Electronegativity and Bond Character: Understanding Polarity and Intermolecular Forces, Exams of Chemistry

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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

Electronegativity Difference Bond Character

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.

 H 2 , N 2 , O 2 , Cl 2

 No difference in the electronegativity

between the atoms.

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.

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

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 CO 2  Energy needed to overcome intermolecular forces will increase with an increase in force strength  Boiling H 2 O will require more energy than boiling H 2 S because H 2 O experiences hydrogen bonding and H 2 S experiences only a slight dipole-dipole