## Aluminum

#### Learning Objective

• Describe the properties of aluminum.

#### Key Points

• Aluminum is a soft, lightweight, and malleable silvery metal that is not soluble in water.
• The vast majority of compounds feature aluminum in the oxidation state 3+, but compounds with +1 and +2 oxidation states are known.
• Aluminum has many known isotopes, whose mass numbers range from 21 to 42.
• Aluminum is the most widely used non-ferrous metal and is mostly alloyed, which improves its mechanical properties.

#### Terms

• aluminumA metallic chemical element (symbol Al) with an atomic number of 13.
• passivationRefers to a material becoming “passive,” that is, being less affected by environmental factors such as air or water.

## Physical Properties of Aluminum

Aluminum is:

• relatively soft
• durable
• lightweight
• ductile
• malleable
• appearance ranging from silvery to dull gray
• not soluble in water under normal circumstances
• nonmagnetic
• does not ignite easily
• capable of being a superconductor

## Chemical Properties

Aluminum is resistant to corrosion due to the phenomenon of passivation. A thin surface layer of aluminum oxide is formed when the metal is exposed to air. This oxide layer protects the aluminum beneath the surface from further oxidation. Like many other metals, aluminum can also be oxidized by water to produce hydrogen and heat:

$2Al\quad +\quad 3{ H }_{ 2 }O\quad \longrightarrow \quad { Al }_{ 2 }{ O }_{ 3 }+3{ H }_{ 2 }$

Although aluminum is extremely easily oxidized, it is possible to remove the oxide layer from a sample without it immediately reforming. The simplest and safest way is to connect a battery to the sample and perform electrolysis under either an inert atmosphere (like argon gas) or vacuum conditions.

The vast majority of aluminum compounds feature the metal in the oxidation state 3+. The coordination number of aluminum can vary , but generally Al3+ is tetra- or hexacoordinate. This means it will have 4 or 6 ligands.

## Aluminum Halides: Use as Lewis Acids

Aluminum is a very reactive metal that readily reacts to product trivalent compounds. Its halides (AlF3, AlCl3, AlBr3 and AlI3) are common examples. Trivalent aluminum is electron-deficient and therefore exceptionally useful as a Lewis acid, particularly in organic synthesis.

## Aluminum Hydrides and Organoaluminum Compounds

A variety of compounds of empirical formula AlR3 and AlR1.5Cl1.5 exist. These species usually feature tetrahedral Al centers. With large organic groups, triorganoaluminum exist as three-coordinate monomers, such as triisobutylaluminum.

The important aluminum hydride is lithium aluminum hydride (LiAlH4), which is used as a reducing agent in organic chemistry. It can be produced from lithium hydride and aluminum trichloride:

$4LiH\quad +\quad Al{ Cl }_{ 3 }\quad \longrightarrow \quad LiAl{ H }_{ 4 }\quad +\quad 3LiCl$

## General Use of Aluminum

Aluminum is the most widely used non-ferrous metal. Aluminum is almost always alloyed, which markedly improves its mechanical properties, especially when tempered. For example, the common aluminum foils and beverage cans are alloys of 92% to 99% aluminum. Some of the many uses for aluminum metal are in:

• Transportation as sheet, tube, castings, etc
• Packaging (cans, foil, etc. )
• Construction (windows, doors, siding, building wire, etc. )
• A wide range of household items, from cooking utensils to baseball bats, and watches
• Street lighting poles, sailing ship masts, walking poles, etc.
• Outer shells of consumer electronics, also cases for equipment (e.g. photographic equipment)
• Electrical transmission lines for power distribution
• Super purity aluminum, used in electronics and CDs
• Heat sinks for electronic appliances, such as transistors and CPUs
• Substrate material of metal-core copper clad laminates used in high brightness LED lighting
• Powdered aluminum used in paint and in pyrotechnics
• A variety of countries, including France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia, have issued coins struck in aluminum or aluminum-copper alloys