Which of the following classifications is incorrect ?

Options

A. The representative elements are metals and nonmetals in which s and p orbitals are filled. These elements are in groups 1, 2 and 13 – 18.
B. The representative elements are metals and nonmetals in which s and p orbitals are filled. These elements are in groups1 and 2 only.
C. The noble gases (group 18) are nonmetals and are placed at the end of each period.
D. The transition elements are the metallic elements in which the d orbitals are filled. These elements are placed in group 3 – 12.

Explanation

To determine which classification is incorrect among the given options, we will analyze each option step by step.

### Step 1: Analyze Option A
- **Statement**: The representative elements are S and P block elements.
- **Analysis**: This statement is correct. The representative elements indeed consist of the S and P block elements, which include metals and non-metals found in groups 1, 2, and 13 to 18.

### Step 2: Analyze Option B
- **Statement**: The representative elements are metals and non-metals, in which S and P orbitals are filled. These elements are in group 1 and group 2 only.
- **Analysis**: This statement is partially correct. While it correctly identifies that representative elements include metals and non-metals with filled S and P orbitals, it incorrectly states that these elements are only in groups 1 and 2. In reality, representative elements also include groups 13 to 18. Therefore, this statement is incorrect.

### Step 3: Analyze Option C
- **Statement**: The noble gases (group 18) are non-metals placed at the end of the period.
- **Analysis**: This statement is correct. Noble gases are indeed non-metals and are located at the end of each period in the periodic table.

### Step 4: Analyze Option D
- **Statement**: The transition elements are metallic elements in which D orbitals are filled.
- **Analysis**: This statement is correct. Transition elements are indeed D block elements, and they are characterized by the filling of D orbitals.

### Conclusion
After analyzing all options, we find that:
- Option A: Correct
- Option B: Incorrect
- Option C: Correct
- Option D: Correct

Thus, the incorrect classification is **Option B**.

---