Identify the wrong statement in the following:

Let's analyze the statements based on periodic trends and isoelectronic species:

1. Isoelectronic Species: These are species with the same number of electrons. Their size is governed by the nuclear charge ($Z$). A higher nuclear charge exerts a stronger attraction on the electrons, shrinking the radius .

• Cations: Comparing isoelectronic cations (e.g., $Na^+$ and $Mg^{2+}$), the one with the smaller positive charge ($Na^+$) has fewer protons ($Z=11$) than the one with the larger charge ($Mg^{2+}$, $Z=12$). Fewer protons mean less attraction, resulting in a larger radius. Therefore, the statement "smaller the positive charge... smaller is the ionic radius" is incorrect.
• Anions: A greater negative charge means fewer protons relative to electrons, leading to reduced effective nuclear charge and increased electron-electron repulsion, which expands the radius. This statement is correct .

2. Group Trend: Atomic radius increases down a group (e.g., Group 1) due to the addition of electron shells shielding the outer electrons . This statement is correct.
3. Period Trend: Atomic radius decreases across a period (e.g., 2nd period) due to increasing effective nuclear charge pulling valence electrons closer . This statement is correct.