Metals have giant structures of atoms with strong metallic bonding. Overcoming this bonding takes a great deal of energy so this means that most metals have high melting and boiling points. Through this giant structure of metal ions moves the ‘sea of electrons’ which gives the metal the ability to conduct electricity. An electrical current is the movement of delocalised electrons through the lattice of ions. In the giant structures of metals, the layers of atoms are able to slide over each other. This means metals can be bent and shaped. They can be hammered flat (metals are malleable) and they can be pulled into a wire (metals are ductile) as the atoms can be moved over one another. (a)
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‘sea’ of electrons
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positive metal ions
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forces of attraction pull ions together
(b) Figure 2.37
Figure 2.38 Copper is (a) malleable and (b) ductile.
DID YOU KNOW?
The different sizes of atoms in an alloy distort the layers in the structure, making it more difficult for them to slide over each other, so alloys are harder than pure metals. 4
Explain, in terms of the packed atom structure, why copper metal can be extruded into wires.
5
Explain why copper can conduct electricity.
6
Sterling silver contains 92.5% silver, the remainder being copper. a Explain whether pure silver or sterling silver is more malleable. b Calculate the mass of copper in 1.0 g of sterling silver.
Smart alloys are alloys that can return to their original shape if heated to a different temperature. This is called a ‘shape memory’. Bendable glasses can be made from frames consisting of smart alloys. One smart alloy is ‘nitinol’. It is made from nickel and titanium.
Aluminium Aluminium is a very useful metal for use in aircraft as it has such a low density and (once it forms a layer of oxide) it is resistant to corrosion. However, it is too soft if it is not alloyed with other metals. The other metals used can be copper, magnesium, manganese and tin. For example, a aluminium and copper alloy is called Duralumin. 7
Explain, in terms of atoms, why alloys are harder and more useable than the main metal within them.
8
Explain why adding copper to aluminium makes an alloy which is harder than both the constituent metals.
9
Copper is a very good electrical conductor. Suggest why alloying copper decreases its conductivity.
Figure 2.39 Layers of alloy atoms, which are distorted layers due to the different sizes of the atoms.
REMEMBER! Think about the different sizes of atoms.
Google search: 'smart alloys or alloys'
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