Steel is an alloy consisting of Fe with a small amount of C. Elemental Cr can be added to steel to m — States of Matter Chemistry Question

STEPS:

1. Understand the types of alloys and relate them to atomic radii: An alloy is a mixture of elements that behaves as a metal. The type of alloy formed is determined by the relative sizes of the component atoms:
* Substitutional Alloy: Forms when the solute atoms have an atomic radius comparable in size to the solvent metal atoms (typically within 15% of each other). In this alloy, the solute atoms directly replace (substitute for) solvent atoms in the regular crystal lattice points.
* Interstitial Alloy: Forms when the solute atoms are significantly smaller than the solvent metal atoms (usually less than 60% of the solvent's radius). In this alloy, the small solute atoms fit into the interstitial spaces (the "holes" or gaps) between the larger metal atoms.
2. Analyze the atomic radii of the components in the table:
* Iron (Fe): $125\text{ pm}$ (solvent metal lattice)
* Chromium (Cr): $127\text{ pm}$ (alloying element)
* Carbon (C): $77\text{ pm}$ (alloying element)
* Oxygen (O): $73\text{ pm}$ (surface reactant)
3. Determine the structural role of each component in the bulk interior:
* Since the atomic radius of Cr ($127\text{ pm}$) is nearly identical to that of Fe ($125\text{ pm}$), chromium must form a substitutional alloy with iron. Therefore, Cr atoms should replace Fe atoms at regular lattice sites.
* Since the atomic radius of C ($77\text{ pm}$) is much smaller than Fe, carbon must form an interstitial alloy with iron. Therefore, C atoms should reside in the small gaps between the metal atoms.
* Thus, the Interior section (right box) must show a lattice consisting primarily of Fe atoms, with a few Cr atoms substituted into the lattice sites, and small C atoms sitting in the interstitial spaces between them.
4. Determine the structural role of each component at the surface:
* The problem states that 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.
* Because oxygen (O, $73\text{ pm}$) reacts specifically with Cr at the surface, we should observe Cr atoms substituted at the surface boundary, and small oxygen atoms bonded to these surface Cr atoms on the exterior edge.
5. Select the correct diagram: Diagram B is the only representation that correctly depicts:
* Left Box (Surface): Small oxygen (O) atoms bonded specifically to the substituted chromium (Cr) atoms at the top boundary.
* Right Box (Interior): Chromium (Cr) atoms substituting for iron (Fe) in the lattice sites, with carbon (C) atoms sitting in the interstitial gaps.

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WHY_OTHERS_WRONG:

• Diagram A is incorrect: The surface view shows oxygen (O) atoms sitting on top of iron (Fe) atoms without any chromium (Cr) present, and the interior view shows a pure iron-carbon interstitial alloy with no chromium atoms participating in substitution. This fails to depict the role of chromium altogether.
• Diagram C is incorrect: This diagram shows chromium (Cr) atoms trying to occupy the interstitial spaces of the lattice. Because Cr ($127\text{ pm}$) has a comparable size to Fe ($125\text{ pm}$), it is physically too large to fit in the small interstitial gaps of the iron lattice.
• Diagram D is incorrect: This diagram shows oxygen (O) atoms penetrating into the interior of the metal lattice. Oxygen only reacts with the surface layer of chromium to form a protective barrier; it does not diffuse into the bulk metallic crystal lattice as an interstitial component of the steel alloy.