📚 Chapter:
Classification of Elements and Periodicity in Properties
📘 Explanation
🔵 Step 1: What determines the number of elements?
In the modern periodic table, the number of elements in a period is determined by the number of electrons that can be accommodated in the subshells being filled.
Number of elements in a period → Number of electrons accommodated in the subshells being filled
🟢 Step 2: Length of Different Periods
| Period | Subshells Filled | Number of Elements |
|---|---|---|
| 1st | 1s | 2 |
| 2nd | 2s, 2p | 8 |
| 3rd | 3s, 3p | 8 |
| 4th | 4s, 3d, 4p | 18 |
| 5th | 5s, 4d, 5p | 18 |
| 6th | 6s, 4f, 5d, 6p | 32 |
| 7th | 7s, 5f, 6d, 7p | 32 |
🎬 3D-Style Period Animation
As more subshells become available for filling, the length of the period increases.
2
8
8
18
18
32
2 → 8 → 8 → 18 → 18 → 32 → 32
🟣 Step 3: Why are the numbers different?
The maximum number of electrons that can be accommodated in a shell is 2n², but the actual length of a period depends on the order in which different subshells are filled.
1st → 2 elements
2nd & 3rd → 8 elements
4th & 5th → 18 elements
6th & 7th → 32 elements
2nd & 3rd → 8 elements
4th & 5th → 18 elements
6th & 7th → 32 elements
✅ Final Answer
The number of elements in a period is determined by the number of electrons that can be accommodated in the subshells being filled in that period.
Period Length = Number of electrons accommodated in the subshells being filled
📝 MCQ Practice
Q1. The number of elements in a period is mainly determined by:
A) Atomic mass
B) Number of subshells being filled
C) Number of neutrons
D) Atomic mass number
✅ Show Answer
Answer: B) Number of subshells being filled
🎯 Exam Point
Period Length → Subshell Filling
2, 8, 8, 18, 18, 32, 32
2, 8, 8, 18, 18, 32, 32
