Question

Indicate whether each statement is true or false: $$ \begin{array}{l}{\text { (a) Substitutional alloys tend to be more ductile than inter- }} \\ {\text { stitial alloys. }} \\ {\text { (b) Interstitial alloys tend to form between elements with }} \\ {\text { similar ionic radii. }} \\ {\text { (c) Nonmetallic elements are never found in alloys. }}\end{array} $$

Short answer

(a) True: Substitutional alloys are generally more ductile than interstitial alloys, as substitution does not severely disrupt the lattice structure. (b) False: Interstitial alloys usually form between elements with different ionic radii, as smaller atoms need to occupy interstitial spaces in the crystal lattice. (c) False: Non-metallic elements can be found in alloys, typically as part of interstitial alloys, such as carbon in carbon steel or nitrogen in stainless steel.

Step-by-step solution

Statement (a) - Ductility of substitutional and interstitial alloys

In substitutional alloys, atoms of the solute element replace atoms of the solvent element in the lattice, forming a solid solution. On the other hand, interstitial alloys are formed when smaller atoms (usually non-metallic) occupy the interstitial spaces in the crystal lattice of the main element. In general, substitutional alloys have a greater degree of ductility because substitution does not disrupt the lattice structure as severely as interstitial atoms. Therefore, this statement is true.

Statement (b) - Formation of interstitial alloys

Interstitial alloys usually form between elements with different ionic radii. The reason is that one element needs to have small enough atoms to occupy the interstitial spaces in the crystal lattice of the other element. If the elements have similar ionic radii, substitutional alloy is more likely to form than interstitial alloy. Therefore, this statement is false.

Statement (c) - Non-metallic elements in alloys

Non-metallic elements can indeed be found in alloys, typically as part of interstitial alloys. For example, carbon atoms can occupy interstitial spaces in the lattice of iron, forming carbon steel. Another example is nitrogen in stainless steel, which occupies interstitial positions and helps improve the strength and corrosion resistance of the material. Therefore, this statement is false. In conclusion: 1. Statement (a) is true. 2. Statement (b) is false. 3. Statement (c) is false.

Key concepts

Substitutional Alloys

Substitutional alloys form when atoms of the solute replace atoms of the solvent in the crystal lattice. This type of alloy typically involves elements with similar atomic sizes. Picture a grid where some of the original balls are replaced with balls of similar size – it's a pretty smooth transformation, right? That's exactly what happens at the atomic level in substitutional alloys, which often results in materials that retain much of the ductility of the pure metal. Think of brass, where copper atoms are substituted with zinc atoms; the result is a malleable material that's easy to work with.

Why is this important? Well, if you’re trying to bend a metal to shape it, you’d want it to be ductile. That's why substitutional alloys are often favored in applications requiring malleability. For example, since gold is very soft, it's mixed with other metals to create substitutional alloys that are hard enough to shape into jewelry that won't easily get bent out of shape.

Interstitial Alloys

Imagine you're filling a box with large balls and there’s some space left – that's where the smaller balls fit in. That's the essence of interstitial alloys: smaller atoms fitting into the gaps ('interstices') between larger metal atoms. These gaps exist in the lattice structure of metals where smaller non-metal atoms, like carbon or nitrogen, can sneak in. A classic example? Steel. Carbon atoms slip into the interstitial spaces in the iron lattice, significantly increasing its strength and hardness.

However, because those tiny atoms can mess with the metal’s crystal structure, interstitial alloys generally have decreased ductility compared to substitutional alloys. These alloys are less malleable but gain other valuable properties like increased strength or resistance to corrosion. So in the case of steel, even though it’s not as flexible as pure iron, its added strength makes it an indispensable material in construction and manufacturing. These differing atomic arrangements and the resulting material properties are key in choosing the right alloy for a job.

Ductility of Alloys

Ductility is the ability of a material to be stretched or formed without breaking. Essentially, it’s like Play-Doh; the more ductile it is, the more you can play with its shape. Now, an important distinction to remember: substitutional alloys tend to be quite ductile. They can often be bent, stretched, or hammered into thin sheets without breaking because their uniform atomic structure allows layers to slide over each other smoothly.

On the flip side, interstitial alloying messes with this smoothness because when smaller atoms fill the gaps in the metal's lattice, they create distortions that make it harder for layers to move past each other. It's kind of like trying to slide sheets with pebbles sandwiched between them – it's not going to be as easy, and this is why interstitial alloys are usually less ductile. But remember, less ductility doesn't mean a material is inferior; it’s just more suited to applications where toughness trumps flexibility, such as in high-strength steel for bridges and skyscrapers. The secret to successful use and design with metals involves understanding these differences in ductility and selecting the right alloy for the job.