Animated Solution for Chemistry - Periodicity in Properties: In general, the properties that decrease and increase down a group in the periodic table, respectively are

Imagine standing at the very top of a column in the periodic table—a group. As you take a step down from one period to the next, you are fundamentally changing the architecture of the atom. With every downward step, the principal quantum number, denoted by $n$, increases by one.

This isn't just a mathematical increment; it represents the physical addition of an entirely new shell of electrons around the nucleus. The atom is literally expanding its territory, building new layers further and further out into space.

Because new electron shells are being added, the outermost electrons—the valence electrons—find themselves at a much greater distance from the positively charged nucleus.

Think of it like a city expanding its borders; the suburbs are much further from the city center than the inner neighborhoods. Consequently, the atomic radius strictly increases as you move down a group. The atoms become larger and bulkier.

Now, let's consider electronegativity, which is an atom's ability to attract a shared pair of electrons in a chemical bond.

As the atom grows larger, two things happen. First, the distance between the nucleus and the shared electrons increases, weakening the electrostatic pull. Second, the inner shells of electrons act like a thick fog, shielding the outermost electrons from the full attractive force of the nucleus—a phenomenon known as the shielding or screening effect.

Because of this increased distance and enhanced shielding, the nucleus loses its tight grip. Therefore, the electronegativity steadily decreases as you travel down the group.

In competitive exams like JEE, the word "respectively" is a notorious trap. It demands strict adherence to the sequence asked in the question.

The question asks for the properties that decrease and increase, in that exact order.

From our analysis: 1. Decreases: Electronegativity 2. Increases: Atomic Radius

Looking at the options, only option (a) provides this exact sequence: electronegativity and atomic radius. If you were to choose option (d), you would be providing the increasing property first, which would cost you precious marks despite knowing the underlying chemistry perfectly. Always respect the sequence!