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, T-shaped, linear

6 Octahedral Octah, sq pyrami, sq planar

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 3-dimensional 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