Which one is the most abundant of all the elements on Earth?

Correct — D, Oxygen. Oxygen makes up roughly 46 per cent of the Earth's crust by mass, close to half of it, because almost every rock-forming mineral is a silicate or an oxide in which oxygen is the anion holding the structure together. Silicon comes next at about 28 per cent and aluminium third at about 8 per cent, so the order of the top three — oxygen, silicon, aluminium — is worth committing to memory. Carbon, for all its importance to life, is a trace constituent of the crust at well under one per cent. Read strictly as the whole planet, core included, iron would edge ahead of oxygen by mass; iron is not on offer here, and the paper is using the crustal sense in which this fact is normally taught.

• (a)Silicon — Second, not first — about 28 per cent of the crust by mass. Silicon is the most abundant element after oxygen, and the two together account for nearly three quarters of the crust.
• (b)Aluminium — Third at roughly 8 per cent. Aluminium is the most abundant metal in the crust, which is the fact this option is baiting, but it is not the most abundant element.
• (c)Carbon — A trace element in the crust, far below a single per cent. Carbon's dominance in living matter and in fuels has nothing to do with its share of the Earth's rocks.

Crustal abundance is quoted by mass, and the standard figures are oxygen about 46 per cent, silicon about 28, aluminium about 8, iron about 5, then calcium, sodium, potassium and magnesium in the low single figures. Those eight account for close to 99 per cent of the crust. The reason oxygen leads is structural rather than chemical accident: the silicate tetrahedron that builds feldspars, quartz, micas and pyroxenes carries four oxygen atoms for each silicon.

Questions on abundance are only answerable if you know which reservoir is meant, and the four common answers differ. In the universe as a whole hydrogen dominates. In the Earth taken whole, iron leads on mass because the core is iron and nickel. In the crust, oxygen leads. In the atmosphere, nitrogen leads by volume. In the human body, oxygen leads by mass and hydrogen by number of atoms. This item offers no metal heavier than aluminium, which is the drafting clue that the crust is meant.

• Crustal abundance by mass: oxygen about 46 per cent, silicon about 28, aluminium about 8, iron about 5.
• Aluminium is the most abundant metal in the crust; iron is the most abundant element in the Earth taken as a whole.
• Silicate minerals build most of the crust, and each silicon in a silicate tetrahedron carries four oxygens.
• Hydrogen is the most abundant element in the universe; nitrogen the most abundant gas in the atmosphere by volume.
• Carbon is under a tenth of a per cent of the crust by mass.

• Answering aluminium because it is the most abundant metal.
• Applying whole-Earth abundance, where iron leads, to a question about the crust.
• Confusing abundance by mass with abundance by number of atoms.

Asked as a bare one-line recall item, so the examiner is testing whether the crustal order is memorised rather than reasoned.

• practice — not a real PYQ

  Which is the most abundant metal in the Earth's crust?

  • (a)Iron
  • (b)Aluminium
  • (c)Calcium
  • (d)Magnesium

  Answer(b) Aluminium — about 8 per cent of the crust by mass, ahead of iron at roughly 5 per cent.

• practice — not a real PYQ

  Which element is the most abundant in the universe?

  • (a)Helium
  • (b)Oxygen
  • (c)Hydrogen
  • (d)Carbon

  Answer(c) Hydrogen — it accounts for about three quarters of ordinary matter by mass, with helium second.