Metallic bonding
Basics of Chemical Bond
CHM 001
Oluwakemi A Oloba-Whenu, Ph. D
148A Faculty of Science
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Introduction….
• Metallic bond is formed between the attraction between kernels and the
mobile electrons in a metal lattice.
• It is a type of chemical bonding that arises from the electrostatic attractive
force between conduction electrons (in the form of an electron cloud of
delocalized electrons) and positively charged metal ions.
• It may be described as the sharing of free electrons among a lattice of
positively charged ions (cations)
• Metallic bonding is found in metals and their alloys.
• When the atoms give up their valence electrons, they form ions. (kernels)
These ions are held together by the electron cloud surrounding them.
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Introduction….
• When the atoms give up their valence electrons, they form ions. (kernels) These
ions are held together by the electron cloud surrounding them.
• Metal atoms achieve stability by “off-loading” electrons to attain the electronic
structure of the nearest noble gas. These electrons join up to form a mobile
cloud which prevents the newly-formed positive ions from flying apart due to
repulsion between similar charges.
Atoms arrange in regular close
packed 3-dimensional crystal lattices.
The outer shell electrons of each atom
leave to join a mobile “cloud” or “sea” of
electrons which can roam throughout the
metal. The electron cloud binds the
newly-formed positive ions together.
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Some characteritics of metals
• Low ionization energies: This implies that the valence electrons of metal
atoms are not strongly held by the nucleus.
• As a result the valence electrons can move freely out of the influence of
their kernels i.e. metals have free electrons
• The cations within a metallic solid are known as Kernels
• Large number of empty orbitals: in metals a number of valence orbitals
remain empty as the valence shell are usually far less than half-filled
• For example, lithium {(Li, Z = 3) 1s22s1} has 2p-orbitals vacant;
• Sodium {(Na, Z = 11) 1s22s22p6 3s1} has 3p-and 3d-orbitals vacant;
• Magnesium {(Mg, Z = 12) 1s22s22p6 3s2} has 3p-and 3d-orbitals vacant
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Electron Sea Model for Metallic
Bonding
• Bonding in metals, proposed by Lorentz a model known as electron gas model or electron
sea model. This model is based on the afore mentioned characteristics of metals:
• Low ionization energies and Large number of empty orbitals
• The outer energy levels of metal atoms (the s and p orbitals) overlap.
• At least one of the valence electrons participating in a metallic bond is not shared with a
neighbor atom, nor is it lost to form an ion.
• The valence electrons can leave the field of influence of one kernel and enter the field of
influence of the other. This movement can take place through the vacant valence orbitals.
• Thus, the valence electrons are not localised but are mobile or delocalised.
• As the movement of electrons in metallic crystal is just like gas molecules, hence, the model
is called electron gas model.
• The electron sea model is an oversimplification of metallic bonding.
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How Metallic Bonds Work
• The outer energy levels of metal atoms (the s and p orbitals) overlap. At least one of the
valence electrons participating in a metallic bond is not shared with a neighbor atom,
nor is it lost to form an ion. Instead, the electrons form what may be termed an
"electron sea" in which valence electrons are free to move from one atom to another.
• The electron sea model is an oversimplification of metallic bonding. Calculations based
on electronic band structure or density functions are more accurate. Metallic bonding
may be seen as a consequence of a material having many more delocalized energy states
than it has delocalized electrons (electron deficiency), so localized unpaired electrons
may become delocalized and mobile.
• The electrons can change energy states and move throughout a lattice in any direction.
• Bonding can also take the form of metallic cluster formation, in which delocalized
electrons flow around localized cores. Bond formation depends heavily on conditions.
For example, hydrogen is a metal under high pressure.
• As pressure is reduced, bonding changes from metallic to nonpolar covalent.
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The important features of
electron sea model are
• The positively charged kernels of metal atoms are arranged in a regular
fashion in a metallic lattice.
• Loosely held valence electrons, surround each kernel in metallic lattice.
• Being loosely held to its kernel, the valence electrons enjoy complete
freedom in the metallic lattice and are regarded as mobile electrons.
• This explanation is also responsible for its name electron sea model.
• Thus, the simultaneous force of attraction between the mobile electrons
and the positive kernels that binds the metal atoms together, is known as
metallic bond
• The metallic bond is non-directional and is weaker than the covalent bond.
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Strength of a metallic bond
• Depends on the number of outer electrons donated to the cloud and
the size of the metal atom/ion.
• The average attractive force and metal bond strength increases with
the decrease in atomic radius and increase in number of valence
• electrons.
Note: both these factors decrease the metal character because of the
tendency to form metallic crystal decreases. For example metallic
character decreases across the period. Among the elements of 3rd
period metal character decreases from left to right. The metallic
elements are only Na, Mg, AI, but strength of metallic bond increases
from Na -7 AI as reflected from their melting points.
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Strength of a metallic bond
Na
K
Mg
• The strength of the metallic bonding in
sodium is relatively weak because each atom
donates one electron to the cloud.
• The metallic bonding in potassium is weaker
than in sodium because the resulting ion is
larger and the electron cloud has a bigger
volume to cover so is less effective at holding
the ions together.
• The metallic bonding in magnesium is stronger
than in sodium because each atom has
donated two electrons to the cloud. The
greater the electron density holds the ions
together more strongly
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METALLIC PROPERTIES
Metals are excellent conductors of electricity
• For a substance to conduct electricity it must have mobile ions or
electrons.
• Because the ELECTRON CLOUD IS MOBILE, electrons are free to move
throughout its structure. Electrons attracted to the positive end are
replaced by those entering from the negative end.
MOBILE ELECTRON CLOUD ALLOWS THE CONDUCTION OF ELECTRICITY
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METALLIC PROPERTIES
Thermal conductivity
• When a part of the metal is heated, the kinetic energy of the
electrons in that region increases.
• Since the electrons are free and mobile, these energetic electrons
move rapidly to the cooler parts and transfer their kinetic energy by
means of collisions with other electrons.
• Therefore, the heat travels from hotter to cooler parts of the metals.
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METALLIC PROPERTIES
Malleability and ductility
• Metals can be beaten into sheets (malleability) and drawn into
wires (ductility). Metallic bonds are non-directional in nature.
• Whenever any stress is applied on metals, the position of
adjacent layers of metallic kernels is altered without destroying
the crystal.
• The metallic lattice gets deformed but the environment of
kernels does not change and remains the same as before.
• The deforming forces simply move the kernels from one lattice
site to another.
• As the metal is beaten into another shape the delocalised
electron cloud continues to bind the “ions” together.
• Some metals, such as gold, can be hammered into sheets thin
enough to be translucent.
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METALLIC PROPERTIES
HIGH MELTING POINTS
Melting point is a measure of how easy it is to separate individual particles.
In metals it is a measure of how strong the electron cloud holds the + ions.
The ease of separation of ions depends on the...
ELECTRON DENSITY OF THE CLOUD
IONIC / ATOMIC SIZE
PERIODS
m.pt
b.pt
Na (2,8,1)
<
Mg (2,8,2)
<
Al (2,8,3)
98°C
890°C
650°C
1110°C
659°C
2470°C
Na+
Mg2+
Al3+
MELTING POINT INCREASES ACROSS THE PERIOD
THE ELECTRON CLOUD DENSITY INCREASES DUE TO THE
GREATER NUMBER OF ELECTRONS DONATED PER ATOM. AS A
RESULT THE IONS ARE HELD MORE STRONGLY.
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METALLIC PROPERTIES
HIGH MELTING POINTS
Melting point is a measure of how easy it is to separate individual particles.
In metals it is a measure of how strong the electron cloud holds the + ions.
The ease of separation of ions depends on the...
ELECTRON DENSITY OF THE CLOUD
IONIC / ATOMIC SIZE
GROUPS
m.pt
b.pt
Li (2,1)
<
Na (2,8,1)
<
K (2,8,8,1)
181°C
1313°C
98°C
890°C
63°C
774°C
Li+
Na+
K+
MELTING POINT INCREASES DOWN A GROUP
IONIC RADIUS INCREASES DOWN THE GROUP. AS THE IONS GET
BIGGER THE ELECTRON CLOUD BECOMES LESS EFFECTIVE
HOLDING THEM TOGETHER SO THEY ARE EASIER TO SEPARATE.
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METALLIC PROPERTIES
Metals Form Alloys
Metals do not combine with metals. They form
Alloys which is a solution of a metal in a metal.
Examples are steel, brass, bronze and pewter.
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Ionic Bond
The transfer of electrons between
two atoms having different electro
negativities forms this bond.
This is a strong bond due to
electrostatic force of attraction.
This is a non-directional bond.
This bond makes substances hard
and brittle.
Covalent Bond
Metallic Bond
This bond is formed by the mutual
sharing of electrons between same
or different elements .
This bond is formed due to the
attraction between kernels and the
mobile electrons in a metal lattice.
This is a weak bond due to the
This is also a fairly strong bond
simultaneous attraction of the
because the electron pair is strongly
electrons by a large number of
attracted by two nuclei.
kernels
This is a directional bond.
This is a non-directional bond.
This bond makes substances hard
This bond make substances
and incompressible.
malleable and ductile.
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