Close Packed Structures — NEET Importance

NEET Importance Analysis

The topic of Close Packed Structures is of significant importance for the NEET UG Chemistry exam, particularly in the 'Solid State' chapter. Questions from this section appear frequently, often carrying a weightage of 4 marks per question. The common question types include:

1
Conceptual Understanding: — Differentiating between HCP and CCP based on stacking sequence, unit cell type, and examples of elements.
2
Coordination Number: — Asking for the coordination number in 1D, 2D (SCP, HCP), or 3D (HCP, CCP) structures.
3
Packing Efficiency: — Recalling the packing efficiency values for SC, BCC, HCP, and CCP/FCC. While derivations are usually not asked, the final values are crucial.
4
Voids and Stoichiometry: — This is a very common and high-yield area. Questions involve calculating the formula of a compound where one type of atom forms a close-packed lattice and the other type occupies a certain fraction of tetrahedral or octahedral voids. This requires a clear understanding of the N:N:2N relationship for atoms, octahedral voids, and tetrahedral voids.
5
Radius Ratio Rules: — Although less frequent, questions might involve the radius ratio for fitting smaller ions into specific voids (e.g., $0.225r$ for tetrahedral, $0.414r$ for octahedral).

Mastery of this topic ensures a solid foundation in solid-state chemistry and directly contributes to scoring well in the exam. It's not just about memorization but also about applying the concepts to solve numerical problems.

Vyyuha Exam Radar — PYQ Pattern

Analysis of previous year NEET (and AIPMT) questions reveals consistent patterns regarding close-packed structures. The topic is a perennial favorite, with questions appearing almost every year. The difficulty level generally ranges from easy to medium, making it a scoring area if concepts are clear.

Common Patterns Observed:

Void-based Stoichiometry (High Frequency): — A significant number of questions revolve around calculating the formula of a compound where one element forms a close-packed lattice (HCP or CCP/FCC) and the other element occupies a specific fraction of tetrahedral or octahedral voids. These questions test the N:N:2N relationship of atoms, octahedral voids, and tetrahedral voids.
Identification of Structures/Stacking (Medium Frequency): — Questions asking to identify the stacking sequence (A-B-A-B for HCP, A-B-C-A-B-C for CCP) or the type of unit cell (hexagonal for HCP, FCC for CCP) are common. Sometimes, examples of metals adopting these structures are asked.
Coordination Number and Packing Efficiency (Medium Frequency): — Direct questions on the coordination number of atoms in 2D or 3D close-packed structures, or the packing efficiency values for HCP/CCP (74%), BCC (68%), and SC (52.4%) are regularly seen.
Radius Ratio (Low Frequency): — While less common, questions on the radius ratio for fitting ions into tetrahedral ($0.225r$) or octahedral ($0.414r$) voids do appear occasionally.

Trends: There's a consistent emphasis on the quantitative aspects (number of voids, formula derivation) and the qualitative distinctions (stacking patterns, examples). Students who master the void calculations and memorize the key characteristics are well-prepared for this section.