# Density increases with atomic number

> More mass in about the same space


## Objectives

- Read the density column of the book's table (for illustration only).
- State that most of them have high density, which increases with atomic number.
- Explain why: the atomic mass increases while the atomic size stays relatively constant.

## 1. General properties: mass, radius and metallic property




## 2. Heavier cubes

Predict: why is the copper cube heavier?

- Copper atoms are heavier, but about the same size ✓
- Copper's cube has more empty space
- Copper atoms are smaller, so more fit in

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.

[ar-EG] ذرات الـ chromium والـ copper ليهم نفس الـ radius (1.17)، لكن الـ atomic mass بتاع الـ copper أكبر (63.5 قصاد 52). mass أكتر في نفس الـ volume تقريبًا بيدّي density أعلى: 8.92 g/cm³ قصاد 7.19 g/cm³.
## 3. The density column

Density from Sc to Cu (book's table, p.12, for illustration only). Aluminum, for comparison: 2.7 g/cm³. Book point 3c: most of them have **high density**, which **increases with the atomic number**.


## 4. Mass up, size steady

Cr to Cu, from the book's table, p.12 (for illustration only) 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) $\ce{Cr}$: same box, atoms of radius 1.17 atomic mass density $\ce{Mn}$: same box, atoms of radius 1.17 atomic mass density $\ce{Fe}$: same box, atoms of radius 1.16 atomic mass density $\ce{Co}$: same box, atoms of radius 1.16 atomic mass density $\ce{Ni}$: the mass dips slightly (58.7), but the radius is a little smaller (1.15), so the density still rises atomic mass density $\ce{Cu}$: same box, atoms of radius 1.17 atomic mass density


## 6. Smaller atoms?

Atomic radius from Cr to Cu Book's table, p.12 (for illustration only): 1.17 at both ends, while the density rises from 7.19 to 8.92 g/cm³. Remember the radius lesson: two opposite factors keep the size relatively constant.


## 7. New metals

Metals X and Y have atoms of the same radius, packed the same way. Y's atomic mass is 20% larger. Which is denser?

- Equal: the atoms are the same size
- Y ✓
- X

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.

[ar-EG] نفس الـ radius ونفس الترتيب يعني نفس الـ volume لكل ذرة. الـ atomic mass بتاع الـ Y أكبر بـ 20%، فالـ Y بيحط mass أكتر في نفس الـ volume، فالـ density بتاعته أعلى.
## 8. Build the exam answer

Give reasons: the density increases with atomic number. Put the answer in order.


## 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. Book point 3c, p.13 Give reasons: the density increases with atomic number Because the **atomic size is relatively constant** with **increasing atomic mass**. 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?



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-te.density-trend