Arrange the following major elements present in the Earth's crust, in ascending order, in weight percentage: 1. Aluminium 2. Calcium 3. Silicon 4. Oxygen Select the answer using the code given below:

Q: What is the answer to CDS (II) 2025 GK Q89?

C. Correct — C, 2, 1, 3, 4. Of the four elements listed, oxygen is by far the most abundant in the Earth's crust by weight, silicon comes second, aluminium third and calcium fourth. The standard modern figures are oxygen about 46.1 per cent, silicon about 28.2 per cent, aluminium about 8.23 per cent and calcium about 4.15 per cent, with iron sitting between aluminium and calcium at about 5.63 per cent; the older series printed in most Indian textbooks reads 46.6, 27.7, 8.1 and 3.6 per cent for the same four, with iron at 5.0. The decimals differ between the series and the order does not, which is all this question needs. Ascending order runs from least to most, so the sequence is calcium, aluminium, silicon, oxygen. Reading those off the paper's own numbering — 1 for aluminium, 2 for calcium, 3 for silicon, 4 for oxygen — gives 2, 1, 3, 4. The reason oxygen dominates a solid crust is worth holding: nearly all crustal rock is made of silicate minerals, in which silicon and aluminium sit at the centres of frameworks built out of oxygen atoms, and oxygen is both abundant and comparatively light in atomic terms, so it takes an enormous share by weight and a still larger one by volume. (Provisional — pending the official UPSC key.)

(a)3, 4, 1, 2
(b)4, 3, 1, 2
(c)2, 1, 3, 4
(d)1, 2, 3, 4

Answer

Why

Correct — C, 2, 1, 3, 4. Of the four elements listed, oxygen is by far the most abundant in the Earth's crust by weight, silicon comes second, aluminium third and calcium fourth. The standard modern figures are oxygen about 46.1 per cent, silicon about 28.2 per cent, aluminium about 8.23 per cent and calcium about 4.15 per cent, with iron sitting between aluminium and calcium at about 5.63 per cent; the older series printed in most Indian textbooks reads 46.6, 27.7, 8.1 and 3.6 per cent for the same four, with iron at 5.0. The decimals differ between the series and the order does not, which is all this question needs. Ascending order runs from least to most, so the sequence is calcium, aluminium, silicon, oxygen. Reading those off the paper's own numbering — 1 for aluminium, 2 for calcium, 3 for silicon, 4 for oxygen — gives 2, 1, 3, 4. The reason oxygen dominates a solid crust is worth holding: nearly all crustal rock is made of silicate minerals, in which silicon and aluminium sit at the centres of frameworks built out of oxygen atoms, and oxygen is both abundant and comparatively light in atomic terms, so it takes an enormous share by weight and a still larger one by volume.

Why the others are wrong

(a)3, 4, 1, 2 — Descending order with the first two swapped — silicon placed ahead of oxygen. It also runs the wrong way for a question that asks for ascending order.
(b)4, 3, 1, 2 — The exact descending order: oxygen, silicon, aluminium, calcium. Everything about it is correct except the direction the question asked for, which makes it the trap of the set.
(d)1, 2, 3, 4 — Simply the order the paper printed the four elements in. It corresponds to aluminium, calcium, silicon, oxygen, which puts aluminium below calcium and is not the abundance order in either direction.

Concept

The Earth's crust is a thin skin of silicate rock — about 5 to 10 kilometres thick under the oceans and 30 to 70 kilometres under the continents — and its bulk composition is dominated by eight elements. By weight these are oxygen at roughly 46 per cent, silicon at roughly 28 per cent, aluminium at about 8 per cent, iron at about 5 per cent, and then calcium, sodium, potassium and magnesium each at 2 to 4 per cent. Together they account for about 98 per cent of the crust, and everything else, including all the metals that are mined in quantity apart from aluminium and iron, shares the remaining 2 per cent. The chemistry behind the list is the silicate tetrahedron: one silicon atom surrounded by four oxygen atoms, linked into chains, sheets and frameworks, with aluminium frequently substituting for silicon and calcium, sodium, potassium and magnesium balancing the charge. Feldspar, the commonest mineral group in the crust, is built exactly that way.

Ordering questions are decided as much by reading the instruction as by knowing the facts, and this one offers the exact descending sequence as option (b) to punish a candidate who reconstructs the abundances correctly and then stops. Underline the word 'ascending' before touching the options. The abundance list itself is worth memorising in its top four because it recurs across geography, chemistry and geology papers, and a useful check is that it corresponds to the composition of ordinary granite: quartz is silicon dioxide, feldspar adds aluminium with potassium, sodium or calcium, and every one of those minerals is built on an oxygen framework. One caution about sources — figures for crustal abundance vary a little between compilations depending on whether the continental crust alone or the whole crust is meant, so quote the order with confidence and the decimals with care.

Key facts

The eight commonest elements in the crust by weight are oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium, together about 98 per cent of the whole.
A widely used modern series gives oxygen 46.1 per cent, silicon 28.2, aluminium 8.23, iron 5.63 and calcium 4.15 per cent.
The older figures printed in most Indian textbooks give oxygen 46.6, silicon 27.7, aluminium 8.1, iron 5.0 and calcium 3.6 per cent — different decimals, identical order.
Iron sits between aluminium and calcium in the sequence, which is why a question that skips iron can still be ordered correctly.
Oxygen dominates because crustal rock is almost entirely silicate, built on frameworks of silicon and aluminium bonded to oxygen.

Study next

Silicate minerals and the silicon-oxygen tetrahedron, and the mineral families built from itContinental and oceanic crust compared — thickness, density and compositionThe layered Earth: crust, mantle, outer core and inner core, and the discontinuities between themAbundance of elements in the whole Earth as against the crust, and why iron dominates the first list but not the secondOres and the metals worth mining — why aluminium is common in the crust yet expensive to obtain

Common traps

Answering with the descending order when the question asks for ascending. That exact reversal is offered as an option here.
Confusing the composition of the crust with the composition of the Earth as a whole, where iron is the leading element.
Assuming a metal familiar from everyday use must be abundant. Calcium and aluminium are both among the top five; copper, lead and zinc are all far below one-tenth of a per cent.

As an ordering or largest-share item on crustal composition, or as a single-fact item on which element is most abundant in the crust, the atmosphere or the human body.

Related PYQs

UPSC_1997_GS1_Q1041997Open & attempt →

Which one of the following is present in the largest amount in terms of per cent by mass in the earth’s crust?

(a) Silicon
(b) Oxygen
(c) Carbon
(d) Calcium

Answer(b) Oxygen

The same abundance list reduced to its first place, and offering silicon as the trap. Oxygen leads at about 46 per cent by mass; silicon follows at about 28 per cent, and calcium and carbon are far behind.

CDS_GK_2021_II_Q92021Open & attempt →

What is the approximate percentage of carbon in the Earth's crust?

(a) 0.045%
(b) 0.025%
(c) 0.015%
(d) 0.005%

Answer(b) 0.025%

The far end of the same table, and a useful sense of scale. Carbon sits at a few hundredths of a per cent against oxygen's forty-six, which is a reminder that familiarity in daily life says nothing about abundance in rock.

Practice

practice — not a real PYQ
Which one of the following is the second most abundant element by weight in the Earth's crust?
- (a)Aluminium
- (b)Silicon
- (c)Iron
- (d)Calcium
Answer(b) Silicon — at about 28 per cent, behind oxygen at about 46 per cent and ahead of aluminium at about 8 per cent.
practice — not a real PYQ
The dominance of oxygen in the Earth's crust by weight is best explained by which one of the following?
- (a)Oxygen in the atmosphere dissolves into surface rock
- (b)Almost all crustal rock is made of silicate minerals built on oxygen frameworks
- (c)Oxygen is the heaviest of the common crustal elements
- (d)Oxygen is concentrated in ocean water resting on the crust
Answer(b) Almost all crustal rock is made of silicate minerals built on oxygen frameworks — the silicon-oxygen tetrahedron is the building block of feldspar, quartz, mica and the rest.