In the context of periodicity, a unit called picometre is used to measure the __________.

Correct — B. A picometre is 10⁻¹² m, and that is the scale of an atom. Atomic radii are tabulated in the tens to low hundreds of picometres, which is why periodicity tables print them in pm, or in angstroms where 1 Å = 100 pm.

Option (b) is also the only quantity offered that is a length at all. The others are a mass, a mass per unit volume and a pure number, and none of them can carry a picometre.

Why the others are wrong

• (a)
  molar mass — Molar mass is a mass per mole, quoted in g/mol. No length unit fits it.
• (c)
  atomic density — Density is mass per unit volume, so its unit carries a cubed length such as kg/m³, never a bare picometre.
• (d)
  spin quantum number — The spin quantum number takes the values +1/2 and −1/2. It is dimensionless and has no unit at all.

Periodicity is the repeating pattern of properties across a period and down a group, and atomic radius is the property most of that pattern is explained by.

Across a period the nuclear charge rises while electrons keep entering the same shell, so the pull tightens and the radius decreases. Down a group a new shell is added each time, so the radius increases.

Because atoms are around 10⁻¹⁰ m across, chemists work in picometres (10⁻¹² m) or angstroms (10⁻¹⁰ m) rather than in metres.

Key facts

• 1 picometre = 10⁻¹² metre, and 1 angstrom = 100 picometres.
• Atomic radius decreases across a period and increases down a group.
• Molar mass is measured in g/mol and the spin quantum number is dimensionless.

Common traps

• Reading picometre as a mass unit because the stem mentions atoms.
• Assuming atomic radius grows across a period because the atom is gaining electrons.

No directly related past PYQ was found.