# Why chromium resists the effect of atmosphere

> Active metal, protected surface


## Objectives

- Give reasons: chromium is chemically active, yet it resists the effect of atmospheric air.
- Explain that the oxide's molecular volume is larger than that of the metal atoms, so it forms a non-porous layer.
- Contrast the non-porous chromium oxide layer with porous rust on iron.

## 1. Economic importance of the first transition series




## 2. Shiny for years

Predict: why doesn't chromium keep reacting with air?

- Chromium does not meet oxygen in air
- Chromium is an unreactive metal, like gold
- A layer forms on its surface and protects it ✓

Chromium is chemically active, so it does react with oxygen of air. The oxide that forms makes a non-porous layer on the surface, and this layer prevents the continued interaction with oxygen.

[ar-EG] الـ chromium نشيط كيميائيًا، فهو فعلًا بيتفاعل مع الـ oxygen اللي في الهوا. الـ oxide اللي بيتكوّن بيعمل طبقة non-porous على السطح، والطبقة دي بتمنع إن التفاعل مع الـ oxygen يكمّل.
## 3. Active, so it reacts first

Remember: an active metal reacts readily, for example with oxygen. Chromium does react with the oxygen of air. A **metal oxide** forms on its surface. Chromium reacts with oxygen of air at its surface So the protection is not a lack of activity. It comes from this oxide.


## 4. Watch the layer form

Move the slider to follow the surface, stage by stage. Stage Stage 1: oxygen meets the surface Stage 2: the oxide forms, larger than the metal atoms Stage 3: a continuous non-porous layer The **molecular volume of the oxide is larger** than that of the metal atoms. So it forms a **non-porous layer**. This layer prevents the continued interaction with oxygen of air.


## 5. Not every oxide protects

Left: chromium oxide blocks oxygen. Right: rust lets air and moisture in. Any oxide layer protects a metal. This is why chromium is useful in metal plating.


## 6. Give the reason

Aluminum is above iron in the activity series. Yet aluminum window frames do not corrode away. Which is the best explanation?

- Aluminum is less active than iron
- Any oxide on a metal stops further corrosion
- A non-porous oxide layer forms on its surface and blocks oxygen ✓

Aluminum is more active than iron, yet its window frames do not corrode away. As with chromium, an active metal is protected when its oxide forms a non-porous layer that blocks oxygen.

[ar-EG] الـ aluminum أنشط من الـ iron، ومع ذلك الشبابيك اللي معمولة منه مابتتآكلش. زي الـ chromium بالظبط: الـ metal النشيط بيتحمي لما الـ oxide بتاعه يعمل طبقة non-porous تمنع الـ oxygen.
## 7. Build the chain

Put the steps of the explanation in order. The molecular volume of the oxide is ___ than that of the metal atoms, so it forms a ___ layer.


## 8. Summary

Back to the taps: chromium reacts, but its oxide seals it. Chromium is a chemically active metal. Its oxide's molecular volume is larger than that of the metal atoms: a non-porous layer forms. Rust on iron is porous, so it does not protect the iron. Two active metals, two oxide layers Give reasons: Cr resists air although it is active. Key words: **molecular volume of the oxide is larger than that of the metal atoms**. **Non-porous layer**. **Prevents continued interaction with oxygen**. Can you explain why rust does not protect iron, but chromium oxide protects chromium?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-te.chromium-oxide-layer