Following statements regarding the periodic trends of chemical reactivity of the alkali metals and the halogens are given. Which of these statements gives the correct picture?

In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group

The reactivity decreases in the alkali metals but increases in the halogens with increase in atomic number down the group

In both the alkali metals and the halogens the chemical reactivity decreases with increase in atomic number down the group

Chemical reactivity increases with increase in atomic number down the group in both the alkali metals and halogens

Periodic Table and Periodicity PYQ — JEE Main 2006 · Shift 0 · Q48 with Solution

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Correct answer: D

Trend in alkali metals (Group 1):
Alkali metals react by losing one electron to form $M^+$ .
Down the group, atomic size increases and ionization enthalpy decreases, so the outer electron is removed more easily.
Therefore, reactivity increases down the group.

Example:
$\text{Li} < \text{Na} < \text{K} < \text{Rb} < \text{Cs}$
Trend in halogens (Group 17):
Halogens react by gaining one electron to form $X^-$ .
Down the group, atomic size increases and the tendency to gain an electron decreases because the incoming electron feels less attraction from the nucleus.
Therefore, reactivity decreases down the group.

Example:
$\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$
Evaluate the options:
- A: Alkali metals decrease, halogens increase down the group → Incorrect
- B: Both decrease down the group → Incorrect
- C: Both increase down the group → Incorrect
- D: Alkali metals increase, halogens decrease down the group → Correct
Final answer:
$\boxed{\text{D}}$

View written solutionFree

Correct answer: D

Trend in alkali metals (Group 1):
Alkali metals react by losing one electron to form $M^+$ .
Down the group, atomic size increases and ionization enthalpy decreases, so the outer electron is removed more easily.
Therefore, reactivity increases down the group.

Example:
$\text{Li} < \text{Na} < \text{K} < \text{Rb} < \text{Cs}$
Trend in halogens (Group 17):
Halogens react by gaining one electron to form $X^-$ .
Down the group, atomic size increases and the tendency to gain an electron decreases because the incoming electron feels less attraction from the nucleus.
Therefore, reactivity decreases down the group.

Example:
$\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$
Evaluate the options:
- A: Alkali metals decrease, halogens increase down the group → Incorrect
- B: Both decrease down the group → Incorrect
- C: Both increase down the group → Incorrect
- D: Alkali metals increase, halogens decrease down the group → Correct
Final answer:
$\boxed{\text{D}}$