Assertion: The third period contains only 8 electrons and not 18 like 4th period.
Reason: In III perriod filling starts from `3s^(1)` and complete at `3p^(6)` whereas in IV period it starts from `4s^(1)` annd complete after `3d^(10)` and `4s^(2)`.

Options

A. If both assertion and reason are true annd the reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but the reason is false
D. if assertion is false but the reason is true

Answer

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Understanding the Assertion
The assertion states that the third period contains only 8 electrons and not 18 like the fourth period.

- The third period of the periodic table consists of the elements from sodium (Na) to argon (Ar).
- The electronic configuration of these elements shows that they fill the 3s and 3p subshells.
- The filling order is as follows:
- 3s: 2 electrons (from Na to Mg)
- 3p: 6 electrons (from Al to Ar)
- Therefore, the total number of electrons in the third period is 2 (from 3s) + 6 (from 3p) = 8 electrons.

### Step 2: Understanding the Reason
The reason states that in the third period, filling starts from `3s^(1)` and completes at `3p^(6)`, whereas in the fourth period, it starts from `4s^(1)` and completes after `3d^(10)` and `4p^(6)`.

- In the fourth period, the filling starts from the 4s subshell, and then it fills the 3d subshell (which can hold 10 electrons) and the 4p subshell (which can hold 6 electrons).
- The filling order is as follows:
- 4s: 2 electrons
- 3d: 10 electrons
- 4p: 6 electrons
- Therefore, the total number of electrons in the fourth period is 2 (from 4s) + 10 (from 3d) + 6 (from 4p) = 18 electrons.

### Conclusion
- The assertion is **true**: The third period contains only 8 electrons.
- The reason is also **true**: The filling of the subshells in the third period is different from that in the fourth period.

Thus, both the assertion and the reason are correct.

### Final Answer
Both the assertion and the reason are true.

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