The statement that is not correct for periodic classification of element is

Options

A. The properties of element are periodic function of their atomic numbers.
B. Non-metallic elements are less in number than metallic elements.
C. For transition elements, the `3d`-orbitals are filled with electrons after `3p`-orbitals and before `4s`-orbitals.
D. The first ionisation enthalpies of element generally increase with increase in atomic number as we go along a period.

Explanation

To determine the statement that is not correct regarding the periodic classification of elements, we will analyze each statement one by one.

### Step 1: Analyze the first statement
**Statement:** The properties of elements are periodic functions of their atomic numbers.
**Analysis:** This statement is true. The periodicity of elements is based on their atomic numbers, which influence their chemical and physical properties.

### Step 2: Analyze the second statement
**Statement:** Nonmetallic elements are less in number than metallic elements.
**Analysis:** This statement is also true. In the periodic table, there are significantly more metallic elements compared to nonmetals.

### Step 3: Analyze the third statement
**Statement:** For transition elements, the 3d orbitals are filled with electrons after the 3p orbitals and before the 4s orbitals.
**Analysis:** This statement is incorrect. While the filling of orbitals can vary, generally, the 4s orbital is filled before the 3d orbitals. In some cases, such as in transition metals, the 3d orbitals can be filled after the 4s, which makes this statement misleading.

### Step 4: Analyze the fourth statement
**Statement:** The first ionization enthalpies of elements generally increase with an increase in atomic number as we go along a period.
**Analysis:** This statement is true. As we move across a period, the atomic size decreases, leading to an increase in ionization energy.

### Conclusion
After analyzing all four statements, the statement that is not correct is the third one regarding the filling of 3d and 4s orbitals in transition elements.

### Final Answer
**The statement that is not correct for periodic classification of elements is:** "For transition elements, the 3d orbitals are filled with electrons after the 3p orbitals and before the 4s orbitals."

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