5b. The metallic bonding model - giant structure -
giant lattice of metal ions
Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A
level US grade
9-12 level Revision Notes
Metals have giant structures of atoms
with strong metallic bonding leading to them having high melting and boiling
points. In pure metals, atoms are arranged in layers, which allows metals to
be bent and shaped without breaking the bonds. However, pure metals are too
soft for many uses and so are mixed with other metals to make alloys which
are harder. You should be able to explain why alloys are harder than pure
metals in terms of distortion of the layers of atoms in the structure of a
pure metal by adding other elements like carbon in steel alloys. Most metals
are good conductors of electricity because the delocalised electrons in the
metal carry electrical charge through the metal. Metals are also good
conductors of thermal energy because energy is transferred by the kinetic
energy of the delocalised electrons.
METALLIC BONDING isn't
quite like ionic or covalent bonding, although the metal atoms form positive
negative ion is formed from the same metal atoms, but the immobile positive metal
ions/atoms in the lattice are attracted together by the free moving negative electrons between
them. So, like ionic bonding, you do get
attraction between positive and negative particles and this is the metallic
The giant metallic lattice of metal ions
STRUCTURE - another 'giant' structure
- All metals have similar properties
BUT, there can be wide variations in melting point, boiling point,
density, electrical conductivity and physical strength.
explain the physical properties of metals like iron or sodium we need a more
sophisticated picture than a simple particle model of atoms all
lined up in close packed regular rows and layers, though this picture is
correctly described as another example of a giant lattice held
together by metallic bonding.
giant metallic lattice – the crystal lattice of metals consists of ions (NOT atoms)
surrounded by a 'sea of electrons' that form the giant lattice
(2D diagram above right).
- Some of the outer electrons (–) from the
original metal atoms are free to move around between the positive
metal ions formed (+) in the giant metallic structure.
- These 'free' or 'delocalised'
electrons from the outer shell of the metal atoms are the 'electronic glue' holding the particles together.
- There is a strong
electrical force of attraction between these free
electrons or 'sea' of delocalised electrons) (–) and the 'immobile' positive metal ions (+)
that form the structure of the giant metallic lattice
and this is the metallic bond. The attractive force acts in
- Metallic bonding is not
directional like covalent bonding, it is like ionic bonding in the
sense that the force of attraction between the positive metal ions
and the mobile electrons acts in every direction about the fixed
(immobile) metal ions of the metal crystal lattice, but in ionic
lattices none of the ions are mobile. a big difference between a
metal bond and an ionic bond.
In general, the
more electrons are delocalised to form the metal lattice of ions, the stronger the bond
- which obviously will have an effect on physical properties such as
melting point and thermal expansion.
It isn't a strict rule, but if
you compare sodium Na,
magnesium Mg and aluminium in groups 1 to 3 on period 3 of the
periodic table, you find the melting point increases.
As you go from left to right from
groups 1 to 3, the number of outer shell electrons increases,
so potentially donating 1 to 3 outer delocalised electrons to the bonding
between the ions and electrons in the giant metallic lattice of
The positive charge on the
average ion will also increase - so both the negative charge and
positive charge between the particles will increase .
This increases the bond strength
and more energy
(from a higher temperature) is needed to give the particles
sufficient kinetic energy to loosen the lattice bonds sufficiently
to melt the metal.
The weaker the metal bonding in
the giant lattice, the more the metal expands on heating per degree
rise in temperature - a higher thermal expansion coefficient.
Explaining the properties of metals using
the bonding model
Sub-index: Part 5
Metallic Bonding – structure and properties of metals
bonding and structure notes
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