Electron Affinities Tendency of an atom to accept an electron Increases across a period (except 8A) Decreases down a family Greatest affinity = fluorine Related to electronegativity

Electronegativity Assigned # or scale (not measured) Indicates an atom’s ability to attract electrons in a bond Same trends as electron affinity Difference in electronegativities determines the type of bond formed: ionic, polar covalent or nonpolar covalent (pure covalent)

Determining Bond Type Electronegativity difference: –If 0, bond = nonpolar covalent or pure covalent –If >1.70 = ionic –If <1.70 = polar covalent –Look at table & graph (p )

Check for understanding Use the table of electronegativities (p. 263) to determine the type of bond: H 2 S CH 4 KCl N 2 CaO

Answers: H 2 S = polar covalent CH 4 = polar covalent KCl = ionic N 2 = nonpolar covalent (pure cov.) CaO = ionic

Check for understanding Use the graph on page 264: 1.What % ionic character is a bond formed between 2 atoms with an electronegativity difference of 2.00? 2.Where would LiBr be plotted on the graph? 3.What is the % ionic character of a nonpolar covalent bond?

Polarity Result of unequal sharing of electrons (can’t happen with pure covalent bonds) Shared e- spend more time around atom with greater electronegativity This end = partially negative Other end = partially positive Polar covalent molecules are called dipoles

Polar Molecules Depends on location polar covalent bonds Symmetrical molecules = nonpolar Asymmetrical molecules = polar Like dissolves like (polar dissolves polar; nonpolar dissolves nonpolar) Polar also dissolves ionic because of similar charges; attracted to electric fields, etc

Check for understanding Determine the bond type & molecular type: SO 2 CS 2 CH 3 Cl CH 2 OHClH 2 S LiF 2 O 2 O 3

Attractive forces in covalent cpds Attraction btwn atoms = quite strong Known as intramolecular forces Attraction btwn molecules = fairly weak Known as intermolecular forces or van der Waals forces (3 kinds) All are weaker than the intramolecular force or an ionic bond

Van der Waals Forces 3 types (weakest to strongest): 1.Dispersion force = nonpolar stuff 2.Dipole-dipole force = polar stuff (more polar = greater force) 3. Hydrogen bond = hydrogen dipole on one atom and the F, O or N atom on another dipole

Properties of Covalent Cpds Relatively low m.p. & b.p. Often gases or vaporize easily Form soft solids, like paraffin Solids = crystal lattices like ionic compounds

Network Solids Solids composed of ATOMS connected by covalent bonds Brittle Nonconductors High m.p. Extremely hard Ex: quartz & diamonds