Study guide Chapter 8 Covalent Bonding see p.273
8.1 Atoms gain stability when they share electrons and form covalent bonds.
Vocabs: octet rule, covalent bond, molecule, Lewis structure, sigma, π bond,
bond strength and bond length, endothermic exothermic.
8.3 Molecule structures including polyatomic ions, resonance structures,
coordinate bonds and exceptions to the octet rule.
8.4 Hybridization and VSEPR model is used to determine molecular shapes.
8.5 Electronegativity difference determines the character of a bond between atoms.
Polar covalent bonds form a dipole, but spatial arrangement of the polar bonds in a
molecule determines the overall polarity of a molecule. Molecules attract each other by
weak intermolecular forces, which influence physical properties such as melting and
boiling point, solubility (like dissolves like), surface tension etc.
Example questions:
1) Name the seven nonmetallic elements that exist as diatomic molecules
(start at 7 make a 7 + H)
N2, O2, F2, Cl2, Br2, I2, and H2
2) How many single bonds are the following elements likely to form to achieve noble gas configuration?
C4
Si 4
N3 P3
O 2 S 2 Se 2 F 1 Cl 1Br 1 H 1
3) Draw the electron dot structure of
C6H6,
C3H4,
for example:
HCN, Si3H8
replace C for Si
4) Draw how two orbitals overlap when forming
a) a sigma bond σ and b) a π bond.
Sigma: overlap along the shortest distance between atoms
Pi bond from parallel p orbitals (also possible d orbitals)
5) Draw the Lewis dot structure of the following polyatomic ions (remember to count all valence
electrons): PO4 3-, NH4+, ClO4-, (central atom underlined)
and resonance structures of NO3- and NO2- (either all possible structures with double arrows in-between
or the actual “average” structure).
or
nitrite
6) Give one example of each exception to the octet rule:
a) odd # of valence electrons: NO2
b) suboctet:
BeCl2 or BH3
c) expanded octet: PF5 or XeF4
Can elements in period 2 form expanded octets? Explain.
No. Atoms need d orbitals for an expanded octet. Elements in period 2 occupy only energy levels 1
+ 2 with only s or p orbitals. Only elements of period 3 and higher have d (or f) orbitals that can be
used for expanded octets.
7) What is a coordinate covalent bond (which is often sought by boron compounds with a suboctet).
Coordinate covalent bond is a bond in which one atom contributes both bonding electrons.
After the coordinate bond is formed it is a regular covalent bond.
8) Why are lone electron pairs on the central atom important when determining the shape of a molecule?
Lone electron pairs occupy space (even more than bonds to other atoms). All valence electron pairs
(bonding and lone pairs) repel each other and orient themselves to minimize repulsion.
9) What shape form the following hybrid orbitals?
a) two sp linear
b) three sp2 trigonal planar
c) four sp3 tetrahedral
10) Determine the molecular shape, approximate bond angle, and hybrid orbitals for each molecule:
a) PH3, trigonal pyramidal, <109°° (∼
∼107°°), Sp3
2
b) BF3 trigonal planar, 120°°, Sp
∼104°°), Sp3 (2 lone pairs on O)
c) OCl2 bent, <<109°° (∼
d) BeF2 linear, 180°°, Sp
e) CF4 tetrahedral 109.5°°, Sp3
11) Which element has a) the highest electronegativity b) the lowest c) no value assigned (why not?)
a) F
b) Cs + Fr
c) noble gases, because they don’t usually form bonds
12) What determines the bond character as mostly ionic, polar covalent, mostly covalent or nonpolar
covalent?
Difference in electronegativity…
13) Differentiate between a polar bond and polar molecule. Can a molecule with polar bonds be
nonpolar? Give one example.
Polarity of a bond is determined by difference in electronegativity. Polar covalent bonds if ∆EN is
between 0.4 and 1.7. Molecules with all nonpolar bonds will always be nonpolar, molecules with
only one polar bond will be polar. Molecules with more than one polar bonds can be polar or
nonpolar, depending on the symmetry of dipole arrows. Highly symmetric dipoles cancel out and
result in a nonpolar molecule.
14) Explain the picture H2O is a polar molecule with a dipole. The area around
oxygen has a higher electron density, a partial negative charge, while the area around
the two hydrogen atoms has a partial positive charge. The dipoles don’t cancel out
because the shape of a water molecule is bent, rather than linear.
15) How are polarity and intermolecular forces related?
All intermolecular forces (or Van der Waals forces) are weak, much weaker than covalent or ionic
bonds!
- Nonpolar molecules have only very weak intermolecular forces, called dispersion force or induced
dipole.
- Dipole-dipole forces between polar molecules are somewhat stronger making oppositly charged
dipole ends “cling” to each other, which influences many physical properties, such as melting and
boiling point, surface tension, solubility etc.