one single bond with the third O to bring Br’s formal charge to zero. Reassign electron pairs to ensure all atoms achieve their octet. The two double-bonded oxygens will still each carry a \(0\) formal charge, while the single-bonded oxygen will have a \(-1\) formal charge.

Verify Formal Charges for Second Structure

In the restructured ion, check that the bromine has a formal charge of \(0\) by confirming it uses 7 of its valence electrons in bonding (as in sharing 4 from 2 double bonds and 3 from the single bond). Confirm that the single-bonded O has a \(-1\) charge, and the two double-bonded oxygens have formal charges of \(0\). Total charges sum to \(-1\), matching the ion charge.

Key concepts

Bromate Ion

The bromate ion, denoted as \( \text{BrO}_3^- \), is a negatively charged ion derived from bromic acid. It plays a role in various chemical processes and is often encountered in inorganic chemistry. The negative charge signifies that the ion has an extra electron compared to a neutral arrangement of atoms. This extra electron affects both the structure and the bonding of the ion. When understanding bromate ions, it's essential to consider how the electrons are distributed among the atoms. Bromine (- is central atom: sharing and receiving electrons from oxygen.- +3 oxidation state: forms numerical basis for calculating electron distribution.Mastering the bromate ion's structure will help you succeed in calculating related resonance structures and understanding how formal charge operates within an ion.

Lewis Structure

A Lewis structure is a diagrammatic representation of the molecules showing how the atoms are bonded together and the lone pairs of electrons present in the molecule. To draw the Lewis structure of the bromate ion \( \text{BrO}_3^- \):- **Count the valence electrons:** Begin by counting the total number of valence electrons available. For bromate, this includes the electrons from bromine (7 valence electrons), from each oxygen (3 oxygens with 6 electrons each), and 1 additional electron from the negative charge, totaling 26 electrons.- **Arrange the atoms:** Place bromine in the center with the three oxygens surrounding it, as bromine is less electronegative than oxygen and can accommodate more bonds.- **Assign the electrons:** Use the electrons to form bonds and assign any remaining as lone pairs to satisfy each atom's octet rule. Two oxygen atoms might form single bonds, while the third forms a double bond with bromine.The Lewis structure helps illustrate how electrons are shared in the bromate ion, providing foundational insight necessary for further calculations, such as formal charges.

Formal Charge

Calculating the formal charge is crucial to understand the distribution of charge within a molecule. The formal charge can be calculated using the formula:\[ \text{Formal Charge} = (\text{Number of Valence Electrons}) - (\text{Non-bonding Electrons} + \frac{1}{2} \text{Bonding Electrons}) \]For the bromate ion's Lewis structure:- **Single-bonded oxygen atoms** typically have a formal charge of - -1, arising from having 6 valence electrons, sharing 1 bond pair, and holding most of the ion's extra electron charges.- **Double-bonded oxygen** has 0 formal charge, ensuring a balanced electron count around it.- **Central bromine atom** may have a formal charge of + +1 in some structures, but can also be zero in alternative resonance forms designed to minimize the charge on central atoms.Understanding formal charges helps to determine the most stable structure, and aids in predicting how molecules interact during reactions.

Octet Rule

The octet rule is a fundamental principle in chemistry stating that atoms tend to form compounds in ways that give them eight valence electrons, achieving a configuration similar to noble gases. When drawing structures like the bromate ion, the octet rule helps you: - **Ensure stability:** Atoms achieve stability by having full valence shells, generally 8 electrons for main-group elements. - **Guide electron assignment:** When arranging electrons in a Lewis structure, you should aim to satisfy this rule for each atom, except for cases where elements can expand their octet like bromine. Bromine, located in the third period of the periodic table, can accommodate more than eight electrons due to available d orbitals. In bromate, it is possible for bromine to form more bonds and occasionally bypass the strict octet rule, especially in structures aimed at minimizing formal charges. Ensuring all atoms, especially oxygen in this case, have full octets allows the molecule to be more stable and can verify the reasonableness of the drawn structure.