材料科学基础(清华大学)-

Repulsive energy
Example problem 2.1
If the attractive force between a pair of Mg2+and S2- is 1.49×10-8N and if the S2- ion has a radius of 0.184nm, calculate a value for the ionic radius of the Mg2+ ion in nanometers.
in atoms or molecules, we refer to them as London forces.
Debye forces
When an induced dipole interacts with a molecule that has a permanent dipole moment, we refer to this interaction as a Debye interaction.
qd
dipole moment
q magnitude of electric charge d separation distance between the charge centers
1. Van der waals interactions
London forces If the interactions are between two dipoles that are induced
For atomic systems:
EN EA ER
Ⅱ.Primary interatomic bonds
1. Ionic bonding Ionic bonding is always found in compounds that are
composed of both metallic and nonmetallic elements, elements that are situated at the horizontal extremities of the periodic table.
Interionic Forces
Fat
t
ract iv e
Z1Z2e2
4 0 a 2
Frepulsive
nb a n1
Fnet
Z1Z2e2
4 0 a 2
nb a n1
Z1, Z2＝number of electrons removed or added from the atoms during the ion formation
0.184nm
0.065nm
2. Covalent bonding
Materials with covalent bonding are characterized by bonds that are formed by sharing of valence electrons among two or more atoms.
e ＝electron charge
a ＝interionic separation distance
ε0＝permittivity of free space＝8.85×10-12C2/(N·m2)
Interionic Energies
Enet
Z1Z 2e 2
4 0 a
b an
Attractive energy
3. Metallic bonding Metallic bonding occurs in solid metals. In metals in solid state, atoms are packed relatively close
together in a systematic pattern or crystal structure.
Ⅲ.Secondary bonding （Van Der waals bonding)
The driving force for secondary bonding is the attraction of the electric dipoles contained in atoms or molecules.
H
oxygen
H
Keesom forces
If the interactions are between molecules that are permanently polarized, we refer to these as Keesom interaction.
Lesson two
§1.2 Atomic bonding
Ⅰ.Bonding forces and energies
FN FA FR
FN : net force FA : attractive force FR : repulsive force
Bonding energies:
E Fdr
An electric dipole moment is created when two equal and opposite charges are separated.
Neutral atom
E
Βιβλιοθήκη Baidu
Van Der waals bonding
A dipole moment is defined as the charge value multiplied by the separation distance between positive and negative charges, or
Solution
Fat t ract iv e
Z1Z2e2
4 0 a 2
a0
Z1Z2e2
4 0 Fat t ract iv e
0.249nm
(2)(2)(1.601019C)2
4[8.851012 C2 (N m2）](1.49108 N)
又 a0
rMg2
r
S
2
rMg2
a0
r
S
2
0.249nm
材料科学基础
Fundamental of Materials
Prof: Tian Min Bo
Tel: 62795426 ，62772851 E-mail: tmb@mail.tsinghua.edu.cn Department of Material Science and Engineering Tsinghua University. Beijing 100084