Transcript ATOMIC AND MOLECULAR STRUCTURE & BONDING

ATOMIC AND MOLECULAR STRUCTURE & BONDING
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Structure of atoms
A. Protons, neutrons, and electrons
B. Electron configurations: shells and subshells
C. Valence states
D. Atoms and the periodic table
Types of bonding between atoms
A. Ionic bonding
B. Covalent bonding
C. Metallic bonding
D. Secondary bonds
1. Permanent dipoles and the hydrogen bond
2. Temporary dipoles and the van der Waals bond
Influence of Bond Type on Engineering Properties
A. Brittle versus ductile behavior
B. Electrical conductivity
C. Melting temperature of polymers
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Structure of Atoms
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Atomic number of an element is equal to the number of electrons or
protons in each atom.
Atomic mass of an element is equal to the average number of protons and
neutrons in the atom.
Avogadro number of an element is the number of atoms or molecules in a
mole.
Atomic mass unit of an element is the mass of an atom expressed as 1/12
the mass of a carbon atom.
Quantum numbers are the numbers that assign electrons in an atom to
discrete energy levels.
A quantum shell is a set of fixed energy levels to which electrons belong.
Pauli exclusion principle specifies that no more than two electrons in a
material can have the same energy.
The valence of an atom is the number of electrons in an atom that
participate in bonding or chemical reactions.
Electronegativity describes the tendency of an atom to gain an electron.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Electron Quantum Numbers
Principle, n: Allowed values of n are 1, 2, 3…
• Roughly associated with electron's energy
– higher values of n mean larger (less favorable) energies.
• The value of n also tells how "far" from the nucleus the electron is
• n is also called the electron shell (n=1 is the first shell, etc.)
Angular momentum, l: Allowed values of l are 0 to (n-1)
• related to angular momentum of the electron in its orbit
• l is also called the electron orbital within a given shell
• Each value of l is assigned a letter of the alphabet:
Magnetic, m: Allowed values of m are -l to +l (2l + 1 distinct values)
Spin, ms: Allowed values of ms are -1/2 (spin-up) and +1/2 (spin-down).
Pauli Exclusion Principle: No two electrons in the same system can share the
same four (n, l, m, ms) quantum numbers!
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Electron States
shell
n
orbital
(0 to n-1)
m values
(m = -l to l)
spin
(ms = ±1/2)
# electrons in shell
1
0 (1s)
1
2
2
2
0 (2s)
1 (2p)
1
3 (2px, …)
2
2
1x2+3x2=
8
3
0 (3s)
1 (3p)
2 (3d)
1
3 (3px, …)
5 (3dxy, …)
2
2
2
1x2+3x2+5x=
18
4
0 (4s)
1 (4p)
2 (4d)
3 (4f)
1
3 (3px, …)
5 (3dxy, …)
7 (4fxyz,…)
2
2
2
2
1x2+3x2+5x2+7x2=
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ATOMIC AND MOLECULAR STRUCTURE & BONDING
Orbital Filling
Example of Pauli
Exclusion Principle
no 2 electrons share the
same quantum numbers
for the same (n,l) at
same E.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Summary of Periodic Table Trends
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Atomic Bonding
Bonding: There are two types of bonds: primary and secondary.
Primary bonds are the strongest bonds which hold atoms together.
The three types of primary bonds are:
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Metallic bond,
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Covalent bond, and
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Ionic bond
Metallic Bonds
Elements in groups I and II of the periodic table, and some in group III form
metallic crystals. In a metal, the outer electrons are shared among all the atoms
in the solid. Each atom gives up its outer electrons and becomes slightly
positively charged. The negatively charged electrons hold the metal atoms
together. Since the electrons are free to move, they lead to good thermal and
electrical conductivity. The metallic bonding does not have the strongly
directional character of covalent bonds.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Metallic Bonding
electron
Ion cores
Metallic bonds
Bonding energies: range from 68 kJ/mol (0.7 eV/atom) for Hg
to 850 kJ/mol (8.8 eV/atom) for W.
Melting temperatures: -39 C for Hg and 3410 C for W.
Stronger bonds lead to higher melting temperature:
atomic scale property  macroscale property.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Ionic Bonding
Atoms like to have a filled outer shell of electrons. Sometimes, by
transferring electrons from one atom to another, electron shells are filled.
The donor atom will take a positive charge, and the acceptor will have a
negative charge. The charged atoms or ions will be attracted to each other,
and form bonds.
The most common examples of ionic crystals are found in the structures of
the alkalindhalides, of which the most familiar is ordinary salt, NaCl. This
structure in which each ion is surrounded by six nearest neighbors of the
other kind is called Face-Centered-Cubic (FCC).
The distance between like ions in NaCl crystals is 5.63 Ao
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Ionic Bonding
Structure
Bonding
Ionic bond for NaCl
Occurs for electronegative (positive) elements that can form close-shell
configurations. Electronegativity plays a role here, e.g., H-F.
Structure is determined by electrostatic attraction EA = –A/r (Coulomb’s Law)
and collapse is prevented by electronic and nuclear repulsion ER = +B/rn.
For closed-shell gases (Ar, Kr,…) n~12. Generally need quantum mechanics.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Covalent Bonding
The cohesive forces in covalent crystals arise from the sharing of an
electron-pair between each two atoms. Some atoms like to share electrons
to complete their outer shells. Each pair of shared atoms is called a
covalent bond.
Covalent bonds are called directional because the atoms tend to remain in
fixed positions with respect to each other. As a result, covalent bonds
result in poor electrical and thermal conductivity. Examples include
diamond, and the O-O and N-N bonds in oxygen and nitrogen gases.
Diamond is an example of a crystal whose atoms are linked by covalent
bonds. Each carbon atom has four nearest neighbors and shares an
electron pair with each of them. Silicon, germanium, and silicon carbide
are among the crystals having the same structure as that of diamond.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Covalent Bonding
Tetrahedral structure of silica (SiO2)
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Van der Waals Bonding
Example of Permanent Dipole: H-Cl Gas and H2O
The Cl has a large electronegativity (3.0) compared to H (2.1); hence, Cl
grabs the electron away from H. (H-Fl and H20 are other examples of this.)
Notably, such "Hydrogen bonds" with permanent dipoles lead to higher
melting points and boiling temperatures, more than can be expected from
their low molecular weights. Such bonds are crucial for polymer chemistry.
Hydrogen bond (H2O Dipole)
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Linear Thermal Strain
L/L0 =  (T - T0)
  asymmetry at r0
No asymmetry at r0
No affect on r(T) or V(T)
  as E0  (less negative)
Volume Thermal Strain
V/V0 = V (T - T0)
E
r0
r
Smaller E
Larger 
Larger E
Smaller 
Parabolic E vs. r shape
Symmetric well: No expansion possible
Atoms just vibrate back and forth!
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ATOMIC AND MOLECULAR STRUCTURE & BONDING
• Non dense, random packing
• Dense, regular packing
Dense, regular-packed structures tend to have lower energy.
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Atomic Packing
Crystalline materials...
• atoms pack in periodic, 3D arrays
-metals
• typical of:
-many ceramics
-some polymers
crystalline SiO2
Noncrystalline materials...
• atoms have no periodic packing
• occurs for:
-complex structures
-rapid cooling
"Amorphous" = Noncrystalline
noncrystalline SiO2
ATOMIC AND MOLECULAR STRUCTURE & BONDING
Summary: Bonding, Structure, Properties
Ceramics
Large bond energies
Ionic and Covalent bonds
large Tm, E
Metals
Varying bond energy
Metallic bonding
intermediate Tm, E, 
Polymers
directional properties
Covalent and Secondary
Small 
secondary dominates outcome
small Tm, E
large 