42. Steel is an alloy consisting of Fe with a small amount of C. Elemental Cr ca

Question

42. Steel is an alloy consisting of Fe with a small amount of C. Elemental Cr can be added to steel to make the steel less likely to rust: Cr atoms react with oxygen in the air to form a nonreactive layer of chromium oxide on the surface of the steel, preventing the oxidation of underlying Fe atoms. A sample of steel-chromium alloy contains 15 percent Cr by mass. Which of the following diagrams best shows a particle-level view of a surface section and an interior section of the alloy represented below at the left? (The atomic radii of the atoms involved are given in the table below at the right.) Molar Mass Atomic Radius Element 55.85 52.00 12.01 16.00 125 127 Cr Surface Interior 73

Explanation / Answer

Answer: Option A, C is rejected because there is no chromium atom neither in surface nor in the interior side.

Option D is rejected due to the presence of oxygen atom in the interior side of alloy and thus will cause rusting which is not possible since oxygen atoms will be just present at surface forming an oxide layer with chromium and prevent rusting.

Hence Correct answer is Option B because of the presence of oxygen atom just on the surface and proper distribution of chromium atom inside the alloy i.e (15% by mass) approximately 3 chromium atom in 15 Fe and 3to 4 Carbon atoms.

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