JEE Main 2024 · 27 January · morning · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Melting point of Boron (2453 K) is unusually high in group 13 elements.
Reason (R) : Solid Boron has very strong crystalline lattice.
In the light of the above statements, choose the most appropriate answer from the options given below ;
- (a) Both (A) and (R) are correct but (R) Is not the correct explanation of (A)
- (b) Both (A) and (R) are correct and (R) is the correct explanation of (A)
- (c) (A) is true but (R) is false
- (d) (A) is false but (R) is true
Answer and solution
Answer (b) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Solution
Chemistry ma’am · 4 steps
Boron is a giant covalent network of B₁₂ icosahedra, so melting it means breaking covalent bonds.
- Check the assertion. Compare boron with the rest of group 13. Boron melts at 2453 K, while every other member melts below 1000 K. So (A) is true.
Element Melting point (K) Type of solid B 2453 covalent network of B₁₂ icosahedra Al 933 metallic Ga 303 metallic (loosely packed Ga₂ units) In 430 metallic Tl 576 metallic - Check the reason. Boron is a non-metal. In the solid, 12 boron atoms form an icosahedron, and these units are joined to each other by strong covalent bonds into a giant 3D lattice. So (R) is true.
- Link them. To melt boron, you must break this network of covalent bonds, and that needs a lot of energy. The other members are metals held by weaker metallic bonding. So the strong lattice is exactly why boron's melting point is so high.
- So. Both statements are correct and (R) correctly explains (A), which is option (b).
The idea to keep. When one element in a group has a melting point far above the others, look at the kind of solid it forms. A covalent network always melts far higher than a metallic lattice.
Answer from the NTA final answer key. Solution by PrepWiser.