Chemical Bonds, Lewis Structures and Molecular Shapes

1
Chapter 10

• Chemical Bonds, Lewis Structures and Molecular
  Shapes

2
Homework Chapter 10

• Text Homework pages 332 to 339. Problems 6,
  8, 10, 11, 12, 27, 29, 33, 45, 47, 48, 53, 57,
  71, 85
• Quiz Do the practice and graded quiz in
  Blackboard.

3
I. Chemical Bonds A. Introduction (summary of
chapter)

• Atoms can combine to produce new larger units
  called molecules or compounds.
• Molecules held together by chemical bonds.
• Bonds result from either transfer of valence
  electrons (Ionic Bonds) or from sharing of
  valence electrons Covalent Bonds.
• Valence electrons rearrange to mimic closest
  Group 8A (18) structure.
• Molecules resulting from covalent bonding will
  have predictable shapes.

4
I. Chemical Bonds B. Ionic Bonds

• - Metals loose electrons, form cations have e-
  configuration of nearest inert gas. Note that H
  is an exception with no electrons.
• - These electrons are taken up by electronegative
  elements (nonmetals) resulting opposite ions
  attract in a ratio which produces a neutral unit.
  Reduce formula to simplest ratio.
• - Ionic Bond Definition bond formed by
  electrostatic attraction between anions (-) and
  cations ().
• - Write formula with element first do not show
  charges Final compound is neutral.
• - Generality any compound formed from metallic
  and nonmetallic elements is ionic.

5
I. Chemical Bonds B. Ionic Bonds

• Know Metals combine with nonmetals form ionic
  bonds by losing or gaining electrons to mimic
  closest Inert Gas (VIIIA). The following are
  reasonable generalities
• 1A - Na, K, Li, etc become 1 ions Na
• 2A - Ca, Mg, etc become 2 ions Ca2
• 3A - Al, Ga become 3 ions Al3
• 5A - N, P become -3 ions N-3
• 6A - O, S become -2 ions O-2
• 7A - F, Cl, Br, I become -1 ions F-1
• Opposite ions attract in a ratio so that the
  product is neutral do not show charges in final
  formula.

6
I. Chemical Bonds B. Ionic Bonds Example
Do not show charges in final formula. NaCl NOT
NaCl-
7
I. Chemical Bonds B. Ionic Bonds Example
8
I. Chemical Bonds B. Ionic Bonds - Examples

• Na Na O-2 Na2O
• Ca2 F- F- CaF2
• Mg2 S-2 MgS
• Al3 Al3 O-2 O-2 O-2 Al2O3
• Give the formulas for the following Na Br
• Ca O Ba I Li O Al F Mg
  N
• Transition metals form ionic bonds and can have
  several charges such as Fe2 and Fe3

9
I. Chemical Bonds C. Electron Dot (Lewis)
Strucures

• - A Lewis electron dot structure is a symbol in
  which the valence electrons are shown as dots.
• - Examples
• Na. Mg Na Ca2
• H1- (Called Hydride) C Si
• - How many valence electrons (dots) would
• N3- O2- F- or Ne have? What
  about Mg2?

10
II. Covalent Bonds A. Introduction Note
EN electronegativity

• - Definition of a covalent bond A bond formed
  by sharing two electrons.
• - When two atoms of similar EN combine, neither
  has the pull to take electrons away a sharing
  of valence electrons results.
• - This occurs when NONMETALS combine with
  NONMETALS.
• - Example H. H. ---) HH H2
• - The atoms share valence electrons to get stable
  group 8A e- configurations.

11
II. Covalent Bonds A. Introduction

• 2 shared electrons represented with a dash
  (Single Bond).
• 4 shared electrons with (Double Bond).
• 6 shared electrons with (Triple Bond).
• We frequently show the structure as a Lewis
  Structure covalent bonds with lines and
  nonbonding valence electrons as dots.
• - Note Group 4A usually forms 4 bonds 5A three
  bonds 6A two bonds and 7A (along with H) one
  bond.

12
II. Covalent Bonds B. Examples

• H F ---) H F
• H O H ---) H O
  H
• N N ---) N N
• Cl O Cl ---) Cl O
  Cl
• O C O -----) O C
  O

13
II. Covalent Bonds C. Lewis Structures
1. Rules for Drawing Lewis Structures

• 1. Calculate total of valence electrons take
  into account charge if the sample is an ion.
• 2. Place atom that forms most bonds at center
  (Closest to Group IVA Lowest if in same group).
  If is a charge, then take this into account with
  the central atom.
• 3. Arrange other atoms around central atom
  allow sharing so that each atom has stable
  electron configuration. Show bonding pairs as
  dashes nonbonding valence e- as dots.
• 4. Double check a) each atom has a stable
  electron configuration b) have the same number
  of valence electrons as in step 1.

14
II. Covalent Bonds C. Lewis Structures
2. Examples

• H I H2O NH4
• H2O2 CH4 SO2
• AlCl4- NO2- CN-

15
Bonding SummaryTwo General Bonding Types

• 1. Ionic Compound containing metallic element.
  Atoms lose/gain e to look like nearest inert gas.
  Add together ions such that neutralize charge.
• 1a 2a 3a 5a
  6a 7a
• 1 2 3 -3
  -2 -1
• 2. Covalent Compound containing nonmetals.
• Atoms obtain inert gas configuration by
• sharing valence electrons.
  

16
III. Shapes

• Molecular Shapes play a major role in
• 1) Physical Properties
• 2) Chemical Properties
• 3) Biochemical Properties
• To Obtain the shape of a molecule draw the Lewis
  Structure, count the number of things around
  the central atom, and use simple geometry to
  predict the shape. things atoms or pairs of
  nonbonding valence electrons.

17
III. Shapes C. Electron Geometry Shape
Bond angle 180o
Bond angle 120o
Bond angle 109o
18
III. Shapes D. Polarity

• Nonpolar Bond A covalent bond with no
  separation of charge. Example H - H
• Polar Bond A covalent bond with a partial
  separation of charge. Example H-Cl H
  is partially Cl is partially
• Polar Nonpolar Molecules
• Polar molecules result when have one or more
  polar bonds with a shape that does not cancel the
  electron pull. Examples H-Cl
  H2O
• Nonpolar molecules result when have all nonpolar
  bonds or have polar bonds a shape that cancels
  electron pull. Examples CH4
  CO2 ( OCO )
• Note Like dissolves Like with respect to
  polarities.