Covalent Bonding

Sec. 8.5: Electronegativity

and Bond Polarity

Objectives

• Describe how electronegativity is used to determine bond type.

• Compare and contrast polar and nonpolar covalent bonds and polar and nonpolar molecules.

• Describe the characteristics of compounds that are covalently bonded.

The character of the bond formed is determined by electron affinity or electronegativity.

Electron affinity measures the tendency of an atom to accept an electron. Electronegativity (ability to attract electrons in a bond) and electronegativity values allow scientists to evaluate affinity.

Pg. 194 in your text

INCREASESD

EC

RE

AS

ES

The electronegativity difference between H and Cl is 0.7. Because the difference is NOT zero, electrons are NOT

shared equally.

The shared electron pair is pulled towards the Cl atom. The electrons spend moretime with the Cl atom than the H atom.

Pg. 266

Bonds between different atoms are never completely

covalent or ionic • The bond character is determined by

electronegativity difference.

• As electronegativity difference increases, the bond becomes more ionic in character.

• At an electronegativity of 1.7, the bond is considered to be 50% covalent and 50% ionic. (see graph pg. 266)

Practice Problems

Determine the electronegativity difference between the atoms listed. Indicate what type of bond would form between them.

1. C-Cl

2. N-H

3. O-S

4. Na-Br

Molecular Polarity

• Polarity of a molecule depends on the polarity of the bonds in the molecule AND the shape of the molecule.

• Molecules that are polar are dipoles - they have a partial negative charge on one side of the molecule and a partial positive charge on the other side.

CCl4: Polar or not?• Each C-Cl bond is polar

covalent with an electro- negativity difference of 0.61

• The molecular shape is tetrahedral. The bonds are symmetrical. The electric charge measured at any distance from the center is identical on all sides and partial charges are balanced.

• The molecule is nonpolar.

H2O: Polar or not?

• Each O-H bond is polar covalent with an electronegativity difference of 1.24.

• The molecular shape is bent. The polar O-H bonds are NOT symmetrical; there are definite negative and positive ends.

• The molecule is polar.

Polar or not?If bonds are polar, •asymmetrical molecules are polar•symmetrical molecules are nonpolar

Practice: NH3, CHCl3, SF2, SF4, SF6

Properties ofCovalent Compounds

• Solubility is the ability of a substance to dissolve another substance. Following the phrase “like dissolves like”– Polar (or ionic) molecules dissolve in other

polar (or ionic) substances.– Nonpolar molecules dissolve ONLY in

nonpolar substances.

Intermolecular Forces• Covalently bonded compounds have weak

intermolecular or van der Waals forces.– Non-polar molecules exhibit a weak dispersion

force, or induced dipole. These forces are the weakest.

– Between polar molecules, these forces are called dipole-dipole forces. One end of the dipole attracts the oppositely charged end of another dipole. The strength of attraction increases with the polarity of the molecule.

Intermolecular Forces

• The third type of van der Waals force is called a hydrogen bond.– This is an especially strong dipole-dipole force.– It is formed between the H end of a dipole and

a F, O, or N atom on another dipole.

Properties ofCovalent Compounds

• Many physical properties depend on the strength of intermolecular forces.– Molecular compounds have LOW melting &

boiling points (compared to ionic compounds)

– Molecular substances exist mostly as gases OR vaporize easily at room temperature.

– Molecular substances are SOFT if solid.• Solids form crystal lattice structures but have much less

attraction between particles than in an ionic crystal.

Covalent Network Solids• Solids composed only of atoms

interconnected by a network of covalent bonds.– Network solids are brittle and

extremely hard.– They do not conduct electricity or heat.– They have very high melting points.

• Examples are graphite and diamonds – both are networks of C atoms.

Graphite

Diamond