Why the third period of Modern periodic table contains `8` elements and not `18`?

Explanation

To understand why the third period of the Modern Periodic Table contains 8 elements and not 18, we can break down the explanation into clear steps:

### Step 1: Identify the Energy Levels
The Modern Periodic Table is organized by periods, where each period corresponds to a new principal energy level (n). The third period corresponds to n = 3.

### Step 2: Determine the Orbitals in the Third Period
For n = 3, the possible values of the azimuthal quantum number (l) are 0, 1, and 2. This means the orbitals present in the third period are:
- 3s (l = 0)
- 3p (l = 1)
- 3d (l = 2)

### Step 3: Apply the Aufbau Principle
According to the Aufbau principle, electrons fill orbitals starting from the lowest energy level to the highest. In the case of the third period:
- The 3s orbital is filled first.
- After the 3s orbital is filled, the 3p orbital is filled next.

### Step 4: Count the Electrons in Each Orbital
- The 3s orbital can hold a maximum of 2 electrons.
- The 3p orbital can hold a maximum of 6 electrons (since there are 3 p orbitals, each can hold 2 electrons).

Thus, the total number of electrons that can be accommodated in the third period is:
- 2 (from 3s) + 6 (from 3p) = 8 electrons.

### Step 5: Understand the Role of the 3d Orbital
Although the 3d orbital exists, it is not filled in the third period. The 3d orbital starts filling in the fourth period after the 4s orbital is filled. This is due to the energy levels of the orbitals, where the 4s orbital is lower in energy than the 3d orbital.

### Conclusion
Therefore, the third period contains only 8 elements because it only includes the filling of the 3s and 3p orbitals, while the 3d orbital does not contribute to the electron count in this period.

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