Chemical Bonds: Structure, Types, and Properties

Chemical Bonds

Introduction to Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. Understanding these bonds is essential for explaining the properties and behaviors of substances, such as the transformation of carbon from soot to diamond under high temperature and pressure.

• Chemical bonds determine the structure and stability of matter.

• Different types of bonds (ionic, covalent, metallic) result in different physical and chemical properties.

Stable Electron Configurations

Noble Gases and the Octet Rule

Noble gases (helium, neon, argon) are chemically inert due to their stable electron configurations, typically with eight electrons in their outermost shell (an octet).

• Fact: Noble gases are unreactive because of their full valence shells.

• Theory: Atoms become less reactive when they achieve a noble gas electron configuration.

• Deduction: Atoms tend to gain, lose, or share electrons to achieve stability.

Lewis (Electron Dot) Symbols

Visualizing Valence Electrons

G. N. Lewis introduced a method to represent valence electrons as dots around the atomic symbol, helping visualize bonding and electron transfer.

• Lewis dot symbols show only the valence electrons.

• Example: Sodium (Na) has one dot; Oxygen (O) has six dots.

Group	1A	2A	3A	4A	5A	6A	7A	Noble Gases
Element	Li·	Be··	B···	C····	N·····	O······	F·······	Ne········

Ionic Bonds and Compounds

Formation of Ions

Atoms form ions by gaining or losing electrons to achieve stable electron configurations. Metals lose electrons to form cations, while nonmetals gain electrons to form anions.

• Cation: Positively charged ion (e.g., Na+).

• Anion: Negatively charged ion (e.g., Cl-).

Octet Rule

Atoms tend to gain, lose, or share electrons to have eight valence electrons.

Crystal Lattice Structure

Ionic compounds are held together by electrostatic attraction in a repeating three-dimensional pattern called a crystal lattice.

Symbols and Names for Simple Ions

Element	Name of Ion	Symbol
Sodium	Sodium ion	Na+
Chlorine	Chloride ion	Cl-
Oxygen	Oxide ion	O2-
Iron	Iron(II) ion	Fe2+
Iron	Iron(III) ion	Fe3+

Naming Binary Ionic Compounds

• Name the cation first, then the anion (e.g., NaCl = sodium chloride).

• For transition metals with multiple charges, use Roman numerals (e.g., FeCl3 = iron(III) chloride).

Covalent Bonds

Formation and Types

Nonmetallic elements often form covalent bonds by sharing electrons. Atoms can share one, two, or three pairs of electrons, resulting in single, double, or triple bonds.

• Single bond: One pair of shared electrons (e.g., H–H).

• Double bond: Two pairs of shared electrons (e.g., O=O).

• Triple bond: Three pairs of shared electrons (e.g., N≡N).

Naming Binary Covalent Compounds

• Use prefixes to indicate the number of atoms (mono-, di-, tri-, tetra-, etc.).

• First element: prefix + name (drop 'mono-' if only one atom).

• Second element: prefix + root + '-ide'.

Prefix	Number
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6

• Example: SBr4 = sulfur tetrabromide; P2O3 = diphosphorus trioxide.

Electronegativity and Bond Polarity

Electronegativity

Electronegativity is a measure of an atom's ability to attract electrons in a bond. Differences in electronegativity determine bond type:

Electronegativity Difference	Bond Type
< 0.5	Nonpolar covalent
0.5–2.0	Polar covalent
> 2.0	Ionic

• Polar covalent bond: Electrons are shared unequally, creating partial charges (δ+ and δ-).

• Nonpolar covalent bond: Electrons are shared equally.

Polyatomic Ions

Definition and Examples

Polyatomic ions are groups of covalently bonded atoms with an overall charge.

Name	Formula
Ammonium ion	NH4+
Sulfate ion	SO42-
Nitrate ion	NO3-
Carbonate ion	CO32-

• When writing formulas, use parentheses if more than one polyatomic ion is needed (e.g., Ca(NO3)2).

Lewis Structures

Rules for Sketching Lewis Structures

1. Count total valence electrons.

2. Draw a skeletal structure.

3. Place lone pairs on outer atoms to fulfill the octet rule.

4. Place remaining electrons on the central atom.

5. If the central atom lacks an octet, form double or triple bonds as needed.

Molecular Shapes: The VSEPR Theory

Predicting Molecular Geometry

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on repulsions between electron pairs around a central atom.

• Linear: 2 electron sets, 180° bond angle

• Trigonal planar: 3 electron sets, 120° bond angle

• Tetrahedral: 4 electron sets, 109.5° bond angle

Polarity of Molecules

Polar vs. Nonpolar Molecules

A molecule is polar if it contains polar bonds arranged asymmetrically, resulting in a net dipole moment.

• Both bond polarity and molecular shape determine overall polarity.

• Example: H2O is polar; CO2 is nonpolar.

Odd Electron Molecules: Free Radicals

Free radicals are atoms or molecules with an unpaired electron, making them highly reactive. Examples include NO, NO2, and ClO2.

Summary Table: Types of Chemical Bonds

Bond Type	Electron Behavior	Example
Ionic	Transfer	NaCl
Covalent	Sharing	H2O
Polar Covalent	Unequal sharing	HCl