Is aluminum a metalloid?

On the New York State Chemistry state exam there was a question like this:

*Which of the following is a metalloid?

  1. Au
  2. Al
  3. As
  4. Ar*
    The answer is obviously 3 and all of my students got it right. But a few of my colleagues say the question is flawed because Al is also considered a metalloid.

Really? I’ve never heard anyone say that before. Aluminum does have a few interesting chemical properties (aluminum chloride is considered a covalent compound and aluminum hydroxide is amphoteric) but these are fairly obscure and in my mind do not negate all of aluminums other properties which are clearly metallic (luster, conductivity, malleability etc). This is all debateable, I suppose, so I did some digging.

Every authoritative source I can find up to and including the American Chemical Society (CAS Registry Number 7429-90-5) state that aluminum is a metal. NIST agrees. A claim was made that in Europe aluminium is considered a metalloid. According to the ESIS (EC# 231-072-3) aluminium is a metal.

So where is this coming from? Does anyone here remember being taught that aluminum is a metalloid?

A few people try to artificially impose patterns on the periodic table that aren’t really there, and to impose one particular pattern they need to call aluminum a metalloid. Which doesn’t change the fact that it’s ductile, malleable, lustrous, and conductive, and that it generally donates electrons in chemical reactions.

The metalloid series is a staircase on the periodic table, and sometimes color-coded. It would be perfectly 2 up-1 over, except for a glaring hole where alumin(i)um sits.

Thanks for the replies. I totally agree. (I’m going to bump this once hoping a few more people can add to this thread)

I taught college-level chemistry for five years, and I’ve never seen aluminum classified as a metalloid in any textbook or reference.

Down through Period 6 on the periodic table, the metalloids consist of: boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), polonium (Po), and astatine (At).

The only reason anyone would attempt to include aluminum on the list of metalloids would be that it would complete the pattern of metalloids straddling the stairstep line that separates the metals from the nonmetals. As Chronos states, however, this is imposing a pattern to the exclusion of actual observations.

Like other metals, aluminum is lustrous, conducts heat and electricity, malleable, and ductile. It would not make sense to classify it as a metalloid.

Never let data mar a beautiful theory. :wink: