Chemical Bonding and Molecular Geometry

Compounds and Ionic Bonding

Compounds are substances composed of two or more elements combined in a specific ratio and held together by chemical bonds. Ionic bonding refers to the electrostatic attraction that holds oppositely charged ions together in an ionic compound. Typically, ionic compounds are formed between metals and nonmetals, resulting in electrically neutral compounds.

• Cation: A positively charged ion (usually a metal).

• Anion: A negatively charged ion (usually a nonmetal).

• Chemical Formula: Denotes the constituent elements and their ratio (e.g., NaCl for sodium chloride).

Classification of Elements: Metals, Nonmetals, and Metalloids

Elements are classified based on their physical and chemical properties:

• Metals: Good conductors of heat and electricity, ductile, malleable, and mostly solid at room temperature (except mercury).

• Nonmetals: Poor conductors, can exist in all physical states, and are generally brittle when solid.

• Metalloids: Exhibit properties intermediate between metals and nonmetals; often semiconductors.

Lattice Energy and Ionic Compound Stability

Lattice Energy

Lattice energy is the energy required to convert one mole of an ionic solid into its constituent ions in the gas phase. It is a measure of the stability of an ionic compound and depends on the magnitude of the charges and the distance between ions.

• Formula: , where is the charge and is the distance between ions.

• Higher lattice energy indicates greater stability and higher melting points.

Electron Configurations of Ions

Writing Electron Configurations

To write the electron configuration of an ion:

1. Write the configuration for the neutral atom.

2. Add electrons for negative ions or remove electrons for positive ions.

Species with the same electron configuration as a noble gas are called isoelectronic.

Bond Types and Electronegativity

Covalent, Polar Covalent, and Ionic Bonds

Bonds can be classified based on the difference in electronegativity between the bonded atoms:

• Pure Covalent: Electrons are shared equally (difference < 0.5).

• Polar Covalent: Electrons are shared unequally (difference 0.5–2.0).

• Ionic: Electrons are transferred (difference > 2.0).

Electronegativity and Bond Polarity

Electronegativity is the ability of an atom to attract electrons in a chemical bond. The greater the difference, the more polar the bond.

• Partial charges are denoted by δ+ (partial positive) and δ− (partial negative).

• Dipole moment () quantifies bond polarity:

Percent Ionic Character

The percent ionic character of a bond is calculated by comparing the observed dipole moment to the calculated value for complete electron transfer:

Naming and Writing Formulas for Ionic Compounds

Types of Ionic Compounds

• Metal + Nonmetal (e.g., NaCl)

• Metal + Polyatomic Ion (e.g., Ca(BrO3)2)

• Polyatomic Ion + Nonmetal (e.g., NH4Cl)

• Polyatomic Ion + Polyatomic Ion (e.g., NH4NO3)

Naming Rules

• Name the cation first (use Roman numerals for variable charge metals).

• Name the anion, changing the ending to -ide for monatomic ions or using the polyatomic ion name.

• For formulas, ensure the compound is electrically neutral by balancing charges (cross-multiplication or sum of charges).

Polyatomic Ions and Oxoanions

Polyatomic Ions

Polyatomic ions are covalently bonded groups of atoms with an overall charge. Their formulas and charges must be memorized for effective naming and formula writing.

• Oxoanions contain oxygen and another element (e.g., SO42−, NO3−).

• Naming conventions: per-...-ate (most O), -ate, -ite (less O), hypo-...-ite (least O).

Naming Molecular Compounds

Binary Molecular Compounds

Binary molecular compounds consist of two nonmetals. Greek prefixes indicate the number of atoms:

• Mono- (1), Di- (2), Tri- (3), Tetra- (4), Penta- (5), Hexa- (6), Hepta- (7), Octa- (8), Nona- (9), Deca- (10)

• First element keeps its name; second element ends in -ide.

• Omit "mono-" for the first element.

Lewis Dot Symbols and Structures

Valence Electrons and Lewis Dot Symbols

Valence electrons are the outermost electrons involved in bonding. Lewis dot symbols represent these electrons as dots around the element symbol.

Lewis Structures and Multiple Bonds

Lewis structures depict covalent bonding using dots for electrons and dashes for bonds. Multiple bonds (double, triple) are shorter and stronger than single bonds.

Resonance Structures

Some molecules cannot be represented by a single Lewis structure. Resonance structures are multiple valid Lewis structures for the same molecule, differing only in electron placement.

Exceptions to the Octet Rule

• Fewer than 8 electrons (e.g., BF3)

• Odd number of electrons (free radicals, e.g., NO)

• More than 8 electrons (expanded octet, e.g., SF6)

Coordinate Covalent Bonds

A coordinate covalent (dative) bond occurs when both electrons in a bond are donated by one atom, as in the reaction of BF3 with NH3.

Molecular Geometry: VSEPR Theory

VSEPR Model

The Valence Shell Electron Pair Repulsion (VSEPR) model predicts molecular shapes based on electron domain repulsion. Electron domains include bonds (single, double, triple) and lone pairs.

• 2 domains: Linear (180°)

• 3 domains: Trigonal planar (120°)

• 4 domains: Tetrahedral (109.5°)

• 5 domains: Trigonal bipyramidal (90°, 120°)

• 6 domains: Octahedral (90°)

Electron-Domain vs. Molecular Geometry

Electron-domain geometry considers all electron domains, while molecular geometry considers only the arrangement of atoms. Lone pairs affect the molecular shape by repelling bonding pairs more strongly.

Bond Angles and Deviations

Lone pairs and multiple bonds cause deviations from ideal bond angles due to increased repulsion.

Molecular Polarity

Bond and Molecular Polarity

A molecule is polar if the vector sum of its bond dipoles does not cancel out. Molecular geometry determines whether polar bonds result in a polar molecule.

Intermolecular Forces (IMFs)

Types of Intermolecular Forces

• Dispersion (London) Forces: Weakest, present in all molecules, increase with molecular size and surface area.

• Dipole-Dipole: Between polar molecules, stronger than dispersion.

• Hydrogen Bonding: Strongest dipole-dipole, occurs when H is bonded to N, O, or F.

Trends in IMFs

• Stronger IMFs lead to higher melting and boiling points.

• Order of strength: Ionic > H-bonding > Dipole-dipole > Dispersion

Summary Table: Electron Domain and Molecular Geometries

*Additional info: More complex geometries (trigonal bipyramidal, octahedral) follow similar logic with more electron domains and lone pairs affecting the shape.*