Question

Each compound contains both ionic and covalent bonds. Write ionic Lewis structures for each, including the covalent structure for the ion in brackets. Write resonance structures if necessary. \begin{equation}\mathrm{ a. }{RbIO}_{2} \quad \text { b. } \mathrm{NH}_{4} \mathrm{Cl} \quad \text { c. KOH } \quad \text { d. } \operatorname{Sr}(\mathrm{CN})_{2}\end{equation}

Short answer

RbIO2: Rb+ [I=O-O-]; NH4Cl: [H-N-H]+ Cl-; KOH: K+ [O-H]-; Sr(CN)2: Sr2+ 2[−C≡N:]

Step-by-step solution

Identify Ionic and Covalent Parts - RbIO2

Establish that RbIO2 consists of the Rb+ cation and the IO2- anion. Rb+ loses an electron (ionic), while IO2- involves covalent bonding between I and O atoms. The Lewis structure for IO2- will be enclosed in brackets.

Draw Lewis Structure for IO2-

Draw Iodine (I) with 7 valence electrons, and each Oxygen (O) with 6 valence electrons. Connect I with two O atoms using single bonds, then fulfill the octet for oxygen atoms by adding lone pairs. Place a negative charge on the ion, to indicate the extra electron.

Include Resonance Structures if Necessary

Since IO2- is a bent molecule, resonance structures are necessary, showing the double bond with each oxygen in alternating structures. Include a negative formal charge on the oxygen atom not double-bonded to iodine in each resonance structure.

Identify Ionic and Covalent Parts - NH4Cl

Recognize NH4Cl consists of NH4+ cation and Cl- anion. NH4+ features covalent bonding within the ion, while the bond between NH4+ and Cl- is ionic. The Lewis structure for NH4+ will be enclosed in brackets.

Draw Lewis Structure for NH4+

Draw Nitrogen (N) with 5 valence electrons, and Hydrogen (H) with one valence electron each. Connect N with four H atoms using single bonds. There are no lone pairs on nitrogen, and the ion has a positive charge reflecting the lost electron.

Identify Ionic and Covalent Parts - KOH

KOH is composed of the K+ cation and the OH- anion. K+ loses an electron (ionic), while OH- involves covalent bonding between O and H atoms. The Lewis structure for OH- will be enclosed in brackets.

Draw Lewis Structure for OH-

Draw Oxygen (O) with 6 valence electrons and Hydrogen (H) with one valence electron. Connect O and H using a single bond and complete the octet for oxygen by adding lone pairs. Include a negative charge on the ion.

Identify Ionic and Covalent Parts - Sr(CN)2

Sr(CN)2 is made up of the Sr2+ cation and two CN- anions. Sr2+ loses two electrons (ionic), while CN- involves covalent bonding between C and N atoms. The Lewis structure for CN- will be enclosed in brackets.

Draw Lewis Structure for CN-

Draw Carbon (C) with 4 valence electrons and Nitrogen (N) with 5 valence electrons. Form a triple bond between C and N to fulfill the octet rule for both atoms. Add a lone pair to N and place a negative charge on the ion.

Key concepts

Ionic and Covalent Bonds

Understanding ionic and covalent bonds is fundamental to grasping the structure of compounds in chemistry.

Ionic bonds form when one atom donates an electron to another, leading to the formation of positively and negatively charged ions. These oppositely charged ions attract each other and create a bond. For example, in the textbook exercise, rubidium (Rb) loses an electron to become Rb+, while in KOH, potassium (K) loses an electron to become K+.

On the other hand, covalent bonds occur when two atoms share electrons, resulting in a more stable electron configuration. Molecules like IO2- and NH4+ in the provided examples demonstrate covalent bonding where atoms share electrons to fill their valence shells.

In many compounds, both ionic and covalent bonds exist, which helps to explain the properties of the substance. For instance, the ionic bond between Rb+ and IO2- in RbIO2 results in a crystalline structure typical of many salts, while the covalent bonds within the IO2- ion allow for multiple bonding arrangements, known as resonance structures.

Resonance Structures

Resonance structures reveal the flexibility in electron sharing within molecules with covalent bonds. They are alternate Lewis structures for the same molecule, showing different possible arrangements of electrons.

Consider the IO2- ion from the exercise. The molecule can have a double bond between the iodine and one of the oxygen atoms while the other oxygen has a single bond. However, the double bond can resonate, or switch, between the two oxygen atoms. This leads to two valid Lewis structures, both accurately representing the molecule's real, averaged structure.

Resonance is crucial for understanding not only the structure but also the stability of the molecule. It spreads out charge and electrons more evenly across the molecule, often leading to increased stability. Whenever you encounter a molecule like the polyatomic ion IO2-, depicting all possible resonance structures helps illustrate the full range of electron delocalization.

Valence Electrons

The outermost electrons of an atom are known as valence electrons, and they play a critical role in chemical bonding. They are the electrons that participate in forming bonds with other atoms.

Knowing how many valence electrons an atom possesses is key to drawing Lewis structures—a graphical representation showing an atom's valence electrons and their interactions. For example, in NH4+, nitrogen has five valence electrons, and each hydrogen atom has one. Together, they share these electrons through covalent bonding to satisfy the octet rule, which states that atoms are most stable when they have eight valence electrons.

An understanding of valence electrons facilitates predictions about how atoms will interact, what kinds of bonds will form, and the geometrical structure of the resulting compound. The exercise shows how atoms like iodine, oxygen, nitrogen, and carbon share or transfer valence electrons to achieve stability and form compounds with distinct properties.