COVALENT BONDS &
MOLECULAR STRUCTURE
CHEMICAL BONDS
• Form between atoms resulting in molecules
(covalent bonds, sharing of electrons).
• Form between ions resulting in ionic cmps
(ionic bonds, electron transfer).
• Form because their formation results in the
lowest possible energy (Figs 6.8, 7.2).
• Chemical bonding model assumes molecule
consists of individual chemical bonds.
COVALENT BONDS
• Most important chemical bond
• Involve electron shared by two nuclei.
• Represent a balance of electrostatic
attractions (+nucleus and -electron(s) and
repulsions (+nuclei, -electrons).
• These bonds can be assigned average bond
lengths (Fig 5.19)
DISSOCIATION BOND
ENERGY
• Each chemical is assigned an average
(±10%) dissociation bond energy, D.
• AB(g)  A(g) + B(g)
• Table 7.1.
• These values are > 0 kJ/mol.
• D is a measure of bond strength
• Note single vs double vs triple bonds
COVALENT BONDS
• Determine physical and chemical properties
of cmps.
• Determine the likelihood and products of
chemical reactions.
• Determine molecular shape (Sec 7.5-7.9).
ELECTRONEGATIVITY
• Defined as the ability of an atom to attract
shared electrons in a covalent bond to itself.
• EN > 0; Fig 7.4
• EN largest in upper right hand corner of PT.
• This unequally sharing leads to unequal
charges on the atoms. Fig 7.3
• Use δ+ and δ- to indicate partial charges on
the atoms.
BOND POLARITY
• Polar covalent bond forms when electron
pair is not shared equally due to bonded
atoms having different EN values.
• ΔEN = difference in EN ~ 0, nonpolar
covalent bond. E.g. H2, O2
• If ΔEN < 2, polar covalent bond; e-pair is
held more closely by atom with greater EN
• If ΔEN > 2, bond is ionic and electron is
transferred to form anion and cation
LOCALIZED ELECTRON (LE)
BONDING MODEL
• Electrons participate in the formation of
chemical bonds (esp. valence electrons).
• Electron pairs are localized between (shared
or bonding pair) or on (lone pair) atoms.
• Main group atoms want to achieve noble
gas config.(octet rule) except H, Li, Be
want to achieve He config. (duet rule).
• VSEPR model predicts molecular geometry
based on LE bonding model.
LEWIS SYMBOLS and
STRUCTURES
• Lewis symbol : picture of atom showing its
valence electrons.
• Lewis structure: picture of molecule
showing bonding electrons as lines and
nonbonding electrons as dots or lines.
• Especially used for main group elements
(T7.3)
COVALENT BONDS (3)
• Form when electron pairs are shared so that
each atom achieves an octet (duet).
• Coordinate covalent bond forms when one
atom provides both bonding electrons.
• Multiple covalent bond forms when more
than one electron pair is shared between two
atoms (double bond, bond order 2 [CO2]
and triple bond, bond order 3 [N2]).
WRITING LEWIS
STRUCTURES
• Determine total # of valence electrons.
• Write skeletal structure with central atom
[lowest EN]; terminal atoms [H, higher EN]
• Use electron pairs to form bonds.
• Achieve octet rule for terminal atoms
• Add the remaining to the central atom;
Central atom may have > octet.
• Form multiple bonds if needed.
WRITING LEWIS
STRUCTURES (2)
• Exceptions to octet rule (odd # of valence
electrons, free radicals, incomplete octets,
more than 8 electrons (expanded valence
shell).
• Resonance hybrid of resonance structures
showing different but equivalent
distributions of electrons; this leads to the
delocalization of electrons.
• Be guided by experimental observations.
FORMAL CHARGE (FC)
• FC = [VE] - [BE]
• FC is a hypothetical charge for electron loss
(+) or gain (-) due to bond formation.
• [VE] = # valence e’s for Group A atoms
• [BE] = all lone pair electrons on atom + 1/2
shared electrons
• Best Lewis structure has minimum FC and
is consistent with EN info.
VSEPR MODEL
• VALENCE-SHELL ELECTRON-PAIR
REPULSION (VSEPR) Method helps us
determine molecular geometry.
• Molecular geometry: 3-D shape of the
molecule.
• This method assumes that the final positions
of nuclei are the ones that minimizes
electron repulsions because this is the one
associated with the lowest energy.
VSEPR METHOD (2)
• Determine Lewis structure of molecule.
• Count charge clouds (ch-cl) around the
central atom: single e, lone pair, single
bond, double bond, triple bond.
• Determine charge cloud geometry.
• Determine molecular group geometry with
A = central atom; X = terminal atom; E =
lone pair of electrons.
• Table 7.4
MOLECULAR GEOMETRY
# ch-cl ch-cl Geom
Molecular Geom
2
Linear
Linear
3
Trigonal planar
Trigonal planar, bent
4
Tetrahedral
Tetrah, trig pyram, bent
5
Trig bipyramidal Trig bipyra, seesaw, Tshaped, linear
Octahedral
Octah, sq pyrami, sq
planar
6
MOLECULAR GEOMETRY (2)
• Charge cloud geometry differs from
molecular geometry when there are lone
electron pairs.
• Electron-electron repulsions decrease as
E-A-E> E-A-X> X-A-X
• Resonance structures
• Note bond angles
VALENCE BOND THEORY
• There are two bonding theories: Valence
Bond (VB) and Molecular Orbital (MO)
• VB: Assumes that atomic orbitals for
electrons not involved in bonding do not
change. However, the AOs of the bonding
electrons overlap and the bonding electrons
in the overlapping orbitals are shared. The
greater the overlap, the stronger the bond.
HYBRIDIZATION
• Consider CH4
• C 1s2 2s2 2p2 suggests one lone pair (2s2)
and two electrons (2p2. )available for
bonding creating two covalent bonds
• Expt evidence confirms that carbon can
bond with four atoms and in methane, the
four C-H bonds are identical
HYBRIDIZATION (2)
• To resolve this conflict, promote a 2s
electron to 2 p, then mix or hybridize the 2s
(1) and 2p (3) orbitals to form four
identical hybrid AOs named sp3
• These hybrid atomic orbitals overlap with
the 1s orbital on hydrogen to form the
covalent C-H bond. Fig 7.6
• Hybrids form to minimize total energy.
HYBRIDIZATION (3)
• Using the VSEPR rules, carbon would have
four covalent bonds and therefore be
tetrahedral with the H-C-H bond angle =
109.5o. This agrees with exptal
measurements.
• Other hybrids: sp2 (3 charge clouds), sp
(2), dsp3 (5), d2sp3 (6)
HYBRIDS AND MOLECULAR
STRUCTURE
• Write Lewis structure and use VSEPR
method to predict charge cloud geometry
• Select hybridization scheme this is
consistent with VSEPR prediction (T7.5)
• Identify orbital overlap
• Form multiple bonds if needed
• Determine molecular geometry
HYBRIDS AND MULTIPLE
BONDS
• Use Valence Bond method to determine 3dimensional structure of hydrocarbons with
double and triple bonds (planar)
• Sigma () or end-to-end orbital overlap
bond
• Pi () or side-by-side orbital overlap bond
• Geometric isomers (2-butene)
• Benzene and other aromatic compounds