Let's proceed precisely as requested, focusing on quantitative data from the provided properties. I will enumerate, summarize, and use the most significant quantitative properties **for grouping**, clearly indicating each parametric value and my logic:

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## 1. **Summary of Key Quantitative Properties (from PubChem)**

| Substance          | Molecular Weight | XLogP | TPSA | H-bond Donors | H-bond Acceptors | Rotatable Bonds | Heavy Atoms | Formula   |
|--------------------|-----------------|-------|------|---------------|------------------|-----------------|-------------|-----------|
| 2-bromopyridine    | 158.0           | 1.4   | 12.9 | 0             | 1                | 0               | 7           | C5H4BrN   |
| 2-chloropyridine   | 113.54          | 1.2   | 12.9 | 0             | 1                | 0               | 7           | C5H4ClN   |
| 2-iodopyridine     | 205.0           | 1.4   | 12.9 | 0             | 1                | 0               | 7           | C5H4IN    |

#### Key Numeric Ranges:
- **Molecular Weight:** 113.54 (Cl) – 205.0 (I)
- **XLogP:** 1.2–1.4 (all similar, minimal spread)
- **TPSA:** 12.9 (identical)
- **H-bond Donors/Acceptors:** 0/1 for all
- **Rotatable Bonds:** 0 for all
- **Heavy Atoms:** 7 for all

## 2. **Analysis of Quantitative Properties**

### a. **Polarity (XLogP, TPSA):**
- **XLogP** (octanol-water partition): Ranges narrowly from 1.2–1.4. **All are mildly lipophilic.**
- **TPSA (Topological Polar Surface Area):** Exact same value (12.9 Å²), indicating **identical capacity for polar interactions** (mainly from the pyridine N).

### b. **Hydrogen Bonding:**
- **Donor:** None have H-bond donors.
- **Acceptor:** Each has 1 acceptor (the pyridine N).
- **Conclusion:** *No divergence for grouping based on basicity/H-bonding.*

### c. **Size/Shape:**
- **Heavy Atom Count:** All 7 (no difference).
- **Rotatable Bonds:** All 0 (all rigid, planar aromatics).

### d. **Halogen Variation (Molecular weight only):**
- The only **quantitative differentiator** in standard properties is the atomic weight (Cl < Br < I):
  - Cl: 113.54
  - Br: 158.0
  - I: 205.0

### e. **Other Physicochemical Properties:**
None of the other listed properties (e.g., complexity, covalent units, etc.) show significant quantitative differences for standard classification.

## 3. **Classification**

All substances have:
- Identical H-bonding properties
- Same Topological Polar Surface Area
- Same rotatable bonds (i.e., identical rigidity/shape)
- Almost identical moderate lipophilicity (XLogP)
- Same count of heavy atoms

**The only significant quantitative difference is the halogen identity, reflected as molecular weight.**
> However, molecular weight alone, in the absence of strong functional/chemical divergence or significant logP/polarity jumps, is not a common criterion for separating physicochemical behavior classes (e.g., function, partitioning, basicity, etc.).

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### **Conclusion / Classification Table**

#### **These three compounds should all be placed in a single class, since:**
- Their **physicochemical parameters are nearly identical or truly identical** on all relevant, quantitative, operationally significant axes (polarity, hydrogen bonding, shape/rigidity, heavy atom count, polar surface area, and partitioning).
- **Halogen identity is a structural, not physicochemical, classifier at this level given the properties listed.**

| **Class** | **2-bromopyridine** | **2-chloropyridine** | **2-iodopyridine** |
|-----------|---------------------|----------------------|--------------------|
| **A**     | ✔                   | ✔                    | ✔                  |

**Justification:** Quantitative database values (TPSA, XLogP, H-bonding, rigidity) do not support further subdivision.

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**If a class based purely on molecular weight or halogen type (not a direct physicochemical property in the sense asked) were allowed, three groups would be possible, but this would not reflect functional/partitioning/interaction differences from a quantitative physicochemical dataset perspective.**

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**Final Assignment: All three substances belong to ONE class under the standard, database-derived quantitative physicochemical property lens.**