I'm confused by what element(s) can be identified to have these properties.

The following is text from my 2019-2020 Kaplan MCAT General Chemistry book under the metalloids section

Silicon (Si) has a metallic luster but is brittle and a poor conductor

So onto the MCAT Concept Check 2.2:

Identify a few elements that likely possess the following properties

  • Good conductivity but brittle

I flip to the answer key and it says:

Metalloids exhibit brittleness but good conductivity

So I'm confused how this makes sense because Si was the main example they provided in that passage, and it seems to contrast the two points they make.

So what is an actual example of good conductivity but brittle??

  • 1
    $\begingroup$ Graphite, an allotrope of carbon, is a decent conductor and brittle $\endgroup$
    – Waylander
    Sep 23 '19 at 18:37
  • $\begingroup$ Sorry what I meant specifically is what elements on a periodic table, not allotropes, isoforms, different configurations, etc $\endgroup$ Sep 23 '19 at 18:42
  • 2
    $\begingroup$ What is the cutoff value for "good conductivity"? The Wikipedia article on metalloids gives conductivity of arsenic as 3.9 × 10⁴ S•cm⁻¹ and the conductivity of antimony as 3.1 × 10⁴ S•cm⁻¹. This looks like it might make them qualify as "good conductors". $\endgroup$
    – njuffa
    Sep 23 '19 at 19:12
  • $\begingroup$ There's no contradiction there. Silicon, in and of itself, is not a good conductor, as the passage says. Silicon in semiconductors is doped with a number of elements to introduce "holes" for electrons. $\endgroup$
    – jonsca
    Sep 23 '19 at 20:13
  • $\begingroup$ @user3665690 Different allotropes of elements have different properties including conductivity so it makes no sense to ask for the properties of "an element". Graphite and diamond are not the same but are both carbon. $\endgroup$
    – matt_black
    Apr 30 '20 at 11:22

Whether a "metalloid" is both brittle and a good conductor depends on what you mean by "metalloid", which varies in literature, what is a "good" level of (electrical) conductivity, and even what exactly you mean by "brittle" versus "ductile". Silicon, for instance, is only a semiconductor, with much less electrical conductivity than metals, when pure.

Wikipedia identifies beryllium and magnesium as brittle metals, although the latter can be made ductile with minor alloying. Zinc is called "slightly brittle".


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