# Atomic radius trend

> Why atoms shrink across a period and grow down a group


## Objectives

- Describe the trend: across a period the atomic radius decreases; down a group it increases.
- Explain it with the attraction of the nucleus (more protons, same level) and the number of main energy levels.
- Name the two opposite factors: the nucleus pulls electrons in; repulsion between electrons pushes them apart.

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




## 2. More electrons, bigger atom?

Predict: which atom is larger?

- Chlorine, because it has more electrons
- Same size: both use three main levels
- Sodium ✓

Sodium is the larger atom. Both atoms use three main levels, but chlorine has six more protons pulling the electrons of the same level closer, so its radius is smaller.

[ar-EG] الـ sodium هو الذرة الأكبر. الذرتين بيستخدموا تلات مستويات رئيسية، لكن الـ chlorine عنده 6 protons زيادة بيشدّوا electrons نفس المستوى لقرب، فالـ radius بتاعه أصغر.
## 3. Across a period: a stronger pull

Atomic radius Half the distance between the nuclei of two identical atoms bonded together. Typical atomic radii across period 3, in Å (1 Å = $10^{-10}$ m) From Na to Cl, each atom adds one proton and one electron. The new electron enters the same outer level (n = 3). More protons pull that level harder. So the radius decreases across the period.


## 4. Down a group: more levels

Move down group IA, from Li to Rb. Watch the occupied main energy levels. period occupied main levels radius of Li radius of Na radius of K radius of Rb Li: 2 main levels, radius 1.52 Å Na: 3 main levels, radius 1.86 Å K: 4 main levels, radius 2.27 Å Rb: 5 main levels, radius 2.48 Å Down a group Each period down adds one main energy level. The outer electron is farther from the nucleus, so the atom is larger.


## 5. Two opposite factors

Remember: like charges repel. All electrons are negative, so they push each other apart. Pulls in: the nucleus More protons attract the electrons more strongly. This decreases the radius. Pushes out: repulsion More electrons repel each other more. This increases the radius. Size is a balance of the two. Across period 3, the pull wins, so the radius decreases.


## 6. Your turn

Bromine (period 4) and chlorine (period 3) are both in group VIIA. Which is larger, and why?

- Bromine: it has more protons
- Bromine: its outer electrons are in an extra main level, farther from the nucleus ✓
- Chlorine: it has fewer protons, so a weaker pull
- Same size: same group

Br uses four main levels, Cl uses three. The extra level wins over the extra protons.

[ar-EG] الـ Br بيستخدم 4 مستويات رئيسية، والـ Cl بيستخدم 3. والمستوى الزيادة بيغلب الـ protons الزيادة.
## 7. Summary

Across a period, the radius decreases: more protons pull the same outer level harder. Down a group, the radius increases: each period adds one main energy level. Two opposite factors: the nucleus pulls electrons in; repulsion between electrons pushes them apart. Next in Chapter 1: from Sc to Cu, both factors grow together. You will see what that does to the radius. Can you explain why chlorine is smaller than sodium, though it has more electrons? From Na to Cl, each new electron enters the same outer level (n = 3), while the protons increase. The pull of the nucleus wins over the repulsion between electrons, so chlorine is smaller.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-pt.atomic-radius-trend