Which one of the following is not a monatomic element?

Correct — C, iodine. Iodine is a non-metal that exists as diatomic molecules (I2), not as single atoms, so it is not monatomic. Copper and barium are metals whose atoms sit individually in a metallic lattice, and helium is a noble gas that occurs naturally as single atoms — only iodine here is molecular (diatomic).

Why the others are wrong

• (a)
  Copper — Copper is a metal; its atoms exist individually in a metallic lattice, so it is treated as monatomic, not molecular.
• (b)
  Helium — Helium is a noble gas and occurs as separate single atoms — it is monatomic.
• (d)
  Barium — Barium is an alkaline-earth metal whose atoms occur singly in a metallic lattice, so it is monatomic, not molecular.

Atomicity is the number of atoms that make up one molecule of an element. Noble gases and metals are treated as monatomic (single atoms), whereas several non-metals exist as diatomic molecules — the halogens (fluorine, chlorine, bromine, iodine) and hydrogen, nitrogen and oxygen. Iodine therefore exists as I2.

Key facts

• Iodine exists as diatomic molecules, I2 — so it is not monatomic.
• Noble gases (helium, neon, argon and so on) exist as single atoms — they are monatomic.
• Metals such as copper and barium are treated as monatomic (atoms in a metallic lattice).
• Common diatomic elements include H2, N2, O2, F2, Cl2, Br2 and I2.

Common traps

• Assuming all elements are monatomic — many non-metals occur as diatomic molecules.
• Thinking a noble gas such as helium is diatomic; noble gases are monatomic.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following exists as diatomic molecules?

  • (a)Helium
  • (b)Argon
  • (c)Chlorine
  • (d)Copper
  Answer(c) Chlorine — it exists as Cl2 molecules.
• practice — not a real PYQ

  Noble gases such as helium and neon are

  • (a)diatomic
  • (b)monatomic
  • (c)triatomic
  • (d)polyatomic
  Answer(b) monatomic — they occur as single, unbonded atoms.