Covalent Bonding

• Topics covered– Covalent bonding– Electronegativity– Bond polarity– Dipole moments

• Homework– Exercises 8.33, 35,

39, 40– Read 8.5-8.7

Covalent Bonding

• In order for covalent bonds to exist, the attractive forces between molecules must exceed the repulsive ones

• Atoms are held together because the nuclei are mutually attracted to the electrons between them

Lewis Structures

• The formation of covalent bonds can be shown using Lewis symbols

• Shared pairs of electrons are shown as dashes (single, double, or triple)

+ or

Lewis Structures

3 + or

2 or

Covalent Bonding

• For the nonmetals, the number of valence electrons on a neutral atom is the same as the group number

• You can predict that 7A elements, such as F, would form one covalent bond to have an octet; 6A elements, such as O, would from two bonds; 5A elements, such as N, would form three bonds; and 4A elements, such as C, would from 4 bonds

• These predictions are true in many compounds, but keep in mind that these are just guidelines and that there are many exceptions

Multiple Bonds

• Sharing a single pair of electrons constitutes a single bond (single line drawn)

• When two pairs of electrons are shared between two atoms it is a double bond (two lines are drawn)

• When three pairs are shared it is a triple bond (three lines are drawn)

Multiple Bonds

• As a general rule, the distance between bonded atoms decreases as the number of shared electron pairs (bond order) increases

Electronegativity

• The ability of an atom in a molecule to attract electrons to itself is called electronegativity

• We use electronegativity to estimate whether a given bond will be nonpolar covalent, polar covalent, or ionic

• Electronegativity is related to electron affinity and follows the same trends across the periodic table

Electronegativity

Bond Polarity

• A nonpolar covalent bond is one in which electrons are shared equally between two atoms

• In a polar covalent bond one of the atoms exerts a greater attraction for the bonding electrons and the sharing is unequal

Bond Polarity

• We can use the difference in electronegativity to gauge the polarity of the bonding between atoms

• Examples– F2, 4.0 - 4.0 = 0, nonpolar covalent

– HF, 4.0 - 2.1 = 1.9, polar covalent– LiF, 4.0 - 1.0 = 3.0, ionic

Sample Problem

• Which bond is more polar:– B-Cl or C-Cl?– P-F or P-Cl?– Indicate in each case which atom has the partial

negative charge

• Answer:– B-Cl, chlorine has partial negative charge– P-F, fluorine has the partial negative charge

Dipole Moments

• Sometimes the concentration of negative charge on one side of a polar covalent bond can lead to an overall partially negative charge on a molecule

• A molecule in which the center of positive and negative charge do not coincide is said to be a polar molecule

Dipole Moments

• We can represent the polarity of molecules in two ways

• The symbol is the lower-case Greek letter “delta” and denotes “partially” in chemistry

Dipole Moments

• Polarity helps determine many of the properties of substances that we observe on the macroscopic level

• Polar molecules align themselves with each other and with ions

• These interactions account for many properties of solids, liquids, and solutions

Dipole Moments

• Whenever two electrical charges of equal magnitude but opposite sign are separate by a distance a dipole is established

• The quantitative measure of the magnitude of a dipole is called its dipole moment ()

• For equal and opposite charges separated by a distance r, = Qr

Dipole Moments

• Dipole moments are usually reported debyes (D), a unit that equals 3.34 x 10-30 C-m

• For molecules, we usually measure in units of electronic charge e, 1.60 x 10-19 C, and distance in angstoms

• To make things easier, use the following equation with built-in conversions:

Qr (Qine)1.6010 19C

1e

(r in)

10 10m

1.00

1D

3.3410 30Cm

Sample Problem

• The distance between the centers of the H and Cl atoms in HCl (called bond length) is 1.27 Å. (a) Calculate the dipole moment, in D, that would result if the charges on the H and Cl atoms were 1+ and 1-. (b) The experimental dipole of HCl is 1.08 D. What are the partial charges, in e, of H and Cl?

• Answer:– (a) 6.08 D– (b) 0.178 e

Sample Problem

• The dipole moment of chlorine monofluoride is 0.88 D. The bond length of the molecule is 1.63 Å. (a) Which atom is expected to have the negative charge? (b) What is the charge on that atom, in e?

• Answer:– (a) F– (b) 0.11- e