2. Bonds in Crystals
Results from the redistribution of e- that leads to a more
stable configuration between two or more atoms
The valence e- are those involved in bonding
When the e- have reorganized themselves, and if the
energy configuration is lower, then the atoms will stay
together
Electrical forces are chemical bonds, i.e., the attraction of
+ and - charges
3. Chemical bonds
Can belong to one of five principal bond types:
‣ ionic
‣ metallic
‣ covalent
‣ van der Waals
‣ hydrogen
Ionic, covalent, and metallic bonds involve valence e-,
while van der Waals and hydrogen bonds do not.
4. Ionic Bonds
Ionic bond: forms when one or more e- in the
valence shell of an atom are transferred to the
valence shell of another, so that both
elements achieve an noble gas configuration
9. Ionic Bond
The attraction between oppositely charged ions
constitutes the ionic (or electrostatic) bond
This bond forms as the result of the exchange of e-
(s) of the metal atom to the nonmetal atom
10. Ionic Bond
Ionic bonds commonly
form between atoms of
columns I and VII and
columns II and VI
As the distance (due to
the size of the anion)
increases, the bond
strength decreases, as a
function of increasing
inter-ionic distances
fig. 3.13
11. Ionic Bonds - Summary
are generally of moderate hardness and have
fairly high melting points
are strong when forced together, but weak
when cleaved or sheared
are poor conductors of electricity and heat
the symmetry of the resultant crystals is
generally high
12. Metallic Bonds
Metallic bond: the attractive force between
positively charged nuclei with filled e- orbitals
and the cloud of negative e- that holds such
crystal structures together
14. a) Schematic cross section
fig. 3.15
through the structure of a
metal. Each circle with a
positive charge represents a
nucleus with filled, nonvalence
e- orbitals of the metal atoms.
The mobile e- are represented
by the cloud around the atoms
(light gray). A possible e- path
between the nuclei is shown by
the line. b) An electron density
map of copper atoms in copper
metal showing the spherical
nature and packing of the
positively charged nuclei (white
circles) surrounded by a less
dense cloud of e- (contour
lines)
15. Covalent Bonds
Covalent bond: when two (or more) atoms
share their outer valence e-
it is an intermediate bond type, between ionic
and metallic
16. Covalent Bonds
The force of the bond is derived from the mutual
sharing of e-
Involves the merging and overlap of e- orbitals to
achieve an octet configuration
Cl- Cl-
17. Covalent Bonds
The force of the bond is derived from the mutual
sharing of e-
Involves the merging and overlap of e- orbitals to
achieve an octet configuration
Cl- Cl-
21. Covalent Bonds
Elements near the middle of the periodic table, such
as C, Si, Al, and S, have 2, 3, and 4 vacancies in
their outer orbitals
# of covalent bonds = # of shared e-
The energy of the bonds produces a very rigid
structure - it is the strongest of all the chemical bonds
22. Covalent Bonds
insoluble - generally slow reactivity
chemical and mechanical stability
very high melting points
nonconductors of electricity
the ions are no longer spherical, and the
symmetry of the resulting crystals is likely
to be lower
23. Electronegativity
Ability of an atom in crystal structure or molecule to
attract e- into its outer shell
EN can be used as a basis for distinguishing
elements:
‣ metals: < 1.9
‣ metalloid: 1.9 < X < 2.1
‣ non-metals: > 2.1
25. Electronegativity
An atom whose EN exceeds that of the other atom
by 2 or more, will generally be able to attract valence
e- from the other atom
If not a lot of contrast - nearly equal EN - neither
atom is able to capture an e- from the other; they
share
26. Bonds without Valence e-
Van der Waals bond:
force of the bond is
derived from residual
surface charges on an
otherwise relatively
neutral structure
28. van der Waals Bond
Could be considered as a weak dipole effect
A small concentration of positive charge are one
end, leads to a small concentration of negative
charge at the other:
29. van der Waals Bond
effective bonding over large distances in
molecular structures
generally defines a zone of cleavage and low
hardness
it is the weakest of the chemical bonds
30. Bonds without Valence e-
Hydrogen bond: an electrostatic bond
between a positively charged hydrogen ion
and a negatively charged ion, such as O2- or
N3-
the hydrogen bond is considerably stronger
than the van der Waals bond
31. Hydrogen bond
These bonds are weak, but there are many of them per
unit volume of structure, which results in an overall,
relatively strong material
Bonding is common in hydroxides (OH-)
Also present in many of the layer silicates, such as
micas and clay minerals
32.
33. Bonds in Crystals
In general, the stronger
the average bond, the
harder the crystal and the
higher its melting point
Hardness, cleavage,
fusibility, electrical and
thermal conductivity, and
compressibility are all
directly related to bond
strength
