1.7 - Metallic Bonding and Alloys

What is a metallic bond?

the electrostatic attration between the positive ions and the delocalised electrons

How are the positive metal ions arranged?

in a regular layered pattern

What are the positive metal ions surrounded by?

delocalised electrons

How does metallic bonding work?

• metal atoms lose outer shell electrons to form positive ions

• the outer electrons separate from the atoms and create a sea of delocalised electrons

• the positive metal ions and are attracted to the delocalised electrons

• there is an electrostatic attraction between is called a metallic bond

How is metallic structure formed?

the positive metal ions are in tightly-packed layers, which form a giant metallic lattice structure with a regular positive-negative arrangement

What are the properties of metallic structures?

• good conductors of heat and electricity

• high melting and boiling points

• malleable

• ductile

• strong

How do metals conduct heat and electricity

• when a metal is heated, the delocalised electrons gain kinetic energy

• these electrons then move faster and so transfer the gained energy throughout the metal

• delocalised electrons also conduct electricity through metals in a similar way

• delocalised electrons carry electrical current and thermal energy through the whole structure

Why do metals have high melting and boiling points?

metals are in a giant lattice with many strong metallic bonds (electrostatic attraction) between positive metal ions and delocalised electrons

What is the melting point of gold?

1064 C

What is the boiling point of gold?

2807 C

Why are metals malleable/ductile?

• positive ions are arranged in neat layers, which easily slide over each other

• meaning metals can be easily shaped (malleable) or pulled into wires (ductile)

Why are metals strong?

• When a metal is hit, the layers of metal ions are able to slide over each other, and so the structure does not shatter

• The metallic bonds do not break because the delocalised electrons are free to move throughout the structure.

What are alloys?

alloys are mixture of a metal with at least one other element (this can be another metal or a non metal)

Why are alloys harder than pure metals?

• by mixing two or more metals together, the regular arrangement of the atoms is disrupted (alloy formed)

• this larger/smaller element prevents the original metal ions forming layers and the ions cannot slide past each other any longer

Why are alloys used?

pure metals don’t always have the properties needed

What are some examples of alloys?

• steel

• magnalium

• nitinol

How are steels made?

by adding small amounts of carbon to the pure iron

What is a benefit of steel?

• iron will rust fairly quickly, but steel is much less likely to rust

• steel is a lot stronger than iron

What is magnalium?

an aluminnum alloy containing 5% magnesium

What are the benefits of magnalium?

is stronger, lighter and more resistant to corrorosion than aluminum

What are aluminum alloys used for?

aircrafts, car parts, soda cans and aluminum foil

What is a shape memory alloy?

an alloy that "remembers" its original, cold-forged shape: returning to the pre-deformed shape when heated

What is nitinol?

a shape memory alloy of nickel and titanium

What are the uses of shape memory alloys?

• glasses frames

• triggers to start the sprinklers in fire alarm system

• controllers for hot water valves in showers or coffee machines