Anand Classes provides a detailed NCERT Intext 3.2 solution for Class 11 Chemistry: How would you justify the presence of 18 elements in the 5th period of the Periodic Table? The explanation is based on the electronic configuration, filling of orbitals, and periodic trends that determine why exactly 18 elements are present in the 5th period. This makes it easier for JEE, NEET, and CBSE students to understand the concept clearly with proper reasoning. Click the print button to download study material and notes.
How would you justify presence of 18 elements in 5th period of Periodic Table ?
🧪 Justification for 18 Elements in the 5th Period
- Principle Quantum Number (n = 5)
- The 5th period begins with filling of the 5th principal energy level (n = 5).
- Subshells Available
According to quantum mechanics, the order of filling (Aufbau principle) is:- 5s → 4d → 5p
- (Notice: 4d subshell is filled after 5s because of relative energy order).
- Electron Capacity of Each Subshell
- 5s subshell → 2 electrons → 2 elements
- 4d subshell → 10 electrons → 10 elements
- 5p subshell → 6 electrons → 6 elements
- Therefore
The 5th period contains 18 elements because filling of electrons occurs in 5s, 4d, and 5p subshells, accommodating a maximum of 18 electrons → hence 18 elements in the 5th period.
Short exam-ready answer
How would you justify presence of 18 elements in 5th period of Periodic Table ? NCERT Intext 3.2 Question Solution
The 5th period contains 18 elements because electrons occupy the 5s (2 electrons), then the 4d (10 electrons) and finally the 5p (6 electrons) subshells. The capacities 2 + 10 + 6 add up to 18, so there are 18 elements (Z = 37 to 54).
Why the 5th period has 18 elements — detailed explanation
1. Quantum rules and subshell capacities
A few basic quantum rules explain how many electrons each subshell can hold:
- Each orbital can hold 2 electrons (Pauli exclusion principle).
- Number of orbitals in a subshell = 2 ℓ + 1 (where ℓ = 0 for s, 1 for p, 2 for d).
- So subshell capacities = 2(2 ℓ + 1) :
- s (ℓ = 0) → 2(1) = 2 electrons
- p (ℓ = 1) → 2(3) = 6 electrons
- d (ℓ = 2) → 2(5) = 10 electrons
For period 5 the relevant subshells are 5s (2), 4d (10) and 5p (6) — total 18.
2. Energy ordering and the role of the (n–1)d subshell
Why does the 5th period use a 4d subshell (n–1 d) instead of a 5d right away?
- Orbital energies depend on both principal quantum number n and angular momentum ℓ.
- For periods ≥ 4, the (n−1)d orbitals have energies comparable to the ns orbital of the next shell. In practice the filling order is : 5s→4d→5p
- The 5s orbital penetrates closer to the nucleus and is lower in energy than empty 4d, so 5s fills first (Rb, Sr). After 5s is filled, electrons begin to occupy 4d (the d-block), then finally 5p (the p-block).
- This is why period 5 has an s-block (2 elements), a d-block (10 elements), and a p-block (6 elements) — totaling 18.
3. Stepwise filling across period 5 (conceptual)
Start from the noble-gas core [Kr] (Z = 36). Add electrons in order:
- 5s fills → 2 elements
- Rb (Z=37): [Kr] 5s¹
- Sr (Z=38): [Kr] 5s²
- 4d fills → 10 elements (d-block; Y → Cd; Z = 39–48)
- Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd
- (These ten slots correspond to up to 10 electrons in 4d.)
- 5p fills → 6 elements (p-block; In → Xe; Z = 49–54)
- In, Sn, Sb, Te, I, Xe
Total = 2 (5s) + 10 (4d) + 6 (5p) = 18 elements (Z = 37 → 54).
4. A note on transition-metal configuration anomalies
Some transition metals show small deviations from the simple Aufbau sequence because half-filled (d⁵) or completely filled (d¹⁰) subshells are especially stable. Typical examples (period 5 / 4d series) include unusual electron distributions for elements like Nb, Mo, Pd, Ag etc. These exceptions do not change the total number of elements in the period — they only affect how electrons are arranged between 5s and 4d for particular atoms.
5. Why period 5 is not longer or shorter
- It’s not shorter because additional available orbitals (4d and 5p) allow more electrons than just 5s and 5p.
- It’s not longer because the next inner subshell that could add more positions is 4f, but 4f orbitals belong to period 6 (they are lower in energy than many 5d orbitals), so they are not available for period 5. Period 6 therefore becomes longer (32 elements) because it has 6s, 4f, 5d and 6p.
6. List of period-5 elements (Z = 37 → 54)
- 37 Rb (rubidium)
- 38 Sr (strontium)
- 39 Y (yttrium)
- 40 Zr (zirconium)
- 41 Nb (niobium)
- 42 Mo (molybdenum)
- 43 Tc (technetium)
- 44 Ru (ruthenium)
- 45 Rh (rhodium)
- 46 Pd (palladium)
- 47 Ag (silver)
- 48 Cd (cadmium)
- 49 In (indium)
- 50 Sn (tin)
- 51 Sb (antimony)
- 52 Te (tellurium)
- 53 I (iodine)
- 54 Xe (xenon)
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