chapter 4 - molecular geometry, polarity, & intermolecular forces of attraction

4.1 predicting electron geometry & molecular geometry

• lewis theory in combo w valence-shell electron-pair repulsion theory, can be used to predict the shapes of molecules

• 3 step process used to determine shape of a molecule

• lewis dot structure → electron geometry → molecular geometry (all depend on each other)

• electron geometry - relative position of the groups of electrons on the central atom

• molecular geometry - relative position of the atoms in a molecule, determined by electron geometry

electron geometry

• VSEPR predicts the arrangements of valence electron pairs (group of electrons) around a central atom that minimizes their mutual repulsion

• a group of electrons are found in various domains

• lone pairs or nonbonding pair, single blonds, double bonds, triple bonds

• group of valence electrons = domains = regions

linear

• # of domains: 2 double bonds → 2 regions/domains

trigonal planar

• 1 double bond, 1 lone pair, 1 single bond → 3 regions/domains

tetrahedral

• 3 single bonds

• 1 lone pair

• 4 regions/domains

for molecular geometry, look at lone pairs (nonbonding regions)

4.2 - bond dipoles & molecular polarity

molecular polarity

• determines how molecules interact w each other at the molecular level

electronegativity

• ability of an atom when part of a covalent bond to draw electrons toward its nucleus

bond dipoles

• electronegativity differences determine if a pole is polar or nonpolar

• difference in electronegativity values for 2 atoms can be used to predict type of chemical bond    

• 2 atoms in a molecule/compound

  • nonpolar covalent - 0-0.4, electrons shared equally

  • polar covalent bond - 0.5-1.9, electrons shared unequally

  • ionic bond - 2.0 or higher, electrons transferred

K-N - 0.8-3.0 = 2.2 → ionic

N-O - 3.0-3.5 = 0.5 → polar covalent

Cl-Cl - 3.0-3.0 = 0 → nonpolar

H-Cl - 2.1-3.0 = 0.9 → polar

ionic & covalent for metalloids or semimetals

• ionic compound - metal & nonmetal

• covalent compound - nonmetal & nonmetal

nonpolar & polar molecules

• have very diff physical & chemical properties

• entire molecules can be polar or nonpolar depending on its molecular shape

  • polar → overall charge separation

  • nonpolar → even distribution of electrons

molecular polarity

• nonpolar → even, homogenous

• polar → uneven

4.3 - intermolecular forces of attraction in a compound

• intramolecular forces of attraction

  • forces that keep a molecule together

    • ionic

    • covalent

• intermolecular forces of attraction

  • forces or repulsion which act between neighboring particles (atoms, molecules, or ions)

    • much weaker than covalent bonds

    • determine physical properties of a compound

      • london (dispersion) forces

      • dipole-dipole forces

      • hydrogen bonding

dispersion forces

• weak attractions between nonpolar molecules

• caused by temporary dipoles

• effect of shape & size

  • propane C3H8 → methane CH4

  • iodine I2 → fluorine F2

  • alkanes exhibit an increase in boiling point as their molar mass increases

  • higher boiling point = stronger the force

• constitutional isomers

  • same molecular formula but diff connections

hydrogen bonding forces

• are dipole-dipole forces forming between molecules w strongest bond dipoles

• form between the partial positive pole of

melting points, boiling points, & attractive forces

• melting & boiling points are related to the strength of attractive forces between molecules or compounds