Post on 19-Dec-2015
Covalent Bonding and Molecular Structure (OWLBook Chapter 8)
• Covalent Bonding• Lewis Structures• Bond Properties• Electron Distribution• VSEPR and Molecular Shapes• Bond Polarity• OWL Due 27-November (I am aware of some of
the problems in Chapter 8, they are being addressed)
• MarvinSketch 5 Tutorial• Exam #4 (Chapters 8 and 9) about 7-December
Covalent BondingCoulombs Law (Ionic Bonds)
◦Describes the forces most involved in ionic bonds Bonds based almost solely on
electrostatic forces
Covalent BondingMetallic Bonding
◦Forces between metal atoms’ nuclei and electrons dominate bonding the atoms together
Covalent Bonding◦Forces between nuclei and electrons
in two atoms in a molecule. It is generally described as sharing of a pair of electrons between to atoms to form a single bond
Evaluating Covalent BondingLewis Structures
◦Used to examine the number of valence electrons and how they may bond
◦Generally, atoms prefer to have eight valence electrons around them when bonded When atoms are not bonded (by
themselves) Group number for the “A” or main group group
elements More complicated for transition elements
◦Elements preferring other than 8? H (2), B (6), S, P, Br
Drawing Lewis StructuresFor atoms and ions
◦Determine the number of valence electrons Usually the group number Can be evaluated from the electron
configuration also
◦Write the atomic symbol◦Arrange electrons about the symbol◦Add or subtract electrons for ions
Enclose in brackets and add charge
Lewis Structures and MoleculesLewis structures help describe
modes of bonding AND shapesTry to follow the octet (8) rule when
it is appropriate◦Evaluate the total number of electrons◦Determine which is the central atom
(usually the single one) Furthest from fluorine (lowest e- affinity)
◦Add one pair (bond between atoms)◦Distribute electrons
Evaluate compliance with the octet rule
Examples….Remember that a line ( - ) is the
same as a pair of electrons (:)◦Hydrochloric acid
◦Ammonia
◦Boron trifluoride
◦Ammonium ion
◦Sulfate ion
Resonance StructuresDifferent (visually) but
ordinarily equivalent Lewis Structures
Help distribute electrons (and thus charge) throughout an ion or molecule◦Stabilizes the ion or molecule
Help satisfy the Octet RuleVery important in Organic
Chemistry
CH3COO-
Valence electrons: 2(4) + 3(1) + 2(6) + 1 = 24
C is the central atom.
A double bond is needed between C—O.
There are two equivalent places for it, so two resonance structures are required.
C
H
H
H CO
O
-
C
H
H
H CO
O
-
Bond Properties
Bond length (or bond distance) is the distance between nuclei in a bond.
Bond order is, defined in terms of the Lewis formula, the number of pairs of electrons in a bond.
Bond energy is, defined in terms of thermodynamics, the energy required to break a bond (separate the nuclei so they no longer can “detect” each others presence) in a gas-phase molecule.
Copyright © Cengage Learning. All rights reserved. 9 | 21
Consider the propylene molecule:
C C C
H
H
H
H
H
H
134 pm 150 pm
The shorter bond is the double bond; the longer bond is the single bond.
One of the carbon–carbon bonds has a length of 150 pm; the other 134 pm. Identify each bond with a bond length.
Formal ChargeRepresents the distribution
of charge in a bond◦ Relative to the # of valence
electrons each atom had originally◦ A value assuming all electrons in a
bond are equally shared Usefully in determining where electrons
might be in a Lewis Dot Structure for a molecule
Total of Formal Charges = Overall Charge on Molecule (0) or Ion (+ or -)
ElectronegativityThe tendency of an atom to draw electrons in a bond towards it◦Electrons are not normally shared equally
The difference in electronegativity between the two atoms in a bond is a rough measure of bond polarity. Using electronegativities,
arrange the following bonds in order by increasing polarity: C—N, Na—F, O—H.
For Na—F, the difference is 4.0 (F) – 0.9 (Na) = 3.1.
For C—N, the difference is 3.0 (N) – 2.5 (C) =
0.5.
For O—H, the difference is 3.5 (O) – 2.1 (H) = 1.4.
C—N <Bond polarities:
O—H <Na—F
Resonance, Formal Charge and ElectronegativityAll of these factor into the distribution of
electrons in a molecule or ion.◦ Formal charge shows the best distribution of
electrons-more towards more electronegative atoms◦ Electronegativity describes bond polarity◦ Resonance helps stabilize molecules and ions
Which is more likely?
In general……Lowest value of formal charges
◦1,0,-1 is preferred over 3, 0, -3Charge distributed somewhat
throughout the moleculeNegative formal charges on more
electronegative elements Practice Problem 8.4.3
VSEPR (Valence Shell Electron Pair Repulsion) Theory
Allows us to predict the shapes of non-metal containing ions and molecules
Central atom is surrounded by structural electron pairs
Structural e- pairs can be◦ Non-bonding (lone pairs)◦ Bonding (between atoms)
Electron pair geometry is the arrangement of structural e- pairs around the central atom(s)
Molecular geometry is the arrangement of atoms around the central atom(s)
Molecular GeometryDifferent than
electron pair geometry when there are lone pairs
Lone pairs want to be physically as far apart as possible in 3-dimensional space
Molecular PolarityFor a molecule to
be polar, it must contain polar bonds, and they must be distributed asymmetrically
Polarity impacts◦ Reactivity◦ Crystal structure◦ Solubility◦ Properties
BP, MP, etc.
Molecular PolarityDetermining
Polarity◦ Draw Lewis
Structure◦ Determine Molecular
Shape◦ Assign polarity to
bonds◦ Determine if there is
an uneven distribution of charge
The dipole moment is measure of polarity.
Let’s draw these molecules.