Which one of the following is a metalloid?

Correct — A, Germanium. Metalloids are the handful of elements that lie along the stepped diagonal separating metals from non-metals in the periodic table — boron, silicon, germanium, arsenic, antimony and tellurium — and they are called that because they behave partly like each side. Germanium is the clearest case in the list. It has a grey-white metallic sheen and looks like a metal, but it is brittle and shatters rather than bending, and its electrical behaviour is the giveaway: its conductivity rises as it is heated, whereas a true metal conducts less well when hot. That is semiconductor behaviour, and it made germanium the material of the first transistor at Bell Labs in 1947, before silicon displaced it. Its place in the table is itself a piece of history — Mendeleev left a gap for an element he called eka-silicon and predicted its properties, and Clemens Winkler found it in 1886 almost exactly as described. The other three are not metalloids for three quite different reasons, and separating those reasons is the real work of the question. Coke is not even an element in the sense the question intends; iodine is a non-metal that merely looks metallic; sugar is a compound.

• (b)Coke — Not an element as such but a material — the porous carbon left when coal is heated out of contact with air. Carbon is a non-metal, and graphite conducting electricity is what makes people hesitate here.
• (c)Iodine — A non-metal, and the most attractive wrong answer because it is genuinely lustrous — the shiny grey-violet crystals look metallic. UPSC has keyed exactly that point before, on a CDS paper asking which element is a non-metal but lustrous.
• (d)Sugar — A compound, sucrose, not an element at all. The metal, non-metal and metalloid scheme classifies elements, so sugar cannot be placed anywhere in it.

The periodic table is crossed by a stepped line running from boron down towards astatine. Metals lie to its left — lustrous, malleable, ductile, good conductors whose conductivity falls as temperature rises, and formers of basic oxides. Non-metals lie to its right — mostly dull and brittle when solid, poor conductors, formers of acidic oxides. The elements sitting on the line itself are the metalloids: boron, silicon, germanium, arsenic, antimony and tellurium. They look metallic, break like non-metals, and conduct in the intermediate way that defines a semiconductor, which is why almost all of electronics is built on two of them.

This question is unusually well built, because each wrong option fails a different test and a candidate who sees that pattern answers in seconds. Ask first whether the option is an element at all: sugar is not, and coke is a form of one rather than the element as such. Then ask, of what remains, whether it sits on the diagonal: iodine does not, it is a halogen well over on the non-metal side. Germanium is the only survivor. The lustre of iodine is worth carrying away as a separate fact, because lustre alone never settles whether something is a metal — graphite is lustrous and conducts, and it is a non-metal too.

• The metalloids are boron, silicon, germanium, arsenic, antimony and tellurium, lying along the stepped line between metals and non-metals.
• Germanium has atomic number 32; Mendeleev predicted it as eka-silicon and Clemens Winkler discovered it in 1886.
• A semiconductor conducts better as its temperature rises; a metal conducts worse.
• The first transistor, built at Bell Labs in 1947, used germanium, and silicon later replaced it in mass production.
• Coke is the porous carbon residue produced by heating coal in the absence of air.
• Iodine is a non-metal with a distinct metallic lustre — appearance alone does not decide the classification.

• Choosing iodine because it is lustrous; lustre does not make an element a metal or a metalloid.
• Treating coke as an element in its own right rather than as a form of carbon.
• Forgetting that a compound such as sugar cannot be classified in a scheme meant for elements.

As a which-is-a-metalloid item, as its mirror image — which element is a non-metal but lustrous — or through semiconductors, where germanium and silicon arrive from the physics side.

• practice — not a real PYQ

  Which one of the following sets contains only metalloids?

  • (a)Boron, silicon, arsenic
  • (b)Sodium, magnesium, aluminium
  • (c)Chlorine, bromine, iodine
  • (d)Copper, silver, gold

  Answer(a) Boron, silicon, arsenic — all three lie on the stepped line between metals and non-metals.

• practice — not a real PYQ

  As temperature rises, the electrical conductivity of a semiconductor such as germanium

  • (a)increases
  • (b)decreases
  • (c)stays unchanged
  • (d)falls to zero

  Answer(a) increases — the opposite of a metal, whose conductivity falls as it gets hotter.