An element in period-3 whose electron affinity is zero

The element in period-3 whose electron affinity is zero is Neon (Ne).

Here's the step by step explanation:

1. Period-3 elements are those in the third row of the periodic table. They include Sodium (Na), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl), and Neon (Ne).

2. Electron affinity is the amount of energy released when an electron is added to a neutral atom to form a negative ion. It is usually exothermic, meaning energy is released, making the electron affinity a negative quantity.

3. However, noble gases like Neon (Ne) have full electron shells, so they do not tend to gain or lose electrons. Therefore, their electron affinity is effectively zero because adding an electron would require energy rather than release it.