IB Chemistry 11 Notes on Bonding and Structure

Nature of Science

Scientific theories offer explanations for natural phenomena, emphasizing that the behavior of substances can be understood through testing predictions based on these theories. For example, the ability of molten ionic compounds to conduct electricity while solid ionic compounds cannot is explained by the breaking of ionic lattices. Additionally, compounds made solely of non-metals exhibit different properties compared to those containing both metals and non-metals.

Models of Bonding

The Importance of Models

Scientists utilize various models, like the Valence Shell Electron Pair Repulsion (VSEPR) model, to represent the structure and bonding within molecules. These representations help predict observable properties. Both London dispersion forces and hydrogen bonding serve as examples to explain specific intermolecular interactions, indicating that molecular covalent compounds' market existence in solid or liquid states implies strong attractive forces surpassing gravitational ones.

Key Vocabulary

1. Delocalized: Electrons that are shared among several atoms rather than associated with a specific atom.

2. Cation/Anion: Ions that are positively or negatively charged, respectively, formed when atoms lose or gain electrons.

3. Ionic Bond: The electrostatic force holding oppositely charged ions together.

4. Covalent Bond: A bond formed when two atoms share electrons.

5. Electronegativity: The tendency of an atom to attract electrons in a bond, influencing bond polarity and often termed polar if there is a significant difference between atoms.

6. Intermolecular Forces: Forces that exist between molecules, including dipole-dipole interactions, London dispersion forces, and hydrogen bonding.

Types of Bonds and Their Properties

Metallic Bonding

A metallic bond emerges from the electrostatic attraction between delocalized electrons and a lattice of positively charged metal ions. The strength of metallic bonds depends on both the charge and size (radius) of the metal ion.

• Properties:

  • Conduct electricity and heat due to free-moving electrons.

  • Malleable and ductile as atoms can slide past each other without breaking the metallic bond.

Ionic Bonding

Formation of ions occurs when metal atoms lose electrons to become cations, while non-metals gain electrons to form anions. Ionic bonds result from the attraction between these oppositely charged ions, creating a structured, three-dimensional lattice.

• Properties:

  • High melting and boiling points due to strong ionic bonds.

  • Brittle and hard, do not conduct electricity in solid form, but conduct when molten or dissolved.

Covalent Bonding

Covalent bonds form through the sharing of electron pairs between non-metals. These bonds can exist as single, double, or triple bonds, depending on how many pairs of electrons are shared.

• Properties:

  • Low melting and boiling points compared to ionic compounds.

  • Generally poor electrical conductors as they do not form charged species in solution.

VSEPR Theory

The VSEPR model helps in predicting the molecular shapes based on the repulsion between electron pairs around a central atom, leading to arrangements that minimize repulsion.

• Shapes:

  • Linear: Two electron domains.

  • Trigonal Planar: Three electron domains, bond angles of 120°.

  • Tetrahedral: Four electron domains, bond angles of 109.5°.

  • Trigonal Pyramidal & Bent: Variants of tetrahedral structures with lone pairs.

Intermolecular Forces

Different types of intermolecular forces—hydrogen bonds, dipole-dipole, and London dispersion forces—are crucial in determining the physical properties such as boiling and melting points. For instance, hydrogen bonds are stronger than dipole-dipole interactions, influencing the boiling points of substances like HF and HCl.

Chromatography

Chromatography is a method used for separating mixtures based on their differing affinities for a stationary and a mobile phase. Solutes will move at different rates depending on their solubility and interaction with the stationary phase, which allows for the separation of components within a mixture.