The correct option describing the trend for chemical reactivity within the group for alkali metals and halogens is Option D.

Here is the explanation for the periodic trends in reactivity for alkali metals and halogens:

Alkali Metals (Group 1 elements): For alkali metals, the chemical reactivity increases with an increase in atomic number down the group. This is because alkali metals have only one electron in their outermost shell, and the ease with which this electron is lost (their ionization energy) decreases as the size of the atoms increases down the group. Larger atoms have the valence electron further from the nucleus, which results in less electrostatic attraction between the nucleus and the electron. Therefore, the electron can be removed more easily, making the atom more reactive. As a result, cesium and francium are the most reactive alkali metals.

Halogens (Group 17 elements): For halogens, the chemical reactivity decreases with an increase in atomic number down the group. Halogens have seven electrons in their outermost shell and are one electron short of a full octet. They tend to gain an electron to achieve a stable electronic configuration. The ability to attract an electron is known as electronegativity, which typically decreases down the group as the size of atoms increase and the added inner electron shells shield the outer electrons from the pull of the nucleus. Thus, fluorine is the most reactive halogen, being able to attract electrons more easily than other halogens like chlorine, bromine, and iodine.

Therefore, Option D is correct: "In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group."

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Periodic Table and Periodicity · Chemistry · BITSAT

MCQ (Single Correct Answer)