Atomic radius: nearly constant
Two opposite factors in balance
1. General properties: mass, radius and metallic property
2. Shrink or stay?
Across period 3, atoms clearly shrink from Na to Cl. From Sc to Cu, each atom also gains one proton.
Pick the right answer
Predict: what happens to the atomic radius from Cr to Cu?
- A. It falls sharply, as across period 3
- B. It stays nearly constant
- C. It increases clearly
Explanation
From Cr to Cu the atomic radius stays nearly constant: the growing nuclear attraction is opposed by the growing repulsion between the 3d electrons.
3. The atomic radius column
| Element (Sc → Cu) | Atomic radius |
|---|---|
| Sc | 1.44 |
| Ti | 1.32 |
| V | 1.22 |
| Cr | 1.17 |
| Mn | 1.17 |
| Fe | 1.16 |
| Co | 1.16 |
| Ni | 1.15 |
| Cu | 1.17 |
Book point 2: the atomic radius changes little across the first transition series.
From Cr to Cu it is nearly constant: 1.17, 1.17, 1.16, 1.16, 1.15, 1.17.
4. Factor 1: a stronger pull
Effective nuclear charge
The pull of the nucleus that the outer electrons actually feel.
From Sc to Cr, the effective nuclear charge increases. The nuclear attraction to electrons increases.
This factor decreases the atomic radius. Acting alone, it would keep shrinking the atoms.
- Effective nuclear charge increases → Nuclear attraction to electrons increases
- Nuclear attraction to electrons increases → Atomic radius decreases
5. Factor 2: 3d electrons repel
From Sc to Cu, the number of 3d electrons grows from 1 to 10. More 3d electrons repel each other more.
This factor increases the atomic radius. The two factors oppose each other, so the radius stays relatively constant.
6. Watch the size
Move from Sc to Cu. Watch the 3d electrons grow while the atom stops shrinking.
element (1 = Sc … 9 = Cu) 1
atomic radius (book) 1.44
electrons in 3d 1
7. Remove one factor
Pick the right answer
Imagine the 3d electrons did not repel each other. What would the radius do from Cr to Cu?
- A. Keep decreasing, as across period 3
- B. Stay nearly constant, as in the table
- C. Increase
Explanation
Without the repulsion between the 3d electrons, the growing nuclear attraction would act alone, so the radius would keep decreasing, as across period 3.
True or false?
Across the first transition series, the atomic radius falls sharply, as it does across period 3.
Explanation
Book point 2: little change in atomic radii; nearly constant from Cr to Cu.
8. Build the exam answer
Put in order
Give reasons: the atomic radius is nearly constant from Cr to Cu. Put the answer in order.
- The effective nuclear charge increases, so the nuclear attraction to electrons increases
- This first factor would decrease the atomic radius
- But the number of 3d electrons increases, so the repulsion between them increases, which would increase the radius
- These two opposite factors balance, so the atomic radius is nearly constant
9. Summary
- The atomic radius changes little; it is nearly constant from Cr to Cu.
- Effective nuclear charge increases: the radius tends to decrease.
- More 3d electrons repel more: the radius tends to increase.
- The book adds: this explains their use in making alloys. An alloy is a metal mixed with other elements; you will study it later.
Can you explain why these atoms stop shrinking, though each gains a proton?
Reveal
Each extra proton raises the effective nuclear charge, which would decrease the radius. But the number of 3d electrons grows too, and their repulsion would increase the radius. The two opposite factors balance, so the radius is nearly constant from Cr to Cu.