Major Concepts of Physics PHY102

1

• Major Concepts of Physics PHY102

Lecture 19 Molecules Covalent and Ionic Bonds
April 1st Spring 2009 Prof. Liviu
Movileanu http//physics.syr.edu/courses/PHY102.07
Spring/index.html lmovilea_at_physics.syr.edu Room
211, Physics Bldg., 443-8078
1
Major Concepts of Physics PHY 102 Lecture 19
2009?Syracuse University
2
Lecture objectives

• Review Conceptual example 28.1
• Electron diffraction experiment
• 2. Covalent bond. Lewis structures of molecules
• 3. Ionic bonds
• 4. Potential-energy diagrams
• 5. Announcements

2
Major Concepts of Physics PHY 102 Lecture 19
2009?Syracuse University
3
Conceptual example
3
Major Concepts of Physics PHY 102 Lecture 19
2009?Syracuse University
4
Conceptual example
4
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
5
1. Strong interactions Covalent bonds Ionic
bonds2. Weak interactions van der Waals
interactions Hydrogen bonding Electrostatic
interactions Hydrophobic interactions
Interactions between atoms in molecules
5
Major Concepts of Physics PHY102 Lecture 19
2005?Syracuse University
6
Chemical Bonding Classical Representation

• Properties of the compounds depend on the nature
  of the bonds between their elements and the
  strength of these bonds.
• In 1916 G.N. Lewis suggested that covalent
  compounds are the result of electrons that become
  paired and share the space between the bonded
  atoms in molecules.
• He developed a simple representation of bonding
  in molecules. This representation is called the
  LEWIS ELECTRON-DOT MODEL

6
Major Concepts of Physics PHY102 Lecture 19
2005?Syracuse University
7
Chemical Bonding Classical Representation
The Lewis model begins by recognizing that not
all electrons in an atom participate in chemical
bonding. Electrons occupy a set of SHELLS or
ORBITALS surrounding the nucleus. Electrons in
INNER shells, called CORE electrons, do not
significantly participate in bonding. The
outermost, partially filled shell (called the
VALENCE shell) contains VALENCE electrons that
are directly involved in bonding.
7
Major Concepts of Physics PHY102 Lecture 19
2005?Syracuse University
8
Example Chloride
Chloride atom 17 2 8 7
8
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
9
Example Chloride
Cl
Cl
2 Chloride atoms Outer shells only
9
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
10
Example Chloride
Chlorine molecule Cl2
Electrons shared
Each outer shell has 8 electrons
Forces (bonds) between atoms in the molecule very
strong
Forces between molecules very weak
Molecular or covalent compounds are usually gases
or liquids they have low melting points and
low boiling points
Molecules have no overall electric charge
10
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
11

• Lewis structures for covalent compounds show the
  shared pairs and the unshared pairs of electrons
  for each atom.
• The electrons shared in the bond are called the
  BONDING electron and those unshared the
  NON-BONDING electrons, or LONE PAIRS

11
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
12

• LEWIS DIAGRAMS FOR COVALENT COMPOUNDS
• Nonmetallic element nonmetallic element -gt
  covalent compound
• Formation of Cl2
• Cl Cl ? Cl2

By forming the Cl2 molecule, each Cl atom has an
octet of electrons, with two electrons shared by
both Cl atoms
12
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
13

• With the exception of Helium, for the Group IA to
  VIIIA the number of valence electrons in a
  neutral atom is equal to the elements group
  number.

13
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
14
Valence and core electrons
To determine the number of valence electrons and
core electrons Total number of electrons
atomic number Number of Valence electrons for the
A block equals the group number Core electrons
Atomic Number of Valence electrons For
example Cl atomic number 17 Total number of
electrons 17 Number of valence electrons 7
(since Cl is located in Gr VII A) Core electrons
17- 7 10 Lewis diagrams involve just the
valence electrons and assumes that the core
electrons are not involved in bonding and
reactivity.
14
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
15
To construct a Lewis diagram dots are used to
represent each electron in the valence shell of
the atom.

• The Lewis dot diagrams for the first 10 elements
  are

For elements with less than four valence
electrons the dots (electrons) are displayed
singly around the four sides of the atoms (with
the exception of Helium). For elements with
greater than 4 valence electrons, pair electrons
15
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
16
Example Oxygen
Oxygen
Oxygen atom 2,6
16
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
17
Oxygen
2 Oxygen atoms (outer shells only)
Double covalent bond
17
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
18
Lewis Structure, Stability, Multiple Bonds, and
Bond Energies

• Single bond one pair of electrons are shared
  between two atoms
• Double bond two pairs of electrons are shared
  between two atoms
• Triple bond three pairs of electrons are shared
  between two atoms

18
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
19
Bond energy the amount of energy required to
break a bond holding two atoms together. triple
bond gt double bond gt single bond Bond length the
distance separating the nuclei of two adjacent
atoms. single bond gt double bond gt triple bond
19
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
20

• IONIC Bonds
• metallic element nonmetallic element -gt ionic
  compound
• Formation of NaCl
• Na --gt Na 1e-
• Cl 1e- --gt Cl-
• Na Cl- --gt NaCl

Atomic number of Na 11 Number of electrons in
Na 11 Total number of electrons in Na is
10 The electronic configuration of Na is the
same as Ne
20
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
21

• Cl has seven valence electrons. If it accepts
  one electron from Na, then Cl- has eight
  electrons completing an octet (same electronic
  configuration as Ar)

Na is electropositive, Cl electronegative Na
transfers an electron to Cl
Lewis diagrams for formation of NaCl ionic bond
--gt 1e-
--gt

21
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
22
22
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
23
Activation energy
r0
(no interaction)
Long-range attractions between e-s(-) and H
nuclei ()
Short-range repulsion between H nuclei
Binding energy
Equilibrium distance
Minimum energy ? stable HH bond
23
Major Concepts of Physics PHY102 Lecture 19
2009?Syracuse University
24
Announcements

• Reading This material is not covered in the
  textbook
• 2. Homework. HW8 is pertinent to Lasers
  (Lecture 18)
• 4. Next week We move on Nuclear Physics

24
Major Concepts of Physics PHY 102 Lecture 19
2009?Syracuse University