Chemical Bonding: Lewis Structures, Molecular Geometry, and Polarity

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Chemical Bonding

Introduction to Chemical Bonding

Chemical bonding describes how atoms combine to form compounds, influencing the properties and behavior of substances. Understanding bonding is essential for predicting molecular structure, reactivity, and physical properties.

Chemical bonds are formed by the transfer or sharing of electrons between atoms.
Bonding theories, such as the Lewis model, help predict which combinations of atoms form stable compounds.
Bonding models are crucial in drug design and understanding biological molecules.

Lewis Model of Chemical Bonding

Valence Electrons and Lewis Structures

The Lewis model represents valence electrons as dots around an element's symbol, providing a visual method to predict bonding and molecular stability.

Valence electrons are the electrons in the outermost shell of an atom and are involved in bonding.
The number of valence electrons for main-group elements equals the group number (except helium).
Lewis structures (dot structures) show how atoms achieve stable electron configurations by sharing or transferring electrons.
Atoms with eight valence electrons (an octet) are particularly stable; helium is an exception, stable with two electrons (a duet).

Types of Chemical Bonds

Atoms bond to achieve stable electron configurations, either by transferring or sharing electrons.

Ionic bonds: Electrons are transferred from a metal to a nonmetal, forming cations and anions.
Covalent bonds: Electrons are shared between nonmetals, forming molecules.
Octet rule: Atoms tend to gain, lose, or share electrons to achieve eight in their valence shell (except H and He).

Lewis Structures for Ionic Compounds

In ionic compounds, electrons are transferred from metals to nonmetals, resulting in charged ions held together by electrostatic attraction.

Example: Formation of KCl involves potassium transferring an electron to chlorine, forming K+ and Cl-.
Lewis structures for ions are shown with brackets and charges.
Stoichiometry is determined by the number of electrons transferred (e.g., Na2S requires two Na atoms for one S atom).

Lewis Structures for Covalent Compounds

Covalent compounds form when nonmetals share electrons to achieve octets or duets.

Shared electrons are called bonding pairs; unshared electrons are lone pairs.
Bonding pairs are often represented by dashes in Lewis structures.
Some elements form double or triple bonds to satisfy the octet rule.

Lewis structure of water showing duets and octet Formation of double bond in oxygen Lewis structure of O2 showing octets Formation of triple bond in nitrogen

Steps to Write Lewis Structures for Covalent Compounds

Follow these steps to construct accurate Lewis structures:

Write the correct skeletal structure, placing the least electronegative atom in the center (except hydrogen).
Calculate the total number of valence electrons, adjusting for ionic charges if necessary.
Distribute electrons to form bonds and complete octets (or duets for hydrogen).
If necessary, convert lone pairs to bonding pairs to satisfy the octet rule.

CCl4 Lewis structure with bonding electrons CCl4 Lewis structure with lone pairs

Lewis Structures for Polyatomic Ions

Polyatomic ions require special attention to charge when calculating total electrons. Their Lewis structures are enclosed in brackets with the charge indicated.

Lewis structure of cyanide ion

Exceptions to the Octet Rule

Some molecules do not follow the octet rule:

Odd-electron molecules (free radicals) cannot have all atoms with octets.
Boron compounds often have only six electrons around boron.
Expanded octets occur in some molecules with more than eight electrons around the central atom (not covered in detail).

Resonance Structures

Some molecules can be represented by more than one valid Lewis structure. These are called resonance structures, and the true structure is an average of all possibilities.

Resonance structures of SO2

Molecular Geometry and VSEPR Theory

Predicting Molecular Shapes

The shape of a molecule is determined by the repulsion between electron groups around the central atom, as described by Valence Shell Electron Pair Repulsion (VSEPR) theory.

Electron geometry considers all electron groups (bonds and lone pairs).
Molecular geometry considers only the arrangement of atoms.
Bond angles depend on the number and type of electron groups.

Linear geometry of CO2 Trigonal planar geometry Tetrahedral geometry of CH4 Space-filling model of CH4

Molecular Geometry with Lone Pairs

Lone pairs on the central atom affect molecular geometry and bond angles.

NH3 has a trigonal pyramidal geometry due to one lone pair.
H2O has a bent geometry due to two lone pairs.

Lewis structure of NH3 Ball-and-stick model of NH3 Trigonal pyramidal structure of NH3 Lewis structure of H2O Ball-and-stick model of H2O Bent structure of H2O

Summary Table: Electron and Molecular Geometries

Electron and molecular geometries can be summarized as follows:

Electron Groups	Electron Geometry	Molecular Geometry	Bond Angles
2	Linear	Linear	180°
3	Trigonal planar	Trigonal planar / Bent	120°
4	Tetrahedral	Tetrahedral / Trigonal pyramidal / Bent	109.5°
Additional info:	Actual bond angles may vary due to lone pairs.

Major molecular geometries

Electronegativity and Polarity

Electronegativity

Electronegativity is the ability of an atom to attract electrons in a covalent bond. It determines bond polarity and influences molecular interactions.

Electronegativity increases across a period and decreases down a group.
Fluorine is the most electronegative element (4.0 on the Pauling scale).

Electronegativity periodic table

Bond Polarity and Dipole Moments

The difference in electronegativity between two atoms determines the type of bond:

Nonpolar covalent bond: Electrons shared equally (e.g., Cl2).
Polar covalent bond: Electrons shared unequally, creating partial charges and a dipole moment (e.g., HCl).
Ionic bond: Electrons transferred completely (e.g., NaCl).

Nonpolar Cl2 molecule Ionic NaCl molecule Polar HCl molecule Continuum of bond types

Polar Molecules

A molecule is polar if its polar bonds do not cancel out, resulting in a net dipole moment. Molecular geometry plays a crucial role in determining overall polarity.

CO2 is nonpolar despite having polar bonds due to its linear geometry.
H2O is polar because its bent geometry prevents dipole cancellation.

Dipole moments in CO2 Dipole moments in H2O

Applications and Everyday Chemistry

Why Oil and Water Don’t Mix

Water molecules are polar, while oil molecules are nonpolar. Polar molecules attract each other and exclude nonpolar molecules, leading to phase separation.

How Soap Works

Soap molecules have a polar head and a nonpolar tail, allowing them to interact with both water and grease, enabling the removal of nonpolar substances from surfaces.

Soap molecule structure

Review and Learning Objectives

Write Lewis structures for elements, ionic, and covalent compounds.
Determine whether a compound is ionic or covalent.
Write resonance structures.
Predict electron and molecular geometry using VSEPR theory.
Determine molecular polarity based on bond polarity and geometry.