Alloys

To compare how an alloy conducts electricity, start by thinking about the type of bonding present. Pure metals and alloys both have metallic bonding: a repeating lattice of positive metal ions with a shared “sea” of delocalized electrons moving through the structure.

Because electrical conduction relies on moving charged particles, these delocalized electrons are the main reason metals can conduct.

An alloy is made by mixing a metal with one or more other elements, but the overall metallic structure remains. The delocalized electrons are still there and still free to move, so an alloy is an electrical conductor even when it is solid.

Even though alloys conduct, their internal arrangement is not as perfectly regular as a pure metal. A pure metal has identical atoms arranged in a very consistent pattern, but an alloy includes atoms with different sizes.

Those size differences disturb the neat pattern, creating lattice distortion. This is why alloys are often harder, and it also changes how easily electrons can travel through the material.

When a voltage is applied, electrons drift through the metal. In a pure metal, the structure is more uniform, so the electrons have a more straightforward route. In an alloy, the distorted lattice provides more irregularities.

These irregularities cause more electron scattering, meaning the moving electrons collide more often with ions in the lattice. More collisions increase resistance, so alloys typically show lower electrical conductivity than the pure parent metal.

Now we can dismiss the incorrect options using the ideas above:

• Option A is wrong because alloys still contain delocalized electrons.
• Option B is wrong because metallic materials conduct when solid as well as when liquid.
• Option C is wrong because extra atom types usually disrupt electron flow rather than improving it.

Putting it all together: alloys are still conductors because they keep delocalized electrons, but the distorted lattice leads to extra scattering and higher resistance. Therefore, alloys usually conduct less well than the pure metal they are based on.

To choose the correct statement, first recall the definition of an alloy. An alloy is a mixture of two or more elements, with at least one being a metal. Since it is mostly metallic, it keeps metallic bonding: positive ions arranged in a lattice with a “sea” of delocalised electrons. This idea appears in Topic: Bonding.

Look at the first two choices:

• Option A says alloys are insulators. That cannot be right because delocalised electrons allow metals and alloys to carry charge when solid.
• Option B says conduction only happens when molten. That description fits ionic substances, not metallic ones. Alloys conduct as solids and also when melted.

Next, consider Option C. Although alloys do include more than one type of atom, those atoms are often different sizes. That unevenness distorts the lattice and creates more interruptions for moving electrons. The delocalised electrons are scattered more, so charge flow becomes harder, not easier.

With A, B, and C removed, the remaining option matches the science best. Alloys still have delocalised electrons so they conduct, but the less regular structure usually reduces conductivity compared with a pure parent metal.

The prompt is about interstitial alloys and specifically names steel. In the IB Bonding content, alloys are used to explain how mixing elements into a metal can change properties like strength and hardness because the metal lattice is altered, not because the bonding type becomes ionic.

In pure iron, the atoms are the same size and line up in a neat, repeating arrangement. When you push or bend the metal, these even layers of atoms can slide across each other fairly easily. That easy sliding is why pure iron is more malleable and not especially hard.

In steel, the carbon atoms are much smaller and sit in the interstices—the small spaces between iron atoms.

Because the carbon atoms do not match the iron atoms in size, they upset the regular pattern of the iron lattice and create a distorted structure. With the lattice no longer perfectly aligned, the iron layers cannot move past one another as smoothly, so the material becomes harder and stronger.

Here is why the other choices don’t explain the hardness of an interstitial alloy:

• Option A: Talking about more delocalised electrons is mainly connected to conductivity, not to stopping layers of atoms from moving.
• Option B: Steel keeps metallic bonding (with possible small covalent effects involving carbon), but it does not become an ionic lattice.
• Option D: The question is about steel being harder than iron; a “soft” coating would not explain increased hardness, and carbon is mixed through the metal rather than acting only on the surface.

The key idea is that hardness increases when the movement of metal layers is physically blocked. Carbon atoms sitting in the gaps act like obstacles, reducing layer slippage, so the correct option is the one describing carbon filling spaces and preventing sliding.

To explain why steel is harder than pure iron, focus on alloy structure. A pure metal has a regular lattice where layers can slide. When another element is added, the lattice becomes less able to slide smoothly. Using elimination helps identify which option describes that structural effect.

Option A claims carbon increases the number of delocalised electrons. Hardness in alloys is mainly due to changes in the lattice arrangement that prevent layers from moving, not because the electron sea becomes larger. Adding a non-metal like carbon doesn’t provide the best explanation for the mechanical hardening effect.

Option B suggests ionic bonding. Steel remains a metallic solid: iron atoms are held by metallic bonding (positive ions attracted to delocalised electrons). Carbon atoms fit into gaps in that metallic structure; they do not form an ionic lattice by electron transfer to or from iron.

Option D talks about a soft protective layer. That conflicts with the question’s focus on increased hardness, since a soft layer would not make the material harder. Also, in steel the carbon is spread throughout the iron lattice, not just forming a surface coating.

After eliminating A, B, and D, Option C remains. In an interstitial alloy, smaller carbon atoms occupy the holes between larger iron atoms, disrupting the neat lattice. This distortion prevents iron layers from sliding easily when force is applied, which is why steel is harder.

To answer this, we need the idea of passivation. Passivation happens when a metal develops a very thin oxide coating that protects the metal underneath. For the coating to actually slow or stop corrosion (as in stainless steel), it needs two key features:

1. Adherent: it grips the metal surface strongly instead of peeling away.
2. Non-porous: it forms a continuous seal so oxygen and water cannot easily reach the metal below.

Pure iron doesn’t “self-protect” well because:

• Iron oxide (rust) tends to be crumbly and full of gaps. Since it doesn’t stay tightly attached and has pores, air and moisture can still get through, so oxidation keeps happening underneath.
• Chromium oxide ($C{r}_2{O}_3$) on stainless steel behaves differently. It forms an extremely thin, hard-to-see passive film that is compact and strongly attached to the alloy’s surface, blocking further oxidation.

Option A describes chromium oxide as a liquid. However, oxide layers that form during corrosion or passivation are solid, ceramic-like coatings at room temperature. If it were a liquid, it would drip or spread away instead of staying as a stable protective film.

Options B and D don’t match how protection works:

• A porous layer (Option B) has holes, so gases like $O_2$ and water can still reach the metal surface. That would keep corrosion going, which is the opposite of protection.
• Chromium oxide does not “turn into” a new metal by reacting with iron (Option D). The oxide’s role is to sit on the surface as a barrier, not to create a new metallic material.

A true passivating film must stay attached and block pathways for oxygen and water. An adherent, non-porous chromium oxide layer does exactly this, preventing further corrosion. That description matches Option C.

This question focuses on how oxide layers behave on metals. We observe that pure iron continues to rust and break down, but stainless steel (with chromium) is much more resistant. Using the Process of Elimination, we can discard choices that conflict with basic chemistry or with the observed behavior.

Consider option A: it says the chromium oxide layer is a liquid.

At room temperature ($25^{\circ }C$), oxides such as $C{r}_2{O}_3$ are solid materials with very high melting points (above $2000^{\circ }C$). A liquid layer would not remain as a durable coating; it would flow away and expose the metal again. So this option cannot be correct.

Option B says the layer is porous and lets the metal “breathe.”

In chemical terms, letting the metal “breathe” means oxygen and water vapor can move through the layer. If the layer is porous, it contains tiny channels that allow $O_2$ and water to reach the metal underneath, which speeds up corrosion. This is exactly why rust fails as protection: it is porous and flakes off.

Option D suggests chromium oxide reacts with iron to form a “new metal.”

An oxide is a compound made from $metal+oxygen$, so it is not a metal itself. The surface film forms because chromium reacts with oxygen in air, creating a protective oxide layer, not because it produces a brand-new metallic substance. Therefore, this description does not fit how stainless steel is protected.

With the other options removed, Option C remains.

This matches the correct science:

1. Adherent: the film stays attached instead of flaking away.
2. Non-porous: it forms a continuous barrier that prevents oxygen and water from contacting the metal atoms below.

This protective surface film is called a passive layer, which explains stainless steel’s corrosion resistance.

To explain why metals can be shaped without snapping (malleable) and stretched into wires (ductile), we need to connect the property to the metal’s internal arrangement.

A metallic lattice is made of positive metal ions arranged in a repeating pattern, surrounded by a mobile “sea” of delocalized valence electrons. These electrons are not tied to one ion; they move through the whole structure and help hold it together.

In a pure metal, every atom is the same element. That means every cation in the lattice is the same size.

Because the ions match, they pack into tidy, regular layers. This idea is part of Bonding: Structure and bonding.

When you hit or bend a pure metal, those even layers can shift and slide past each other.

Metallic bonding is non-directional: the attraction is between the cations and the shared electron sea in all directions. So when layers move, the bonding doesn’t “snap” like a directional bond might. The ions simply end up in new positions while still being attracted to the delocalized electrons.

We can reject the other options by checking whether they match a pure metal lattice:

• Option B refers to particles of different sizes, which is typical of an alloy. Different-sized atoms disrupt smooth layers, making sliding harder, so alloys tend to be harder and less malleable.
• Option C is wrong because metallic bonding is non-directional, not directional.
• Option D is wrong because delocalized electrons are not stuck in one spot; they are free to move throughout the lattice.

The key reason is that in a pure metal the cations are the same size, forming regular layers that can slide smoothly under force while the delocalized electrons continue to hold the structure together.

The question is asking for the structural reason pure metals can be reshaped: malleable (hammered into sheets) and ductile (pulled into wires). The answer must come from how particles are arranged and bonded in a pure metal lattice, a core idea in Topic: Bonding (Structure and bonding).

Option B says there are different-sized ions. That fits an alloy (a mixture), not a pure element. In a pure metal, all atoms are the same element, so the cations are the same size. Mixed sizes actually block layers from sliding easily, which is why alloys are usually harder and less malleable than pure metals.

Option C suggests the bonding works in set directions. Directional bonding is typical of covalent bonds where electrons are shared between particular atoms. Metallic bonding is non-directional because the attraction is between the positive ions and the overall sea of delocalized electrons, allowing ions to move without breaking the bonding.

Option D claims the electrons are in fixed positions, but that conflicts with the meaning of delocalized. Delocalized electrons can move through the structure, so they cannot be described as locked in place.

With B, C, and D eliminated, Option A is left. In a pure metal, same-sized cations form regular layers, and because metallic bonding is non-directional, those layers can slide over one another under force. This explains malleability and ductility.