Electronegativity – The tendency of an atom in a bond to attract shared electrons to itself.

1. Covalent Bond – An attraction between two atoms caused by the sharing of a pair of electrons between two atoms. Polar Covalent – A covalent bond in which electrons are shared unequally. Nonpolar covalent – electrons are shared equally. SYMMETRICAL arrangement of valence electrons.

2. Electronegativity – The tendency of an atom in a bond to attract shared electrons to itself. Look at electronegativity difference to determine bond type:

3. Partially Positive Partially Negative

4. Polar Molecule – A molecule in which valence electrons (bonds and unshared pairs of electrons) are not equally distributed (asymmetrical) To be polar, a molecule must: 1. contain polar covalent bonds 2. be asymmetrical

5. Examples – Is each molecule polar or nonpolar? • CO2 • H2S • CCl4 • NBr3

6. The covalent bond holding a molecule together is an intramolecular force. • The attraction between molecules is an intermolecular force. • Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl). • When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).

7. Intermolecular Forces • The covalent bond holding a molecule together is an intramolecular force. • The attraction between molecules is an intermolecular force. • Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl). • When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).

9. Intermolecular Forces Dipole-dipole forces – Attraction between polar molecules (also called dipoles). Molecules are aligned with the “partial positive” end of one molecule near the “partial negative” end of another.

10. London Dispersion Forces • Weakest of all intermolecular forces. • It is possible for two adjacent neutral molecules to affect each other. • The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). • For an instant, the electron clouds become distorted. • In that instant a dipole is formed (called an instantaneous dipole).

11. One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). • The forces between instantaneous dipoles are called London dispersion forces.

12. Hydrogen Bonding • Special case of dipole-dipole forces. • By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high. • Intermolecular forces are abnormally strong.

13. H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). • Electrons in the H-X (X = electronegative element) lie much closer to X than H. • H has only one electron, so in the H-X bond, the + H presents an almost bare proton to the - X. • Therefore, H-bonds are strong.

14. Hydrogen Bonding

15. Summary of Intermolecular Forces • Nonpolar – contains only London dispersion forces (LDF) • Polar – contains LDF and dipole-dipole forces • Polar with H bonded to N, O, or F (with unshared pair) – contains LDF, dipole-dipole forces, and hydrogen bonds. • Larger molecule, stronger LDF (all other factors equal)