NDA (I) 2017 General Ability Test · Question 125 of 149

Correct — A, an inert gas. Radon is element number 86 and sits at the foot of group 18, the noble or inert gases, below helium, neon, argon, krypton and xenon. Like the rest of that family it has a complete outer electron shell, so it has almost no tendency to gain, lose or share electrons and forms compounds only under extreme laboratory conditions. Two further facts distinguish it from its neighbours. It is radioactive, being produced continuously by the decay of radium in the uranium series, and it is the densest of the common gases, which is why it collects in basements, caves and poorly ventilated mines and is treated as a health hazard there.

• (b)an artificial fibre — Artificial fibres are synthetic polymers such as nylon, rayon, polyester and acrylic. Radon is a chemical element, not a manufactured material, and no fibre is made from it.
• (c)an explosive — Explosives such as TNT, nitroglycerine and RDX are compounds that decompose rapidly to release large volumes of gas. Radon is chemically about as unreactive as an element can be, so it is close to the opposite of an explosive.
• (d)a metal — Radon is a gas at ordinary temperatures and a member of group 18, the non-metals with full valence shells. Its heavy atomic mass makes it dense for a gas, but density is not what makes a substance a metal.

Group 18 of the periodic table holds helium, neon, argon, krypton, xenon and radon. All have complete outermost shells, which makes them monatomic, colourless, odourless and almost entirely unreactive — the property that gives them their older name of inert gases, softened to noble gases once compounds of xenon and krypton were prepared. Argon fills ordinary electric lamps, neon glows in display tubes, helium lifts balloons and is mixed with oxygen for deep-sea divers, and radon, being both heavy and radioactive, is studied mainly as an environmental hazard rather than for any everyday use.

The stem is printed as the two-word fragment 'Radon is', with the four lower-case options completing the sentence, and that is how the booklet sets it. The identification question behind it is easy once you place the name in the periodic table, and the ending itself is a hint — helium, neon, argon, krypton, xenon and radon all close the same way. The three distractors come from three different categories altogether, a fibre, an explosive and a metal, which is the sign of a question testing plain recognition rather than any fine distinction. Carry the radioactivity as the extra fact, because that is where radon differs from the rest of its group and where later questions will go.

• Radon has atomic number 86 and belongs to group 18, the noble or inert gases.
• It is radioactive and is formed in the decay chain of uranium, immediately from radium.
• The commonest isotope, radon-222, has a half-life of about 3.8 days.
• Radon is the densest of the common gases and accumulates in basements, caves and mines, where it is a recognised lung-cancer risk.

• Assuming every heavy element must be a metal.
• Treating all noble gases as absolutely inert; xenon and krypton do form compounds.
• Confusing radon with radium — one is a gas in group 18, the other a metal in group 2.

NDA asks you to identify an element by its family, or matches noble gases with their uses.

• practice — not a real PYQ

  Radon is produced in nature mainly by the radioactive decay of

  • (a)radium
  • (b)sodium
  • (c)carbon
  • (d)potassium

  Answer(a) radium — radon appears in the uranium decay series as the gaseous product of radium's decay.

• practice — not a real PYQ

  Which one of the following gases is used along with oxygen in the cylinders carried by deep-sea divers?

  • (a)Chlorine
  • (b)Helium
  • (c)Ozone
  • (d)Radon

  Answer(b) Helium — it is far less soluble in blood than nitrogen, so a helium-oxygen mixture reduces the risk of decompression sickness.