some of the important properties of solid materials depend on geometrical atomic arrangements the...
TRANSCRIPT
Some of the important properties of solid materials depend on
geometrical atomic arrangements
the interactions that exist among the constituent atoms or molecules
Atomic structureFundamental Concept
Atoms are the basic structural unit of all engineering materials. It is the basic unit of an element that can undergo chemical change
Each atom consists of a very small nucleus composed of protons & neutrons which is encircled by moving electrons.
Atomic number, Z - Number of protons. In a neutral atom the atomic number is equal to the number of electrons (Z~p=e).
Atomic mass, A - Total mass of proton and neutron in the nucleus ( A=Z+N ).
Isotope - atoms that have two or more atomic mass. Same number of proton but different number of neutron.
1 atomic mass unit (a.m.u) – 1/12 of the atomic mass of carbon
1 mole= 6.023 x 1023 atoms ( Avogadro’s number NA ).
1 amu/atom = 1g/mol
Atomic structure
Example 1
Example 2
Classified all the elements – electron configuration in the periodic table
The element situated with increasing atomic number in seven horizontal rows called periods.
All the elements arrayed in 1 column or group have similar :
Periodic Table
i)valence electron
ii)chemical properties
iii)physical properties
The periodic table
He
Ne
Ar
Kr
Xe
Rn
inert
gase
s acc
ep
t 1
e acc
ep
t 2
e
giv
e u
p 1
e g
ive u
p 2e
giv
e u
p 3
e
F Li Be
Metal
Nonmetal
Intermediate
H
Na Cl
Br
I
At
O
S Mg
Ca
Sr
Ba
Ra
K
Rb
Cs
Fr
Sc
Y
Se
Te
Po
Electropositive elements: Metallic in nature & give up electrons in chemical reactions to
produce positive ions (cations). Most electropositive ~ groups 1A & 2A
Electronegative elements: Nonmetallic in nature & accept electrons in chemical
reactions to produce negative ions (anions). Most electronegative ~ groups 6A & 7A
Periodic Table
Groups 4A & 7A can behave either electropositive electronegative manner
Oxidation numbers of the elements with respect to their positions in periodic table
Electronegativity is the degree to which an atom attracts
electrons to itself. Large values: tendency to acquire electrons. Ranges from 0.7 to 4.0,
Smaller electronegativity Larger electronegativity
Metals Nonmetals
Have few electrons in outer shell, usually 3 or less
Have 4 or more electrons in outer shell
Form cations by losing electrons
Form anions by gaining electrons
Have low electronegativities
Have high electronegativities
Summary of some of the electronic structure-chemical property relationships for
metals & nonmetals
Interatomic Bonding
In general, why does bonding between atoms occur?
Bonding between atoms generally occurs because the atoms’ energies are lowered through the bonding process.
Types of atomic & molecular bonds
Chemical bond between atoms
Primary (strong bond) Ionic bonds Covalent bonds Metallic bonds
Secondary (weak bond) Permanent dipole bonds Fluctuating dipole bonds
Depends on their valence electrons.
Ionic bonds eg; NaCl
Ionic bonding arises from the electrostatic attraction between oppositely charged ions.
In the process of ion formation, an electron or a number of electrons may be transferred from a highly electropositive element (eg; Na) to a highly electronegative one(eg; Cl).
The ionic bond in solids is nondirectional.
He -
Ne -
Ar -
Kr -
Xe -
Rn -
F 4.0
Cl 3.0
Br 2.8
I 2.5
At 2.2
Li 1.0
Na 0.9
K 0.8
Rb 0.8
Cs 0.7
Fr 0.7
H 2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ra 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
CsCl
MgO
CaF2
NaCl
O 3.5
Ionic Bonding• In compounds made of metals and
nonmetals.• (Give and take creates bonding !)• e.g. Ceramics, NaCl• Nondirectional bonding: Bond magnitude is
equal in all directions around an ion…THUS, all positive ions should be surrounded by negative ions in 3D.
• Ionic materials: Hard, Brittle, Electrically and thermally insulative (no free electrons).
Ionic bonding
• Involves metal & nonmetal elements
• Metal elements donate e- & nonmetal element gain electron
• Large difference in electronegativity required.
• Large different of electronegativity between element means strong bonding
Na (metal) unstable
Cl (nonmetal) unstable
electron
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
Ionic bonding
Example 3Describe the ionic bonding process between a pair of Na and Cl atoms. Which electrons are involved in the bonding process?
The ionic bonding process between a pair of Na and Cl atoms involves a transfer of the outer 3s1 electron of the Na atom to the 3p vacancy in the Cl atom. Thus, the Na ion formed has the Ne electron configuration while the Cl ion has the Kr electron configuration.
Solution
Example 4After ionization, why is the sodium ion smaller than the sodium atom?
Solution
After ionization to the Na+, the Na atom becomes smaller because the electron-to-proton ratio of the Na atom is decreased when the Na+ ion forms. Also, the outer third shell no longer exists once the 3s 1 electron is lost by the Na atom.
Example 5After ionization, why is the chloride ion larger than the chlorine atom?
Solution
After ionization, the Cl- ion is larger because the electron-to-proton ratio of the chlorine atom is decreased by the ionization process.
Bonding forces
• As two atoms approach each other, two forces exist (each is function of separation distance:– Attractive force (Depends on type of bonding).– Repulsive force (created as atoms approach each
other, outer electron shells overlap).
• At equilibrium:– The two atoms will counteract any attractive or
repulsive forces:
Fattractive + FRepulsive = 0– Atomic centers become separated by r0 (equilibrium
spacing).
Interionic Forces for an Ion Pair
Fnet = Fattractive + Frepulsive
Interionic Forces for an Ion Pair
Example 6
Bonding energies (cont.)
• Bonding energy (Eo): Energy at equilibrium separation, i.e., energy required to separate the two atoms to an infinite separation (i.e., break them apart).
• Magnitude of bonding energy & shape of energy vs. interatomic separation curve depends on material & type of atomic bonding.
Interionic Energies for an Ion Pair
Enet = Eattractive + Erepulsive
Example 7
Calculate the net potential energy for a K+Br- pair by using the b constant calculated from example 8. Assume n = 9.5.
Ion arrangements in Solid
• ionic bond is nondirectional in character.• Ionic solid is
– governed by the geometric arrangement of ions– Electrical Neutrality
Ionic packing arrangements in (a) CsCl and (b) NaCl. 8 Cl- ions can pack around Cs:, but only 6 Cl- can pack around a Na+ ion.
Ion arrangements in Solid
• ionic bond is nondirectional in character.• Ionic solid is
– governed by the geometric arrangement of ions– Electrical Neutrality
Ionic packing arrangements in (a) CsCl and (b) NaCl. 8 Cl- ions can pack around Cs:, but only 6 Cl- can pack around a Na+ ion.
Covalent Bonding
Covalent bonding is a primary type of bonding which arises from the reduction in energy associated with the overlapping of half-filled orbitals of two atoms.
In this bond, there is an electron exchange interaction.
The covalent bond is a directional type of bond.
Covalent Bonding Occur between atoms with small differences in
electronegativity & close to each other in the periodic table.
Atoms share their outer s & p electrons with other.
In a single covalent bond, each of two atoms contributes one electron to form an electron pair bond = energy small ~ more stable.
In multiple electron-pair bonds can be formed by one atom with itself or other atoms.
shared electrons from carbon atom
shared electrons from hydrogen atoms
H
H
H
H
C
CH4
Covalent bonding
He -
Ne -
Ar -
Kr -
Xe -
Rn -
F 4.0
Cl 3.0
Br 2.8
I 2.5
At 2.2
Li 1.0
Na 0.9
K 0.8
Rb 0.8
Cs 0.7
Fr 0.7
H 2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ra 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
SiC
C(diamond)
H2O
C 2.5
H2
Cl2
F2
Si 1.8
Ga 1.6
GaAs
Ge 1.8
O 2.0
colu
mn IVA
Sn 1.8Pb 1.8
Covalent Bonding (Cont.)
Materials with covalent bonding:
• Electrically and thermally insulative.
• Covalently-bonded materials can be:Hard, High melting temperature.Weak, low melting temperature.
Covalent Bonding1. Covalent Bonding in the Hydrogen molecule
Covalent Bonding
2. Covalent Bonding in Other Diatomic Molecules
Covalent Bonding
2. Covalent Bonding by Carbon
Covalent Bonding2. Covalent Bonding by Carbon-containing Molecules
Benzene
Partially ionic Partially covalent
• Few compounds exhibit pure ionic or pure covalent bonding.
• For a compound, the degree depends on relative positions of atoms in periodic table (i.e, electronegativity).
• Close elements bond covalently. Far elements bond ionically. (Distant entities need to sacrifice to get bonded !)
Metallic bonding
Metallic bonding is a primary type of bonding involving the interaction of the valence electron or electrons of one atom with many surrounding atoms.
This interaction leads to a reduction in energy of the system considered.
The valence bonding electrons of these bonds are sometimes regarded as an “electron gas” bonding the positive ion cores (atoms less their valence electrons) of atoms.
The metallic bond is nondirectional
Metallic bonding
e.g., all metals Weak or strong Good conductors for electricity and heat
(free electrons).
Metallic bonding
Atomic arrangement in a metallic copper crystal.
Each copper atom is coordinated with 12 other atoms, producing a crystal structure called face centered-cubic (fcc) structure.
The atoms are bonded together by an “electron gas” of delocalized valence electron
Metallic bonding
Two-dimensional schematic diagram of metallically bonded atoms.
The circles with the inner positive signs represent positive-ion cores,
The charge clouds around the ion cores represent the dispersed valence electrons
Metallic bonding
Formed as a result of the interaction of the electric dipoles contained in atoms or molecules
Can be divided by:
(1) Fluctuating Dipoles
(2) Permanent Dipoles
SECONDARY BONDING
Secondary bonding (Van der Waal)
• Exists between all atoms or molecules
• Electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule.
• Evident for inert gases & molecules covalently bonded.
• Arises from atomic or molecular dipoles.
• Special case: Hydrogen bonding.
Fluctuating Dipoles Fluctuating dipole bonding is a secondary type of
bonding between atoms which contain electric dipoles.
These electric dipoles, formed due to the asymmetrical electron charge distribution within the atoms, change in both direction and magnitude with time.
This type of bond is electrostatic in nature, very weak and nondirectional.
Fluctuating Dipoles
Idealized symmetrical electron charge cloud distribution
Electron charge cloud distribution in a noble-gas atom
Real case with symmetrical electron charge cloud distribution that changes with time, creating a Fluctuating electric dipoles
Permanent Dipoles
Permanent dipole bonding is also a secondary type of bonding between molecules possessing permanent electric dipoles.
The bonds, formed by the electrostatic attraction of the dipoles, are directional in nature.
Permanent Dipoles
Permanent dipole nature of the water molecule
Hydrogen bonding among water molecules due to permanent dipole attraction
Arises from interaction between dipoles
• Permanent dipoles-molecule induced
• Fluctuating dipoles
+ - secondary bonding + -
H Cl H Clsecondary bonding
secondary bonding
HH HH
H2 H2
secondary bonding
ex: liquid H2asymmetric electron clouds
+ - + -secondary bonding
-general case:
-ex: liquid HCl
-ex: polymer
Secondary bonding
Ceramics(Ionic & covalent bonding):
Metals(Metallic bonding):
Polymers(Covalent & Secondary):
secondary bonding
Large bond energylarge Tm
large Esmall
Variable bond energymoderate Tm
moderate Emoderate
Directional PropertiesSecondary bonding dominates
small Tsmall Elarge
SUMMARY: PRIMARY BONDS
Type Bond Energy Comments
Ionic Large Nondirectional (ceramics)
Covalent Variable
large-Diamond
small-Bismuth
Directional(semiconductors, ceramics, polymer chains)
Metallic Variable
large-Tungsten
small-Mercury
Nondirectional (metals)
Secondary smallest Directional
inter-chain (polymer)
inter-molecular
SUMMARY: BONDING
Mixed Bonding
• Ionic-covalent eg; GaAs
• Metallic-covalent eg; transition metal
• Metallic-Ionic eg; intermetallic – NaZn13
Example 81) Explain the following types of primary bonding: (a) ionic, (b) covalent, and (c) metallic.
-refer lecture note
2) Explain the following types of secondary bonding: (a) fluctuating dipole, and (b) permanent dipole.
-refer lecture note
Example 9Describe the hybridization process for the formation of four equivalent sp3 hybrid orbitals in carbon during covalent bonding. Use orbital diagrams.
Solution
Example 10Describe the hybridization process for the formation of four equivalent sp3 hybrid orbitals in carbon during covalent bonding. Use orbital diagrams.
Solution
Example 11
Why is diamond such a hard material?
Solution
Diamond is extremely hard because its carbon atoms are covalently bonded by single sp3 hybrid bonds in a three dimensional arrangement.
Example 12How can the high electrical and thermal conductivities of metals be explained by the “electron gas” model of metallic bonding? Ductility?
Solution
The high electrical and thermal conductivities of metals are explained by the mobility of their outer valence electrons in the presence of an electrical potential or thermal gradient.
The ductility of metals is explained by the bonding “electron gas” which enables atoms to pass over each other during deformation, without severing their bonds.
Example 13
Why is diamond such a hard material?
Solution
Diamond is extremely hard because its carbon atoms are covalently bonded by single sp3 hybrid bonds in a three dimensional arrangement.
Example 14Describe the covalent bonding process between a pair of hydrogen atoms. What is the driving energy for the formation of a diatomic molecule?
Solution
The covalent bonding in the hydrogen molecule involves the interaction and overlapping of the 1s orbitals of the hydrogen atoms.
The covalent bond forms between the two hydrogen atoms because their energies are lowered by the bonding process.