Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
Secondary Bonds
Electric Dipoles
To understand these forces the idea of dipoles is needed. An electric dipole is basically a pair of equal positive and negative charges separated by a small distance.
These dipoles will arise, for example, in a molecule, where atoms share an electron, but the electron spends more time with one atom, because it is bigger, and less time with the smaller atom.
The effective charge separation may be only a fraction (perhaps a tenth) of the electron's charge.
Bonding between dipoles happens when the positive end of one dipole is attracted to the negative end of another.
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
Since the effective charges are small compared with those involved in primary bonding, secondary bonding is about one tenth of primary bond strengths.
Van Der Waals Bonding
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
Secondary bonds are weak in comparison to primary bonds.
They are found in most materials, but their effects are often overshadowed by the strength of the primary bonding.
Secondary bonds are not bonds with a valence electron being shared or donated. They are usually formed when an uneven charge distribution occurs, creating what is known as a dipole (the total charge is zero, but there is slightly more positive or negative charge on one end of the atom than on the other).
These dipoles can be produced by a random fluctuation of the electrons around what is normally an electrically symmetric field in the atom.
Once a random dipole is formed in one atom, an induced dipole is formed in the adjacent atom.
This is the type of bonding present in N
2
molecules, and is known as
Van Der Waals Bonding.
Secondary bonding may also exist when there is a permanent dipole in a molecule due to an asymmetrical arrangement of positive and negative regions.
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
Molecules with a permanent dipole can either induce a dipole in adjacent electrically symmetric molecules, and thus form a weak bond, or they can form bonds with other permanent dipole molecules.
Secondary Bonds 2
Hydrogen Bonding
Hydrogen bonding is the strongest form of secondary bonding and is formed from the polar nature of molecules containing hydrogen.
The hydrogen side of the molecule is more positive than the atom it is bonded to, allowing an attraction to form with the negative end of another molecule.
In hydrogen bonding, the hydrogen is attached directly to an element which is electronegative. This causes the hydrogen to acquire a significant positive charge.
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
The melting and boiling points of materials containing hydrogen bonding, are abnormally high, for their atomic weights, due to hydrogen bonding.
Bonds in periodic table:
The Periodic Table with Electron Configurations
For a background on the periodic table,
Atoms have their electrons confined in potential energy wells around them, but not at any energy , there are only a few discrete energy levels allowed. These levels are like steps on a ladder where your feet cannot be supported at any height, but only on the rungs.
The electrons then cannot be at intermediate energies, they will fall down to the next
"rung" on the energy ladder, grouping in three dimensional "shells".
The periodic table arises because only certain numbers of electrons are allowed in each shell, as also illustrated in the diagram. Electrons are held at upper levels
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
because they cannot all fall down to the bottom.
The valence electrons are in the outer (highest) energy levels and are the main determiners of chemical activity and bonding. "Valence" comes from a Latin word meaning "power" and used for the "combining power" of an atom,
As the outer shells determine most of the chemical properties, the
Periodic Table below shows only the electron numbers in the two outer shells, the colours indicate how shells are filled.
H
1
1
IA IIA IIIB IVB VB VIB
VII
B
{-
VIII
B
-} IB IIB IIIA IVA VA VIA
VII
A
0
He
2
Li
2.1
Be
2.2
2
B
2.3
C
2.4
N
2.5
O
2.6
F
2.7
Ne
2.8
3
Na
.8.
1
Mg
.8.
2
4
K
.8.
1
Ca
.8.
2
Sc
.9.2
Ti
.10.
2
V
.11.
2
Cr
.13.
1
Mn
.13.
2
Fe
.14.
2
Co
.15.
2
Al
.8.3
Si
.8.4
P
.8.5
S
.8.6
Cl
.8.7
Ar
.8.8
Ni
.16.
2
Cu
.18.
1
Zn
.18.
2
Ga
.18.
3
Ge
.18.
4
As
.18.
5
Se
.18.
6
Br
.18.
7
Kr
.18.
8
5
Rb
.8.
1
Sr
.8.
2
Y
.9.2
Zr
.10.
2
Nb
.12.
1
Mo
.13.
1
Tc
.14.
1
Ru
.15.
1
Rh
.16.
1
Pd
.18
Ag
.18.
1
Cd
.18.
2
In
.18.
3
Sn
.18.
4
Sb
.18.
5
Te
.18.
6
I
.18.
7
Xe
.18.
8
6
Cs
.8.
1
Ba
.8.
2
La*
.9.2
Hf
.10.
2
Ta
.11.
2
W
.12.
2
Re
.13.
2
Os
.14.
2
Ir
.17
Pt
.17.
1
Au
.18.
1
Hg
.18.
2
Tl
.18.
3
Pb
.18.
4
Bi
.18.
5
Po
.18.
6
At
.18.
7
Rn
.18.
8
Fr Ra Ac* Rf Ha Sg Ns Hs Mt Uu Uu Uu Uut Uu Uu Uu Uus Uu
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
7
.8.
1
.8.
2
*
.9.2 n u b q p h
Ce*
.8.2
Pr
.8.2
Nd
.8.2
Pm
.8.2
Sm
.8.2
Eu
.8.2
Gd
.9.2
Tb
.8.2
Dy
.8.2
Ho
.8.2
Er
.8.2
Tm
.8.2
Yb
.8.2
Lu
.9.2
Th*
*
.10.
2
Pa
.9.2
U
.9.2
Np
.9.2
Pu
.9.2
Am
.9.2
Cm
.9.2
Bk Cf Es Fm Md No Lr o
The inert gases of Group 0 , namely Helium, Neon, Argon, Krypton,
Xenon, and Radon, each have 8 valence electrons and form few chemical compounds. (Helium is an exception with only 2 outer electrons). This suggests that 8 valence electrons form a stable electronic structure where an atom tends not to share electrons with other atoms.
An element will be chemically reactive if it can get to the stable electronic configuration of an inert gas, either
• by losing one or two electrons to another atom, or
• by gaining one or two electrons from another atom (at most three),or
• by sharing three or more electrons.
Summarising:
*The Periodic Table gives an indication of how electrons will be shared.
*Ionic bonds tend to form between elements at the edges.
*Covalent bonds tend to form between elements in the middle.
*Metallic bonds have overlapping potentials releasing some electrons to form a "glue".
*Secondary bonds are the result of electric dipole interaction.
*Hydrogen bonds form with permament
*Van der Waals bonds generally form from fluctuating dipoles. dipoles.
Ionic
Strong
Covalent
Strong n=1, m=9 n=6, m=12 non-directional Directional long range charge stacking requirements short range no charge stacking requirements
Metallic strong
Van der Waals Hydrogen weak weak n=6, m=12 n=2 non-directional non-directional non-directional long range no charge stacking requirements short range short range dipole orientation dipole orientation
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Lecture 4: Secondary bonds , Materials Physics 2nd class/polymer Dep.
*Ionic crystals tend to be brittle because of the charge restriction.
*Covalent crystals tend to be brittle because of the directional restriction.
*Metals tend to be plastic because there are no directional or charge restrictions.
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