It is not possible to measure the atomic radius prectsely since the electron cloub surrouding the atom does not have a sharp boundary. One practial approach to estimate the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound togther by a signle bond in a covalent molecule and then dividing by two. For metals we define the term "matellic radius" which is taken as half the internuclear distance separating the metal cores in the metallic crystal. Then van der waal's radius represents the over all size of the atoms which includes its valence shell in a non bonded situation. It is the half of the distance between two similar atoms in separate molecules in a solid. The atomis radius decreases across a period and increases down the group. Same trends are observed in case of ionic radius. Ionic radius of the species having same number of electrons depends on the number of protons in their nuclei. Sometimes, atomic and ionic radii give unexpected trends due to poor shielding of nuclear charge by `d`-and `f-`orbital electrons.
Now answer the following three questions:
Select the INCORRECT option regarding atomic//ionic sizes:

Options

A. `Zn gt Cu`
B. `Pb^(2+) gt Pb^(4+)`
C. `Zr ~~ Hf`
D. `N^(3-) lt AI^(3+)`

Explanation

To answer the question regarding the incorrect option related to atomic and ionic sizes, we will analyze each statement provided in the video transcript step by step.

### Step-by-Step Solution:

1. **Statement Analysis**:
- The first statement compares the sizes of zinc (Zn) and copper (Cu). It states that zinc is larger than copper. This is correct because, although both elements are in the same period, zinc has a filled d-subshell (d10) which leads to increased electron-electron repulsion, making it larger than copper which has a d9 configuration.

2. **Stability of Lead Ions**:
- The second statement discusses the stability of lead ions, specifically Pb²⁺ and Pb⁴⁺. It states that Pb²⁺ is more stable than Pb⁴⁺ due to the inert pair effect. This is also correct. As we move down the group in the periodic table, the lower oxidation states tend to be more stable due to the inert pair effect, which is the reluctance of the s-electrons to participate in bonding.

3. **Comparison of Zirconium and Hafnium**:
- The third statement compares the sizes of zirconium (Zr) and hafnium (Hf). It mentions that this is correct due to the lanthanide contraction, which causes the atomic size of Hf to be smaller than expected when compared to Zr. This statement is also correct.

4. **Isoelectronic Species**:
- The final statement compares the ionic sizes of N³⁻ and Al³⁺. It states that N³⁻ is larger than Al³⁺. This is the incorrect statement. For isoelectronic species (species with the same number of electrons), cations (like Al³⁺) are smaller than anions (like N³⁻) due to the greater nuclear charge in cations which pulls the electron cloud closer to the nucleus.

### Conclusion:
The incorrect option regarding atomic/ionic sizes is the comparison between N³⁻ and Al³⁺, where it is stated that N³⁻ is larger than Al³⁺.