Density increases with atomic number
More mass in about the same space
1. General properties: mass, radius and metallic property
2. Heavier cubes
Chromium and copper atoms have the same radius (1.17 in the book's table). Yet a 1 cm³ cube of chromium has mass 7.19 g, and the same cube of copper: 8.92 g.
Pick the right answer
Predict: why is the copper cube heavier?
- A. Copper atoms are smaller, so more fit in
- B. Copper atoms are heavier, but about the same size
- C. Copper's cube has more empty space
الشرح
Chromium and copper atoms have the same radius (1.17), but copper's atomic mass is larger (63.5 against 52). More mass in about the same volume gives the higher density: 8.92 g/cm³ against 7.19 g/cm³.
3. The density column
| Element (Sc → Cu) | Density (g/cm³) |
|---|---|
| Sc | 3.1 |
| Ti | 4.42 |
| V | 6.07 |
| Cr | 7.19 |
| Mn | 7.21 |
| Fe | 7.87 |
| Co | 8.7 |
| Ni | 8.9 |
| Cu | 8.92 |
Book point 3c: most of them have high density, which increases with the atomic number.
4. Mass up, size steady
| Element | Atomic mass | Atomic radius | Density (g/cm³) |
|---|---|---|---|
| Cr | 52 | 1.17 | 7.19 |
| Mn | 54.9 | 1.17 | 7.21 |
| Fe | 55.9 | 1.16 | 7.87 |
| Co | 58.9 | 1.16 | 8.70 |
| Ni | 58.7 | 1.15 | 8.90 |
| Cu | 63.5 | 1.17 | 8.92 |
From Cr to Cu, the atomic radius hardly changes, but the atomic mass rises from 52 to 63.5.
More mass in about the same volume: the density increases.
5. Same box, heavier atoms
Step from Cr to Cu. Same-size atoms, rising mass (shown by the shade).
element (1 = Cr … 6 = Cu) 1
atomic mass 52
density 7.19 g/cm³
6. Smaller atoms?
Atomic radius from Cr to Cu
| Element (Cr → Cu) | Atomic radius |
|---|---|
| Cr | 1.17 |
| Mn | 1.17 |
| Fe | 1.16 |
| Co | 1.16 |
| Ni | 1.15 |
| Cu | 1.17 |
Remember the radius lesson: two opposite factors keep the size relatively constant.
7. New metals
Pick the right answer
Metals X and Y have atoms of the same radius, packed the same way. Y's atomic mass is 20% larger. Which is denser?
- A. Y
- B. X
- C. Equal: the atoms are the same size
الشرح
Same radius and same packing mean the same volume per atom. Y's atomic mass is 20% larger, so Y packs more mass into the same volume and is denser.
True or false?
From Cr to Cu, the density rises mainly because the atoms get much smaller.
الشرح
From Cr to Cu the atomic radius hardly changes (1.17 at both ends), yet the density still rises, so shrinking atoms cannot be the reason. Book point 3c: the atomic size is relatively constant while the atomic mass increases.
8. Build the exam answer
Put in order
Give reasons: the density increases with atomic number. Put the answer in order.
- The atomic number increases across the series
- So the atomic mass increases
- But the atomic size stays relatively constant, so the volume hardly changes
- So more mass is packed into about the same volume, and the density increases
9. Summary
- Most first-series transition elements have high density.
- Density increases with atomic number: Sc 3.10 → Cu 8.92 g/cm³ (book's table).
- Reason: the atomic mass increases while the atomic size is relatively constant.
- Atomic number increases → Atomic mass increases
- Atomic mass increases → Atomic size relatively constant (but)
- Atomic size relatively constant → More mass in about the same volume
- More mass in about the same volume → Density increases
Back to the hook: why is a copper cube heavier than a chromium cube of the same size, when their atoms have the same radius?