Assertion (A): Group 1 elements are the largest in their horizontal periods in the periodic table (exclude noble gases).

Reason (R): The melting and boiling points of group 1 elements increases on moving down from Li to Cs.

Correct Answer: Assertion (A) is true, and Reason (R) is false.

Step-by-step Explanation:

Analyzing Assertion (A):
Assertion (A) states that Group 1 elements are the largest in their horizontal periods in the periodic table (excluding noble gases).

This statement is TRUE. Here's why:

As we move across a period from left to right in the periodic table, the atomic number increases by one at each step, meaning one additional proton is added to the nucleus and one additional electron is added to the same shell. The increased nuclear charge pulls the electron cloud closer to the nucleus, causing the atomic radius to decrease across a period.

Since Group 1 elements (alkali metals: Li, Na, K, Rb, Cs) sit at the extreme left of each period, they experience the least effective nuclear charge among all elements in that period. Their single valence electron is loosely held and the electron cloud is most expanded. Therefore, Group 1 elements have the largest atomic size in their respective periods (excluding noble gases, as specified).

Analyzing Reason (R):
Reason (R) states that the melting and boiling points of Group 1 elements increase on moving down from Li to Cs.

This statement is FALSE. Here's why:

Group 1 elements are metals with a metallic bonding structure. The strength of metallic bonding depends on:

• The charge on the metal cation (which is +1 for all alkali metals)
• The size of the metal cation — a smaller cation leads to stronger metallic bonding

As we move down Group 1 from Li to Cs, the atomic and ionic radii increase significantly. This means the metallic bonds become weaker because the valence electron is farther from the nucleus and the charge density of the cation decreases.

As a result, the melting and boiling points actually decrease down the group:

Li → 181°C
Na → 98°C
K → 63°C
Rb → 39°C
Cs → 28°C

The trend clearly shows a decrease, not an increase, in melting points from Li to Cs. Therefore, Reason (R) is false.

Conclusion:
Since Assertion (A) is true and Reason (R) is false, the correct option is: "Assertion (A) is true, and Reason (R) is false."