Metallic bonding allows metals to have unique properties. Metals form a regular lattice structure where the outer electrons separate from the metal atoms to form a "sea of electrons". This delocalizes the electrons and allows them to move freely through the metal. The metal atoms become positively charged ions that are attracted to the sea of electrons, called metallic bonding. This strong bonding allows metals to conduct heat and electricity well and be hard and malleable. Band theory can explain these properties by describing the states that form from the overlapping atomic orbitals in a metal lattice.
3. The properties of metals
Metals have high melting points
Metals conduct heat and
electricity
Metals are hard, not brittle
Metals are shining/lustrous
4. sea of electrons
metal ions
The atoms in a pure metal are in tightly-
packed layers, which form
a regular lattice structure.
The outer electrons of the metal atoms
separate from the atoms
and create a ‘sea of electrons’.
These electrons are delocalized,
and so are free to move through
the whole structure.
The metal atoms become positively charged ions and
are attracted to the sea of electrons. This attraction is
called metallic bonding.
5. Metals often have high melting points
and boiling points. Gold, for example,
has a melting point of 1064 °C and a
boiling point of 2807 °C.
The properties of metals are related to their
structure.
In metal extraction and other industrial processes, furnaces often run
continuously to maintain the high temperatures needed to work with molten
metals.
This property is due to the strong attraction
between the positively-charged metal ions
and the sea of electrons.
Why do metals have high
melting points?
6. Semakin kuat ikatan logam maka semakin tinggi pula
titik lebur dan titik didihnya.
Logam Titik lebur (oC) Titik didih (oC)
Na 97,8 892
Mg 651 1107
Al 660 2467
Logam Jari2 atom
logam (pm)
Kation logam Jari2 kation
logam (pm)
Titik lebur
(oC)
Titik didih
(oC)
Li 157 Li+ 106 180 1330
Na 191 Na+ 132 97,8 892
K 235 K+ 165 63,7 774
Rb 250 Rb+ 175 38,9 688
Cs 272 Cs+ 188 29,7 690
7. Delocalized electrons in metallic bonding
allow metals to conduct heat and
electricity.
For example, when a metal is heated,
electrons are able to gain kinetic energy in
hotter areas of the metal and are able to
quickly transfer it to other parts of the metal
lattice because of their freedom of
movement. Heat causes the electrons to
move faster and the ‘bumping’ of these
electrons with each other and the electrons
transfers the heat
heat
How do metals conduct heat and
electricity?
8. How do metals conduct heat
and electricity?
When an electric field is applied to a
metal, one end of the metal becomes
positive and the other becomes
negative. All the electrons experience a
force toward the positive end. The
movement of electrons is an electric
current.
9. Daya hantar listrik logam dipengaruhi
dua faktor, yaitu energi ionisasi dan jari-
jari atom logam
Semakin besar energi ionisasi maka
semakin sulit lautan elektron terbentuk
sehingga daya hantar listrik cenderung
lebih rendah
Semakin besar ukuran atom logam,
daya hantar listrik logam cenderung
semakin rendah
10. Metals are usually strong, not brittle. When a metal is hit, the layers of metal
ions are able to slide over each other, and so the structure does not shatter.
Why are metals strong?
The metallic bonds do not break because the delocalized electrons are free to
move throughout the structure.
metal after it is hit
force
force
This also explains why metals are malleable (easy to shape) and ductile (can be
drawn into wires).
metal before it is hit
14. Ionic solids are brittle
Strong Repulsion breaks a crystal apart,
due to similar ions being next to each
other.
+
- +
-
+ - + -
+
- +
-
Force
15. 15
Band Theory for Metals (and Other
Solids)
Thus far, whenever we’ve seen electrons, they’ve
been in orbitals (atomic orbitals for atoms, molecular
orbitals for molecules). What about the electrons in a
metal?
These solids can be treated in a way similar to
molecular orbital theory; however, instead of MOs, we
will produce states. Consider that, in a metal, there
are no distinct molecules. You could almost say that
an entire piece of metal is a molecule. That’s how
we’ll be treating them:
– We combine atomic orbitals from every atom in the
sample to make states which look rather like very
large molecular orbitals.
16. – As in LCAO-MO theory, the number of states
produced must equal the number of atomic orbitals
combined.
– The Pauli exclusion principle still applies, so each
state can only hold two electrons.
– For a metal to conduct electricity, its electrons must
be able to gain enough extra energy to be excited into
higher energy states.
– The highest energy state when no such excitation has
occurred (i.e. in the ground state metal) is called the
Fermi level.
Band Theory for Metals (and Other Solids)
17. 17
Band Theory for Metals (and Other Solids)
Image adapted from “Chemical Structure and Bonding” by R. L. DeKock and H. B. Gray
So, how do states
get formed, and
what do they look
like?
Consider lithium.
The figure at the
right shows the
MOs produced by
linear combination
of the 2s orbitals
in Li2, Li3 and Li4.
18. 18
Band Theory for Metals (and Other Solids)
In an alkali metal, the valence s band is only half
full. e.g. sodium
– If there are N atoms of sodium in a sample,
there
will be N electrons in 3s orbitals.
– There will be N states made from 3s orbitals,
each
able to hold two electrons.
– As such, N /2 states in the 3s band will be full
and N /2 states will be empty (in ground state
Na).
19. Like all other alkali metals,
sodium conducts
electricity well because the
valence band is
only half full. It is therefore
easy for electrons
in the valence band to be
excited into empty
higher energy states.
Since these empty higher
energy states are in the same
band, we can say that the
valence band for sodium is
also the conduction band.
Band Theory for Metals (and Other Solids)
20. 20
Band Theory for Metals (and Other Solids)
In an alkaline earth metal, the valence s band
is full.
e.g. beryllium
– If there are N atoms of beryllium in a
sample, there will be 2N electrons in 2s
orbitals.
– There will be N states made from 2s
orbitals, each able to hold two electrons.
– As such, all states in the 2s band will be full
andnone will be empty (in ground state Be).
21. So, why are alkaline earth metals
conductors?
– While the 2s band in beryllium
is full, it overlaps
with the 2p band.
– As such, some electrons in
the valence band
can easily be excited into the
conduction band.
– In beryllium, the conduction
band (band containing
the lowest energy empty
states) is the 2p band.
Band Theory for Metals (and Other Solids)
22. Since the gap between energy levels are
extremely small, radiation of any frequency in
visible region can be absorbed and emitted.
Lin
Surface lustre
Half-filled 2s
band
Silvery and shiny
23. 23
Band Theory for Metals (and Other Solids)
What do the bands look like for
something that doesn’t conduct
electricity? i.e. for an insulator
e.g. diamond
– If there are N atoms of carbon
in a sample,
there will be 4N valence
electrons.
– The valence orbitals of the
carbon atoms
will combine to make two
bands, each
containing 2N states.
24. 24
Band Theory for Metals (and Other Solids)
There are two broad categories
of semiconductors:
– Intrinsic Semiconductors
• Naturally have a moderate band gap.
A small fraction of the electrons in the
valence band can be excited into the
conduction band. They can carry
current.
• The “holes” these electrons leave in
the valence band can also carry
current as other electrons in the
valence band can be excited into
them.
25. –Extrinsic Semiconductors
• Have had impurities added in order to increase the
amount of current they can conduct. (impurities
called dopants; process called doping)
• The dopants can *either* provide extra electrons
*or* provide extra holes:
– A semiconductor doped to have extra electrons is an n-
type semiconductor (‘n’ is for ‘negative’)
– A semiconductor doped to have extra holes is a p-type
semiconductor (‘p’ is for ‘positive)
Band Theory for Metals (and Other Solids)
26. Semikonduktor Tipe p
Semikonduktor tipe p diperoleh
dengan cara mendoping atom-atom
yang bervalensi satu tingkat lebih
rendah ke dalam semikonduktor
Penambahan pengotor bervalensi tiga
seperti B, Al atau Ga (akseptor
elektron) ke dalam semikonduktor
intrinsik (Si) menghasilkan defesiensi
elektron valensi yang disebut ‘lubang’
(bermuatan positif)
Defesiensi elektron atau lubang
tersebut berada pada tingkat fermi..
Elektron pada pita valensi akan
mengisi rongga tersebut, sehingga
aliran elektron dapat mencapai pita
konduksi
26
27. Semikonduktor Tipe n
Semikonduktor tipe n diperoleh
dengan cara mendoping atom-
atom bervalensi satu tingkat lebih
tinggi ke dalam semikonduktor.
Penambahan pengotor
bervalensi lima seperti Sb, As
atau P menyumbangkan elektron
bebas (donor free elektron).
Elektron bebas itu berada pada
tingkat fermi dan dapat masuk ke
pita konduksi. Kekosongannya
digantikan oleh elektron dari pita
valensi, sehingga terjadi aliran
elektron. Akibatnya konduktifitas
semikonduktor instrinsik
bertambah.
27
28. Metallic Radius
Metallic radius (r) is defined as half of the
internuclear distance between adjacent
atoms in a metal crystal.
