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Chapter 2
ATOMIC STRUCTURE AND
INTERATOMIC BONDING
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Atomic Structure Why shouldwe know about it?
There are only about 100 types of atomsin the whole universe!!!!
But the kind of materials that are possible
are infinite.. Atoms bond in different ways to form
chemically similar materials that may be
physically vastly different
Graphite (soft material) and Diamond (hardest
known material)
Both are forms of Carbon
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Atomic Structure and Properties
Graphite Soft, Black, ExcellentLubricantEasy to separate atoms
Atoms slide past one another
Diamond Transparent, Strong,Very hardAtoms are very strongly held in
their positions
The nature by which atoms are held together (bonded)
influences the properties
An understanding of the atomic structure and the way atoms
are bonded is key to understanding the properties of the
material
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Structures and AtomicStructure
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Nice and Beautiful !!! PlatinumAtoms
http://www.cartage.org.lb/en/themes/Sciences/Physics/SolidStatePhysics/AtomicBonding
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Fundamental Concepts
Atomic number (Z):Number of protons in
the nucleus
Atomic mass (A): sum
of masses of protonsand neutrons
For all atoms of a
same element,
number of neutronsmay be variable -
Isotopes
Nucleus(protons + neutrons)
Electrons
Bohr model of the atom
Mass of protons and
neutrons: 1.67 x 10-27 kg Mass of electrons:
9.11 x 10-31 kg
Charge magnitude:
1.60 x 10-19 C
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Fundamental Concepts
atomic mass unit (amu) = 1/12 mass of12
C Atomic wt = wt of 6.022 x 1023 molecules or atoms
1 amu/atom = 1g/mol
C 12.011 ?H 1.008 ?
Wave-Mechanical model (Refer Fig. 2.3)
Electrons have wavelike and particulate properties.
Two of the wavelike characteristics are
electrons are in orbitals defined by a probability.
each orbital at discrete energy level is determined by
quantum numbers.
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8
Electronic Structure
Quantum number Designationn = principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.)l= subsidiary (orbitals)- shape s,p, d, f (0, 1, 2, 3,, n-1)
ml= magneticno of energy states 1, 3, 5, 7 (-lto +l )ms = spin , -
Pauli exclusion principle?
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Electron energy states
Electrons have discrete energy states
Tend to occupy the lowest energy states ground state
1s
2s2p
K-shell n = 1
L-shell n = 2
3s3p M-shell n = 3
3d
4s
4p4d
Energy
N-shell n = 4
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Stable electron configurations
Complete s and p subshells Tends to be unreactive
Inert gases
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Most elements are unstable ???
Valence (outer shell) not filled completely
Valence electrons participate in bonding
Electron configuration
1s 1
1s 2 (stable)
1s 22s 1
1s 22s 2
1s 22s 22p 1
1s 22s 22p 2
...
1s 22s 22p 6 (stable)
1s 22s 22p 63s 1
1s 22s 22p 63s 2
1s 22s 22p 63s 23p 1
...
1s 22s 22p 63s 23p 6 (stable)...
1s 22s 22p 63s 23p 63d 10 4s 246 (stable)
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12
The Periodic Table
Columns: SimilarValence Structure
Electropositive elements:
Readily give up electrons
to become + ions.Electronegative elements:
Readily acquire electrons
to become - ions.
giveup1
e-
giveup2e-
give
up3e-
inertgas
es
accept1e-
accept2e-
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
ClS
Li Be
H
Na Mg
BaCs
RaFr
CaK Sc
SrRb Y
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Attractive Bonding Forces
Interatomic separation (r)
Attraction
Repulsion
Force
Attractive Force FA
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Repulsive Bonding Forces
Interatomic separation (r)Attraction
Repulsion
Force
Repulsive Force FR
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Net Bonding Force
Interatomic separation (r)
Attraction
Repulsion
Force
Attractive Force FA
Repulsive Force FR
Net Force FN = FA +FR
r0
r0 = 3 Aofor many atoms
NetForce FN
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Attractive Energy
EA= FA.dr
Rep
ulsion
Attra
ction
Po
tentialEne
rgy
E
Attractive Energy EA
Interatomic separation (r)
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Repulsive Energy
Rep
ulsion
Attra
ction
Po
tentialEne
rgy
E
Repulsive Energy ER
Interatomic separation (r)
ER= FR.dr
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Net Energy
Attractive bonding forces are
Coulombic
For 2 isolated atoms, attractive
energy EA is given as:
Repulsive energy ER is given as:
r
AE
A
Net Energy EN
= EA
+ ER
nR
r
BE
))((4
121
0
eZeZA
0 is the permittivity of vacuum (8.854 * 10-12 F/m)Z1 and Z2 are the valences of the two ion typese is the electron charge (1.602 x 10-19 C)
B and n are constantsn~8
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Some Notes on Bonding Energy
and Force
Bonding Energy Energy required to separate twoatoms to an infinite separation
A particular E0 can be associated for each atom
Shape of the energy curve varies
Material properties depend on curve shape, E0, andbonding type
Material Stiffness (Modulus of Elasticity) depends on
shape of Force vs. r curve
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A Quick High School Chemistry Tour
Chemical Bonding
Covalent Bond
Ionic Bond
Metallic Bond
Vander VaalsForce
Hydrogen Bond
Molecule - Ion
attractions
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Ionic Bonding
Bond in which one or more electrons fromone atom are removed and attached to
another atom, resulting in positive and
negative ions which attract each other
Common Example:NaCl
Predominant inCERAMICS
cation
anion
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Covalent Bonding
Bond in which one or more pairs ofelectrons are shared by two atoms.
Common Examples:Methane (CH4)
Predominant in
Elemental solidsNon metallic molecules
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Comparison of Covalent and Ionic
Bonding
Covalent Bonding Ionic Bonding
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Comparison of Covalent and Ionic
Compounds
Ionic Compounds Crystalline solids (made
of ions)
High melting and boiling
points Conduct electricity when
melted
Many soluble in water
but not in nonpolar liquid
Covalent Compounds Gases, liquids, or solids
(made of molecules)
Low melting and boiling
points Poor electrical
conductors in all phases
Many soluble in
nonpolar liquids but notin water
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Metallic Bonding
Both atoms want to lose e-, but
none wants to accept
The lost electrons form a
sea they go nowhere
Primary bonding type in metalsand their alloys
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Secondary bonding
Van der waals bondingNon polar molecules (H2, He)
Randomly moving electrons
Gets piled up on one side of atom
creates a temporary polarity
Hydrogen Bonding Special type of secondary bond between some molecules
containing H
Seen in water
+-
Vander Vaals
attraction
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Announcemets
Discussion class:Thursday 5:00-7:00 pm -CAMP 176
Read:
3.1-3.7
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