To determine the incorrect order of radius among the given options, we will analyze each option step by step.
### Step 1: Analyze Option A
- **Given Species**: Strontium (Sr²⁺), Rubidium (Rb⁺), Bromine (Br⁻), Selenium (Se²⁻)
- **Period**: Sr and Rb are in period 5, while Br and Se are in period 4.
- **Isoelectronic Species**: When electrons are removed or added, we can form isoelectronic species.
- **Effective Nuclear Charge (Z_eff)**: The atomic radius is inversely proportional to Z_eff. Thus, a higher positive charge leads to a smaller radius, while a higher negative charge leads to a larger radius.
- **Order of Radius**: Se²⁻ > Br⁻ > Rb⁺ > Sr²⁺. This order is correct.
### Step 2: Analyze Option B
- **Given Species**: Niobium (Nb⁵⁺), Zirconium (Zr⁴⁺), Ytterbium (Yb³⁺)
- **Electrons**: All species have 36 electrons after ionization.
- **Isoelectronic Species**: They are isoelectronic.
- **Order of Radius**: Yb³⁺ > Zr⁴⁺ > Nb⁵⁺. This order is correct.
### Step 3: Analyze Option C
- **Given Species**: Cobalt (Co), Cobalt (Co²⁺), Cobalt (Co³⁺), Cobalt (Co⁴⁺)
- **Charge and Radius**: The atomic radius decreases with increasing positive charge.
- **Order of Radius**: Co > Co²⁺ > Co³⁺ > Co⁴⁺. This order is correct.
### Step 4: Analyze Option D
- **Given Species**: Barium (Ba²⁺), Cesium (Cs⁺), Selenium (Se²⁻), Arsenide (As³⁻)
- **Period**: Ba and Cs are in period 6, while Se and As are in period 4.
- **Not Isoelectronic**: Since they are not isoelectronic, we compare their atomic sizes based on their periods.
- **Order of Radius**: Ba²⁺ and Cs⁺ will have larger radii compared to Se²⁻ and As³⁻. The expected order should be Cs⁺ > Ba²⁺ > Se²⁻ > As³⁻. This order is incorrect as it does not follow the expected trend.
### Conclusion
The incorrect order of radius is found in **Option D**: Barium (Ba²⁺), Cesium (Cs⁺), Selenium (Se²⁻), and Arsenide (As³⁻).
### Final Answer
**Option D** is the incorrect order of radius.
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