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Aluminium is along the dark line of the Periodic Table and it is $p$-block metal.

Is it metal or a metalloid? Why?

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  • $\begingroup$ Please visit this page, this page and this ‎one on how to make your future posts better.‎ $\endgroup$ – It's Over Jul 8 '15 at 18:34
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    $\begingroup$ A metal. And a highly conductive one at that. No doubt about it. $\endgroup$ – Oscar Lanzi Dec 1 '17 at 13:35
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Aluminium is usually considered to be a metal, as described in the Wikipedia article Metalloids: Aluminium:

Aluminium is ordinarily classified as a metal. It is lustrous, malleable and ductile, and has high electrical and thermal conductivity. Like most metals it has a close-packed crystalline structure, and forms a cation in aqueous solution.

The reason why, in a few cases, aluminium is referred to as a metalloid, from the article:

It has some properties that are unusual for a metal; taken together, these are sometimes used as a basis to classify aluminium as a metalloid. Its crystalline structure shows some evidence of directional bonding. Aluminium bonds covalently in most compounds. The oxide $\ce{Al2O3}$ is amphoteric, and a conditional glass-former. Aluminium can form anionic aluminates, such behaviour being considered nonmetallic in character

The line between metals and non-metals is somewhat arbitrary with

Elements to the lower left of the line generally display increasing metallic behaviour; elements to the upper right display increasing nonmetallic behaviour.

Boron, carbon and silicon (all on the other side of the boundary line) are hard and brittle and are semiconductors, which is a major reason why the majority of resources list Aluminium as a metal, largely due to the metallic properties listed in the first quote.

A related question is Metal Compounds that bond covalently

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  • $\begingroup$ How is there evidence of directional bonding in crystalline aluminum? Reference/link? $\endgroup$ – Oscar Lanzi Oct 30 '19 at 10:24
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Aluminium is a metal because it is good conductor of heat and electricity, it is hard, shiny and it is malleable and ductile. They have high boiling and melting points. They possess metallic lustre. They are solid state. They can produce a metallic clink or sonorous sound. It is used for making paint, wrapping chocolates, medicines, etc.. It is used for making domestic utensils and coins. It is also used for making electric wires, bodies of vehicles and aeroplanes. Nowadays it is used to make doors and windows and their frames.

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