Polarity

Electron Affinity A measure of the tendency of an atom to accept an electron Increases with increasing atomic number within a period Decreases with increasing atomic number within a group

Electronegativity Electronegativity – the relative ability of an atom to attract electrons in a chemical bond Allows us to evaluate the electron affinity of specific atoms in a compound

Bond Character A chemical bond between atoms of different elements is never completely ionic or covalent Bond character depends on how strongly each of the bonded atoms attracts electrons

Polar Covalent Bonds Results from unequal sharing of electrons in the bond In a covalent bond, electrons spend more time near the nucleus of the atom with the higher electronegativity

Polarity The atom that attracts electrons most strongly (has the highest electronegativity) has a slight negative charge δ+ means partially positive δ- means partially negative

Electronegativity Difference Bond Character Electronegativity Difference Bond Character > 1.7 Mostly ionic 0.4 – 1.7 Polar covalent < 0.4 Mostly covalent Nonpolar covalent

Determining bond character Look at HF Electronegativity of F = 3.98 Electronegativity of H = 2.20 Difference: 3.98 - 2.20 = 1.78 Electrons move towards the fluorine atom

Polarity Polarity of molecules depends on Look at H2O and BeH2 The presence of polar bonds The position(s) of polar bonds in the molecule (polarity is a vector quantity) Look at H2O and BeH2 Element Electronegativity Hydrogen 2.20 Oxygen 3.44 Beryllium 1.57

Practice Determine if the following molecules are polar CH4 CH2Cl2 CCl4 CH3Cl Element Electronegativity Carbon 2.55 Hydrogen 2.20 Chlorine 3.16

Nonpolar Polar Polar Nonpolar Polar