Electronegativity and Polarity Section 9-5

Electron affinity Tendency of atom to accept electron Usually increases as atomic numbers increase within period Usually decreases as atomic number increases within a group

Linus Pauling Developed scale of electronegativity Figure 9-15, pg 263 Fluorine has the highest: 3.98 Francium has the lowest: 0.7 Noble gases are not given (0)

Electron Affinity tells us… Character and type of bond – Never completely ionic or covalent – Sharing is not always completely equal

Ionic Bond Large differences in electronegativity indicate that electrons are transferred

Nonpolar covalent bonds Difference in electron affinity of atoms involved is very small Electrons shared fairly equally

Identical Atoms Difference in electronegativity is 0 Electrons are shared equally.

Polar Covalent Bonds Unequal sharing Electron tug-of-war Stronger affinity = greater strength

Differences Less than 0.4 = nonpolar covalent bond 0.4 – 1.7 = polar covalent bond Greater than 1.7 = ionic bond

Examples: H and Br 2.20 and 2.96 = 0.76 polar covalent C and O 2.55 and 3.44 = 0.89 polar covalent Li and F 0.98 and 3.98 = 3.00 ionic bond

Solubility of Polar Molecules Like dissolves like – Polar (and ionic) compounds are soluble in polar substances – Nonpolar only soluble in non-polar substances

Properties of Covalent Compounds Van der Waals forces: weak forces of attraction between individual molecules

Types of Intermolecular Forces Nonpolar – Weak attraction – Dispersion or induced dipole Polar – Stronger attraction – Dipole-dipole force Hydrogen bond – Very strong – Between H and another dipole

Physical properties Melting/boiling points lower than ionic Many are gases at room temp Non-conductors of heat or electricity Extremely hard *Due to the intermolecular forces!!