Four elements A, B, C and D along with their configurations are given below: Element A-2,1: Element B-2, 8, Element C-2, 8, 1, Element D-2, 8, 8
Which two elements belong to the same period?

Explanation

To determine which two elements belong to the same period, we need to analyze the electronic configurations of the given elements A, B, C, and D. The electronic configurations are as follows:

- Element A: 2, 1
- Element B: 2, 8
- Element C: 2, 8, 1
- Element D: 2, 8, 8

### Step 1: Identify the number of shells for each element
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