Question

Write the electron dot formula and draw the structural formula for each of the following polyatomic ions. (Central atoms are indicated in bold.) (a) BrO (b) \(\mathrm{BrO}_{2}\) (c) \(\mathrm{BrO}_{3}^{-}\) (d) \(\mathrm{BrO}_{4}\)

Short answer

BrO involves Br as the central atom; BrO2, BrO3−, and BrO4 follow with similar central Br and additional O atoms, forming single/double bonds to satisfy the octet rule.

Step-by-step solution

Determine Total Valence Electrons for BrO

For BrO, bromine (Br) has 7 valence electrons and oxygen (O) has 6 valence electrons. Thus, the total number of valence electrons is \(7 + 6 = 13\). In the electron dot formula, represent these electrons around the symbols of Br and O while maintaining the valence octet rule where possible.

Draw Lewis Structure for BrO

Place Br at the center since it's indicated as the central atom, and connect it to O with a single bond. Distribute the remaining electrons around O and Br to satisfy the octet rule as much as possible.

Determine Total Valence Electrons for BrO2

For \(\text{BrO}_2\), bromine again has 7 valence electrons and each oxygen has 6, resulting in \(7 + 2\times6 = 19\) valence electrons. Account for these electrons in the electron dot formula with Br centrally placed.

Draw Lewis Structure for BrO2

Place Br in the center, and bind it with two O atoms. Use single bonds initially and then arrange the remaining electrons to satisfy the octet rule using lone pairs. Consider forming double bonds if necessary to fulfill the octet rule around oxygen atoms.

Determine Total Valence Electrons for BrO3−

For \(\text{BrO}_3^-\), add 1 electron for the negative charge, leading to \(7 + 3\times6 + 1 = 26\) electrons. Spread these electrons in the electron dot arrangement with Br as the central atom.

Draw Lewis Structure for BrO3−

Position Br in the middle connected to three O atoms. Begin with single bonds, and adjust octet violations by converting paired electrons into double bonds. Ensure negative charge is represented with the final structure indicating expanded octet possibilities for Br.

Determine Total Valence Electrons for BrO4

For \(\text{BrO}_4\), calculate \(7 + 4\cdot6 = 31\) valence electrons. As before, using Br as the central atom lays the foundation for this structure.

Draw Lewis Structure for BrO4

Center Br and attach all O atoms with single bonds. Distribute the remaining electrons to satisfy all oxygen and the central Br's electron requirements, potentially forming double bonds to fulfill the octet rule and ensure correct electron representation.

Key concepts

Electron Dot Formula

The electron dot formula, often known as the Lewis dot structure, is a simple way to showcase the bonding between atoms in a molecule or a polyatomic ion. It also illustrates all the electrons present, specifically outlining valence electrons which are the outermost electrons of an atom involved in chemical bonds. In these diagrams for polyatomic ions, symbols for the elements are used, with dots surrounding them to represent these electrons.
One of the primary guidelines in drawing these structures is to initially place the central atom, indicated in the exercise as bromine (Br) in each molecule. Bonds are represented with a line, where each line accounts for two shared electrons. Remaining electrons, or lone pairs, are distributed around the atoms, generally prioritizing the more electronegative atoms like oxygen. This setup helps in systematically arranging electrons to satisfy bond requirements and each atom's need for electron shells, mainly following the octet rule.

Valence Electrons

Valence electrons are pivotal in determining how atoms interact with each other. They are the electrons that reside in the outermost shell of an atom and are crucial for bonding between atoms. In our exercise, knowing the number of valence electrons for bromine and oxygen is the first step. Bromine has 7 valence electrons while each oxygen atom has 6.
By understanding this, calculating the total valence electrons in polyatomic ions becomes straightforward:

• BrO: 7 (Br) + 6 (O) = 13 valence electrons
• BrO₂: 7 (Br) + 2✕6 (O) = 19 valence electrons
• BrO_3^-: 7 (Br) + 3✕6 (O) + 1 (extra electron) = 26 valence electrons
• BrO₄: 7 (Br) + 4✕6 (O) = 31 valence electrons

Determining how these electrons are distributed helps in creating the correct electron dot structures as they provide insight into the possible formations and behaviors of the molecules.

Octet Rule

The octet rule is a crucial principle in chemistry used to predict the formation and structure of molecules. It posits that atoms are most stable when they have eight electrons in their valence shell, similar to the electron configuration of noble gases. In the context of covalent bonding, atoms tend to share or exchange electrons to achieve this stable configuration.
The Lewis structures for the bromine-oxygen ions in this exercise illustrate the application of the octet rule. For example, when designing the BrO_3^- ion structure, each oxygen needs to satisfy the octet, potentially forming double bonds with bromine where necessary. Although there are exceptions, and some atoms can expand their octet by involving d-orbitals, bromine is capable of this due to being part of the elements that can exceed the octet when bonded with highly electronegative atoms like oxygen.

Polyatomic Ions

Polyatomic ions are ions that consist of more than one atom, as the name suggests. They are tightly bonded groups of atoms that carry a charge because they have either lost or gained electrons. These ions are prevalent in various chemical compounds, such as those found in nature and industry.
Examples in our exercise include the bromate ions, such as the bromite ion BrO₂, the bromate ion BrO_3^-, and the perbromate ion BrO₄. These ions are known for their stability and reactivity, often forming part of larger compounds.
Understanding their structure via Lewis dot structures helps in visualizing how electrons are shared or distributed among the participating atoms. This understanding is fundamental to predicting the physical and chemical properties of compounds involving polyatomic ions, such as their solubility in water or their propensity to participate in chemical reactions.