Transcript Metallic Bonding

KS4 Chemistry
Metallic Bonding
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Metals in the periodic table
Metal are found at the left and centre of the periodic table.
H
He
Li Be
B C N O F Ne
Na Mg
Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg ? ? ? ? ? ? ?
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Metallic bonding
Atoms of metals are tightly packed together in a giant lattice
similar to the lattice in ionic compounds.
metal
atoms
ions
sea of free
electrons
The outer electrons separate from their atoms and become
delocalized, creating a ‘sea of electrons’. The atoms
become positive ions and are attracted to these electrons.
This attraction is called metallic bonding and is the reason
why the positive metal ions do not repel each other.
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Electrons and metallic bonding
How is the sea of electrons involved in metallic bonding?
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Properties of metals: density
Metal ions form a lattice which is more tightly packed and
denser than the lattices in ionic compounds. They form
crystals called grains.
Metals generally have a very high melting and boiling point
because metallic bonds are very strong and so a large
amount of energy is needed to break them.
Which metal has the lowest melting point?
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Properties of metals: conductivity
Metals are good conductors of:
 heat – the free electrons can take in heat energy, which
makes them move faster. They can then transfer the
energy throughout the lattice.
 electricity – the free electrons can carry an electrical
charge.
Silver is the best conductor of
electricity and copper is the
second best.
Why is copper used instead of
silver for electrical wires?
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Properties of metals: strength
Metals are usually tough, not brittle. When a metal is hit, the
layers of the lattice just slide over each other. The metallic
bonds do not break because the electrons are free to move.
force
This means that metals are:
 malleable – they can be bent and pressed into shape;
 ductile – they can be drawn out into wires.
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Metal grains
Layers of atoms can slip over each other in metal grains when
a force is applied, but this slippage stops at grain boundaries.
grain boundary: where
one grain meets another
The smaller the grains, the shorter the distance the atom
layers can move. This means that metals with smaller grains
are stronger and harder than metals with larger grains.
The faster that molten metal is cooled, the smaller the
grain size.
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Strengthening metals
Metals can be made stronger by adding another element
when the metal is molten. The atoms of the new element
spread through the crystal structure.
force
force
Because the atoms of the added element are larger, they
make it difficult for layers of metal atoms to slide. This
makes the metal less malleable and less ductile.
This process is called alloying, and the new substance is
called an alloy.
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Different types of alloys
Adding different elements to metals creates different alloys,
with different properties. This affects how alloys can be used.
Usually, other metals are added, for example:
 zinc, magnesium and copper are added to aluminium
to create an alloy that is light but very strong. This is used
in building aircraft.
 chromium and nickel are added to iron to make
stainless steel, which is resistant to corrosion.
 tin and lead are mixed together to create solder, which
has a low melting point and is used to join other metals
together.
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