Question

Decreasing order of elements as per their average composition in earth crust (% by weight) is:
A. Al³⁺
B. Fe²⁺
C. Si⁴⁺
D. Ca²⁺
E. Na⁺

• A. A, B, D, E, C
• B. C, A, B, D, E
• C. C, A, B, E, D
• D. A, C, B, D, E

Hint:

Mnemonic for Crustal Abundance: "OSIAL CA SOD POT MAG" (Oxygen, Silicon, Aluminium, Iron, Calcium, Sodium, Potassium, Magnesium). This covers over 99% of the crust!

Answer 1

The Correct Option is B

Concept: The Earth's crust is composed of several key chemical elements. In soil science and geology, understanding the relative abundance of these elements is vital for studying mineral weathering and nutrient availability.

Step 1: Identify the most abundant elements.
Oxygen is the most abundant (approx. 46.6%), followed by Silicon (C) at approx. 27.7%. Since Oxygen is not in the list, Silicon (C) is the highest among the choices.

Step 2: Evaluate the metal cations.
Next is Aluminium (A) at approx. 8.1%, followed by Iron (B) at approx. 5.0%.

Step 3: Distinguish between the remaining alkali and alkaline earth metals.
Calcium (D) makes up approx. 3.6%, while Sodium (E) is slightly lower at approx. 2.8%.

Step 4: Construct the decreasing sequence.
The correct order from most to least abundant is: Silicon (C) $\rightarrow$ Aluminium (A) $\rightarrow$ Iron (B) $\rightarrow$ Calcium (D) $\rightarrow$ Sodium (E). This gives the sequence C, A, B, D, E.