Identify the incorrect statement among the following:

Question

Accepted Answer

1.  **Analyze Isoelectronic Species:** These are species with the same number of electrons. Their ionic radii depend on the effective nuclear charge ($Z_{eff}$). The radius is inversely proportional to the nuclear charge (number of protons).
2.  **Evaluate Option A:** For cations with the same number of electrons (e.g., $Na^+$ and $Mg^{2+}$), the one with the *smaller* positive charge ($Na^+$, Z=11) has *fewer* protons attracting the electrons compared to the one with the higher charge ($Mg^{2+}$, Z=12). Fewer protons mean less attraction and a **larger** radius. Therefore, the statement that 'smaller positive charge... smaller radius' is **incorrect**.
3.  **Evaluate Option B:** Greater negative charge implies fewer protons for the same number of electrons, leading to less attraction and a **larger** radius. This statement is correct.
4.  **Evaluate Option C:** Moving down a group adds new electron shells, increasing the atomic radius. This is correct.
5.  **Evaluate Option D:** Moving left to right across a period increases the nuclear charge without adding shells, pulling electrons closer and decreasing the radius. This is correct.

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