Covalent ForcesUnit 6.6

Two Types of Forces• Intra Molecular Forces - The forces of

attraction that exist within a molecule.• Inter Molecular Forces - The forces of

attraction (or repulsion) between a molecule and a neighboring molecule

Intramolecular Forces

Electronegativity and Polarity

• Electronegativity - the tendency of an atom to draw electrons in a bond toward itself

• There are two periodic trends concerning electronegativity.x As you move down a group, electronegativity decreasesx As you move across a period, electronegativity increases

Electronegativity Trends

Polarity - Polar• Not all electrons are shared equally…• Polar covalent- unequal sharing of e-

between two atoms (kind of like tug-of-war)

• The atom with greater electronegativity will hog the electronsx Symbol:

x δ+ (positive end of molecule) x δ- (negative end of molecule)

x All bent and pyramid shapes are polar

Polarity - Nonpolar• Nonpolar covalent - covalent bond in

which the electrons are shared equallyx Ex. The diatomic elements (Br, I, N, Cl, H, O,

and F) are all have nonpolar covalent bonds

x Cl – Cl Br – Br I – I

x N – N H – H O – O F – F

Calculating Polarity• Polarity can be determined by the

difference in electronegativities of the atoms (EN) x Trend: Increasing EN to the right and up

Calculating Polarity• Determine if the following bonds are polar

or nonpolar. (If polar, label δ+ or δ-)• F – F

• O – C

• C – H

Intermolecular Forces

Intermolecular Forces• Intermolecular Forces (IMF) -attraction between molecules

• Three types (Van der Waals Forces)x Dipole-Dipolex Hydrogen Bondsx London Dispersion

Dipole - Dipole• The positive dipole of one polar molecule is attracted to the negative dipole of another moleculex Ex. H - Cl is polar, the negative Cl is

attracted to the positive H on ANOTHER molecule

Hydrogen Bonding• The attraction between an H atom and

unshared electrons on N, O, or F (on a different molecule)

• H – F or H – O or H– Nx STRONG attraction (important to life)

• Ex. H2O attracts H2O

London Dispersion Forces (LDF)• The attraction due to the motion of

electons in the atoms. (Momentary induced dipole-dipole)x All molecules have this (but especially nonpolar)!

• WEAK attraction force • Ex. I2 attracts I2