How to determine which has greater charge density?

Charge density of an ion = (Magnitude of charge carried by the ion) / (Volume of the ion)
Na⁺ and Cl⁻ ions carry opposite charges but are equal in magnitude.
Therefore, the small ion between Na⁺ and Cl⁻ ions would have a greater charge density.

As the size of Na⁺ ion is greater than that of Cl⁻ ion, Na⁺ has a greater density than Cl⁻.

Na⁺ ion is greater than Cl⁻ ion in size because :

(1) Na⁺ ion (2, 8) has two filled electronic shells, but Cl⁻ (2, 8, 8) ion has three.

(2) Both Na (2, 8, 1) and Cl (2, 8, 7) atoms are comparable in size because each of them has three filled electronic shells. When Na atom becomes Na⁺ ion, the number of filled electronic shells is decreased by one, and thus the size decreases sharply. On the other hand, when Cl atom becomes Cl⁻ ion, one extra electron is gained, and the increase in repulsions between electrons leads to a sharp increase in size.

Cl has a higher ,has the bigger ionic radius. This means the charge of Na is spread out over a smaller volume.

Since both charges are the same, the charge density (charge per volume or charge per area) of Na is bigger.