Chapter 2 Chemical bonds

Introduction

A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of electrons as in covalent bonds. The strength of chemical bonds varies considerably; there are “strong bonds” or “primary bonds” such as covalent, ionic and metallic bonds, and “weak bonds” or “secondary bonds” such as dipole–dipole interactions, the London dispersion force, and hydrogen bonding.

There are several types of chemical bonds known in chemistry, each with distinct characteristics and properties. The main types of chemical bonds include:

  1. Ionic Bonds: Ionic bonds form between atoms when one atom transfers electrons to another, resulting in the formation of positively charged ions (cations) and negatively charged ions (anions). The electrostatic attraction between these oppositely charged ions holds them together in an ionic compound.
  2. Covalent Bonds: Covalent bonds occur when atoms share pairs of electrons to achieve a stable electron configuration. These bonds are typically formed between nonmetal atoms and can be single, double, or triple bonds depending on the number of shared electron pairs.
  3. Polar Covalent Bonds: Polar covalent bonds are a type of covalent bond where electrons are shared unequally between atoms, leading to partial positive and negative charges within the molecule. This occurs when there is a significant difference in electronegativity between the bonded atoms.
  4. Nonpolar Covalent Bonds: Nonpolar covalent bonds are covalent bonds in which electrons are shared equally between atoms, resulting in a balanced distribution of charge within the molecule. This occurs when the electronegativity difference between bonded atoms is minimal.
  5. Hydrogen Bonds: Hydrogen bonds are weak electrostatic attractions that occur between a hydrogen atom covalently bonded to an electronegative atom (such as oxygen, nitrogen, or fluorine) and another electronegative atom in a nearby molecule. Hydrogen bonds play important roles in various biological processes and the properties of water.
  6. Metallic Bonds: Metallic bonds are found in metals and alloys, where positively charged metal ions are held together in a sea of delocalized electrons. This electron “sea” allows metals to conduct electricity and heat, and gives them their characteristic luster and malleability.
  7. Van der Waals Forces: Van der Waals forces are weak attractions between molecules or atoms resulting from temporary fluctuations in electron distribution. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding (a type of dipole-dipole interaction).

These types of chemical bonds play crucial roles in determining the physical and chemical properties of substances, including their melting and boiling points, solubility, conductivity, and reactivity. Understanding these bonds is essential for predicting and explaining the behavior of molecules and compounds in various chemical reactions and interactions.

Ionic Bonding 

An ionic bond can be formed after two or more atoms loss or gain electrons to form an ion. Ionic bonds occur between metals, losing electrons, and nonmetals, gaining electrons.

Lewis Dot Structures

The Ions That Elements Form Lewis Symbols

The Energetics of Ionic Bond Formation


Ionic bond formation is a type of chemical bonding that occurs between atoms when one or more electrons are transferred from one atom to another. This transfer of electrons results in the formation of positively charged ions (cations) and negatively charged ions (anions) that are attracted to each other due to their opposite charges. Ionic bonds are typically formed between metals and nonmetals, where the metal atom loses electrons to form a cation, and the nonmetal atom gains those electrons to form an anion.

Here are the key steps involved in ionic bond formation:

  1. Electron Transfer: In an ionic bond formation, one atom (typically a metal) donates one or more electrons to another atom (typically a nonmetal). The donating atom becomes a positively charged ion (cation) because it loses electrons, and the receiving atom becomes a negatively charged ion (anion) because it gains electrons.
  2. Formation of Oppositely Charged Ions: As a result of electron transfer, oppositely charged ions are formed. For example, if a sodium (Na) atom loses an electron, it becomes a Na⁺ cation, and if a chlorine (Cl) atom gains that electron, it becomes a Cl⁻ anion.
  3. Electrostatic Attraction: The positively charged ions (cations) are attracted to the negatively charged ions (anions) due to electrostatic forces. This attraction is what holds the ions together and forms an ionic bond.
  4. Ionic Compound Formation: The positively and negatively charged ions arrange themselves in a three-dimensional lattice structure, forming an ionic compound. This compound is electrically neutral overall because the total positive charge from the cations balances the total negative charge from the anions.
  5. Properties of Ionic Compounds: Ionic compounds tend to have high melting and boiling points because the ionic bonds are strong and require a lot of energy to break. They also tend to be soluble in water and conduct electricity when dissolved or melted, but not as solids because the ions are free to move in the liquid or molten state.

Examples of ionic compounds formed through ionic bond formation include sodium chloride (NaCl), potassium iodide (KI), calcium oxide (CaO), and magnesium sulfate (MgSO₄).

It’s important to note that ionic bonds are different from covalent bonds, where electrons are shared between atoms rather than transferred completely. Ionic bonding is common in compounds involving elements with large differences in electronegativity, leading to strong attractions between oppositely charged ions.

Interactions Between Ions


Covalent Bonding 

Covalent bonding is a type of chemical bonding in which atoms share pairs of electrons to achieve a stable electron configuration. Unlike ionic bonding, where electrons are transferred between atoms, covalent bonds involve the sharing of electrons between atoms to form molecules. Covalent bonds are typically formed between nonmetal atoms, although they can also occur between nonmetal and metalloid atoms.

Here are the key features and characteristics of covalent bonding:

  1. Sharing of Electrons: In a covalent bond, two or more atoms share pairs of electrons to fill their valence shells and achieve a stable electronic configuration. Each atom contributes one or more electrons to the shared pair, creating a bond.
  2. Formation of Molecules: Covalent bonds are responsible for the formation of molecules, which are collections of atoms held together by covalent bonds. Examples of covalent molecules include water (H₂O), methane (CH₄), oxygen (O₂), and carbon dioxide (CO₂).
  3. Types of Covalent Bonds: Covalent bonds can be classified into two main types based on the sharing of electrons:
    • Single Covalent Bond: In a single covalent bond, two atoms share one pair of electrons. This is the simplest type of covalent bond and is represented by a single line (e.g., H₂, Cl₂).
    • Multiple Covalent Bonds: Atoms can also share multiple pairs of electrons, leading to double (two shared pairs) or triple (three shared pairs) covalent bonds. Examples include oxygen (O₂) with a double bond and nitrogen (N₂) with a triple bond.
  4. Electron Dot Structures: Covalent bonds can be represented using electron dot structures (Lewis structures), where valence electrons are shown as dots around the atomic symbols. Shared pairs of electrons are represented by a line between the atoms.
  5. Bond Length and Bond Strength: The distance between the nuclei of bonded atoms in a covalent bond is known as the bond length. The strength of a covalent bond depends on factors such as the number of shared electrons and the types of atoms involved.
  6. Polar and Nonpolar Covalent Bonds: Covalent bonds can be polar or nonpolar based on the electronegativity difference between the bonded atoms. In polar covalent bonds, electrons are unequally shared, leading to partial charges on the atoms. In nonpolar covalent bonds, electrons are equally shared.
  7. Properties of Covalent Compounds: Covalent compounds often have lower melting and boiling points compared to ionic compounds. They may exist as gases, liquids, or solids at room temperature, depending on the intermolecular forces between molecules.

Overall, covalent bonding plays a crucial role in the formation of molecules and the structure and properties of various compounds in chemistry.

     

Lewis Stnurtures

Species

 Resonance

In chemistry, resonance refers to a phenomenon observed in molecules and polyatomic ions where the actual structure of the molecule is a combination or hybrid of several different resonance structures. Resonance occurs when there are multiple valid Lewis structures that can be drawn for a molecule or ion, and the true electronic structure is represented as a weighted average or blend of these resonance forms.

Key points about resonance in chemistry:

  1. Multiple Lewis Structures: Resonance arises when there are multiple ways to arrange electrons in a molecule without violating the octet rule or the rules of electron pair arrangement. These different arrangements are represented as separate Lewis structures, called resonance structures.
  2. Equivalent Resonance Structures: Resonance structures are equivalent if they have the same placement of atoms but different arrangements of electrons. They contribute equally to the overall electronic structure of the molecule.
  3. Delocalization of Electrons: In a molecule with resonance, electrons are delocalized over multiple atoms or bonds rather than being confined to a single bond or atom. This delocalization stabilizes the molecule and lowers its overall energy.
  4. Resonance Contributors: Resonance structures are often referred to as resonance contributors, and the actual electronic structure of the molecule is a resonance hybrid that incorporates aspects of all valid resonance structures.
  5. Formal Charge Consideration: When drawing resonance structures, it is important to consider formal charges to ensure that the overall charge of the molecule or ion is consistent with its actual charge.
  6. Examples of Resonance:
  1. Effects on Chemical Properties: Resonance can affect the chemical properties of molecules, such as their stability, reactivity, and bond lengths. Molecules with resonance tend to be more stable and have lower energy than would be predicted by a single Lewis structure.

It’s important to note that resonance is a theoretical concept used to describe the electronic structure of molecules and ions. The actual electronic distribution in a molecule is a combination of all valid resonance structures, resulting in a more accurate representation of the molecule’s behavior and properties.

 Formal Charge

The Octet Rule is a fundamental concept in chemistry that states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration similar to the noble gases, which typically have eight electrons in their outermost shell (except for helium, which has two electrons in its outer shell).

Key points about the Octet Rule:

  1. Stable Electron Configuration: Atoms are most stable when their outermost shell (valence shell) is filled with eight electrons, known as an octet. This stable configuration is similar to the noble gases, which have full valence shells.
  2. Tendency to Gain or Lose Electrons: Atoms will gain or lose electrons to achieve an octet in their valence shell. For example, nonmetals tend to gain electrons to form negatively charged ions (anions) with an octet, while metals tend to lose electrons to form positively charged ions (cations) with an octet.
  3. Exceptions to the Octet Rule: While the Octet Rule is a useful guideline, there are several cases and exceptions where atoms do not follow the rule completely:
    • Hydrogen (H) and helium (He) can achieve a stable electron configuration with only two electrons in their outer shell (a duet), so they do not always follow the Octet Rule.
    • Some elements can have expanded octets, meaning they can accommodate more than eight electrons in their valence shell. This occurs in elements from the third period and beyond (e.g., phosphorus, sulfur, chlorine) that can utilize d orbitals to hold additional electrons.
    • Some molecules and ions have an odd number of electrons, making it impossible for all atoms to achieve an octet. Examples include NO, NO2, and O3.
    • Certain compounds and ions, such as boron compounds (e.g., BF3), can have incomplete octets due to a shortage of available electrons for bonding.
    • Some transition metals and metalloids can exhibit variable oxidation states and may not strictly follow the Octet Rule due to their unique electronic configurations.
  4. Importance in Chemical Bonding: The Octet Rule helps explain the formation of chemical bonds, including ionic bonds (where electrons are transferred) and covalent bonds (where electrons are shared). It provides a basis for understanding the stability and reactivity of atoms and molecules.

In summary, while the Octet Rule is a valuable concept for understanding chemical bonding and electron configurations, there are cases and exceptions where atoms deviate from achieving an octet in their valence shell. These exceptions are important to consider in certain chemical reactions and structures.

Beyond the Octet Rule

Radicals and Biradicals

What Has This tn Da

With . . . Staying Alive? Chern cal Salf-Pwianration

Expanded Valence Shells

 Incwnplata Octets i

Topic D The Properties of Bonds

Correcting the Covalent Model:

Electronegativity

Correcting the tonic Model:

Polarizab Iity ! Bo nd Strengths

Band Lengths