Intermolecular Forces Learning Goal 3: Part A

Intermolecular Forces Forces of attraction between different molecules rather than bonding forces within the same molecule. Dipole-dipole attraction Hydrogen bonds Dispersion forces

Forces and Phases of Matter Substances with very little intermolecular attraction exist as gases Substances with strong intermolecular attraction exist as liquids Substances with very strong intermolecular (or ionic) attraction exist as solids

Types of Intermolecular Forces 1. Hydrogen Bond (strongest) Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen. IT IS NOT A BOND. A H … B or A & B are N, O, or F 11.2

Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment. H F + -

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

Types of Intermolecular Forces 2. Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction 11.2

Types of Intermolecular Forces 3. Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

11.2

Ion-Dipole Forces

Types of Intermolecular Forces 4. Dispersion Forces – van der Walls forces/London forces (weakest) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction 11.2

Summary: Intermolecular Forces Formed by the attraction between … Examples Ion-dipole an ion and a polar molecule Na+ and H2O Hydrogen bond molecules which have H on N, O, or F atoms H2O and H2O; H2O and CH3CH2OH Dipole - Dipole two polar molecules CH3Br and ICl; CH3Br and H2O Ion - Induced dipole an ion and a nonpolar Fe2+ and O2 Dipole - Induced dipole a polar molecule and a nonpolar molecule HCl and Cl2 London (dispersion) forces two nonpolar molecules CH4 and CH4; F2 and F2; CH4 and F2

MOLECULAR GEOMETRY Learning Goal #3 Part B

VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory. Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.

Some Common Geometries Linear Trigonal Planar Tetrahedral

VSEPR charts Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Molecule takes the shape of that portion of the electron arrangement Charts look at the CENTRAL atom for all data!

Structure Determination by VSEPR Water, H2O The electron pair geometry is TETRAHEDRAL 2 bond pairs 2 lone pairs The molecular geometry is BENT.

Methane, CH4

Ammonia, NH3

Water, H2O

Rules for Predicting Molecular Structure using the VSEPR Model Two pairs of electron on a central atom in a molecule are always placed 180o apart (linear). Three pairs of electrons on a central atom in a molecule are always placed 120o apart on the same plane as the central atom (trigonal Planar). Four pairs of electrons on a central atom in a molecule are always placed 109.5o apart (tetrahedral).

Rules for Predicting Molecular Structure using the VSEPR Model When every pair of electrons on the central atom is shared with another atom, the molecular structure has the same name as the arrangement of the electron pairs. Number of Pairs Name of Arrangement 2 Linear 3 Trigonal Planar 4 tetrahedral

Rules for Predicting Molecular Structure using the VSEPR Model When one or more of the electron pairs around a central atom are lone pairs, the name for the molecular structure is different from the arrangement of electron pairs.

Determining polarity of a molecule based on the symmetry of the molecule.