To identify the wrong statement among the given options, we need to analyze each statement related to isoelectronic species and atomic radius trends in the periodic table.
### Step-by-Step Solution:
1. **Understanding Isoelectronic Species:**
Isoelectronic species are atoms or ions that have the same number of electrons. For example, \( \text{Ca}^{2+} \), \( \text{K}^{+} \), and \( \text{Ar} \) are isoelectronic because they all have 18 electrons.
2. **Analyzing Option A:**
- The statement claims: "Among isoelectronic species, smaller the positive charge in cation, smaller the ionic radius."
- This statement is incorrect. In fact, the opposite is true: the greater the positive charge on a cation, the smaller its ionic radius. This is because a higher positive charge means a greater effective nuclear charge attracting the electrons closer, thus reducing the size of the ion.
3. **Analyzing Option B:**
- The statement claims: "Among isoelectronic species, greater the negative charge on the ion, larger the radius."
- This statement is correct. For example, \( \text{N}^{3-} \) is larger than \( \text{O}^{2-} \) and \( \text{F}^{-} \) because it has more electrons than protons, resulting in a larger size.
4. **Analyzing Option C:**
- The statement claims: "Atomic radius of an element increases as one moves down the first group of the periodic table."
- This statement is correct. As we move down a group, additional electron shells are added, which increases the atomic radius.
5. **Analyzing Option D:**
- The statement claims: "Atomic radius of an element decreases as one moves across from left to right in the periodic table."
- This statement is also correct. As we move from left to right, the effective nuclear charge increases, pulling the electrons closer to the nucleus and decreasing the atomic radius.
6. **Conclusion:**
- The only incorrect statement is Option A. Therefore, the answer to the question is that Option A is the wrong statement.
### Final Answer:
The wrong statement is: **"Among isoelectronic species, smaller the positive charge in cation, smaller the ionic radius."**
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