Atomic radius trend
Why atoms shrink across a period and grow down a group
1. General properties: mass, radius and metallic property
2. More electrons, bigger atom?
Sodium (Z = 11) and chlorine (Z = 17) are both in period 3. Chlorine has six more electrons.
Pick the right answer
Predict: which atom is larger?
- A. Sodium
- B. Chlorine, because it has more electrons
- C. Same size: both use three main levels
الشرح
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.
3. Across a period: a stronger pull
Atomic radius
Half the distance between the nuclei of two identical atoms bonded together.
| Period 3 (left to right) | Atomic radius (Å) |
|---|---|
| Na | 1.86 |
| Mg | 1.6 |
| Al | 1.43 |
| Si | 1.18 |
| P | 1.1 |
| S | 1.04 |
| Cl | 0.99 |
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 2
occupied main levels 2
radius of Li 1.52 Å
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.
| Factor | Effect on the radius |
|---|---|
| Pull of the nucleus (more protons) | decreases it |
| Repulsion between electrons (more electrons) | increases it |
| Across period 3 | the pull wins: the radius decreases |
Size is a balance of the two. Across period 3, the pull wins, so the radius decreases.
6. Your turn
Put in order
Order these period 2 atoms from the largest radius to the smallest.
- Li (Z = 3)
- B (Z = 5)
- N (Z = 7)
- F (Z = 9)
Pick the right answer
Bromine (period 4) and chlorine (period 3) are both in group VIIA. Which is larger, and why?
- A. Bromine: its outer electrons are in an extra main level, farther from the nucleus
- B. Chlorine: it has fewer protons, so a weaker pull
- C. Bromine: it has more protons
- D. Same size: same group
الشرح
Br uses four main levels, Cl uses three. The extra level wins over the extra protons.
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?
Reveal
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.