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
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
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