POLARITY & FORCES ELECTRONEGATIVITY Electron Affinity – a measure of the tendency of an atom to accept an electron Increases across a period Decreases down a group This is why your halogens create anions – they have high electron affinities! Electronegativity – measure of the tendency of an atom to attract electrons when in a compound and sharing electrons Increases across a period Decreases down a group Most electronegative = F BOND CHARACTER A chemical bond is never completely ionic or covalent Electronegativity Bond Character Bond character depends on how strongly the Difference bonded atoms attract electrons > 1.7 Mostly Ionic Use electronegativity difference between 0.4 – 1.7 Polar Covalent two atoms to determine the type of bond/ bond character < 0.4 Mostly Covalent Nonpolar The greater the electronegativity 0 Covalent difference, the greater the percentage of ionic character! BOND CHARACTER Electronegativity Bond Character Difference > 1.7 Mostly Ionic 0.4 – 1.7 Polar Covalent < 0.4 Mostly Covalent 0 Nonpolar Covalent NONPOLAR COVALENT BOND Nonpolar covalent bond - nuclei of atoms pull on electrons equally; bonding electrons are shared equally. H2, N2, O2, Cl2 No difference in the electronegativity between the atoms. POLAR COVALENT BONDS Polar covalent bond - nuclei of atoms pull on electrons unequally The electrons are shared unequally. The more electronegative the atom, the stronger they pull on the electrons and take a partial negative charge. The less electronegative atom will have a partial positive charge. POLAR COVALENT BONDS POLARITY OF MOLECULES Polarity of a molecule depends on the individual bonds and the shape of the overall molecule. If the polar bonds are symmetrical, the polarity cancels out and the overall molecule is nonpolar. If the polar bonds are asymmetric, the polarity persists in the molecule. INTRAMOLECULAR FORCES Intramolecular Forces – attractive forces that hold particles together in ionic, covalent and metallic bonds Intra – within Metallic bond – Strongest Ionic Bond – 2nd Strongest Covalent Bonds Polar Covalent Bond– 3rd strongest Nonpolar Covalent Bond– 4th strongest INTERMOLECULAR FORCES Intermolecular - attractive forces between molecules Inter – between All intermolecular forces are weaker than intramolecular forces Dispersion Dipole-dipole Hydrogen bonds INTERMOLECULAR FORCES Dispersion Forces – weak forces that result from temporary shifts in the density of electrons in electron clouds Larger molecules experience larger dispersion forces Dipole-dipole – attractions between oppositely charged regions of polar molecules Hydrogen Bonds – dipole-dipole attraction between molecules with a H bonded to a small, electronegative atom that has one lone electron pair H bonded to a F, O, or N From weakest to strongest: Dispersion Dipole-dipole Hydrogen Bonds INTERMOLECULAR FORCES INTER & INTRA MOLECULAR FORCES & PROPERTIES Stronger the force, the more energy is needed to break the bond Energy needed to break a metallic or an ionic bond will be much higher than the energy needed to break a covalent bond Separating NaCl will require more energy than separating CO2 Energy needed to overcome intermolecular forces will increase with an increase in force strength Boiling H2O will require more energy than boiling H2S because H2O experiences hydrogen bonding and H2S experiences only a slight dipole-dipole.