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

Options

A. The properties of elements are the periodic functions of their atomic numbers.
B. Non-metallic elements are less in number than metallic elements.
C. The first ionisation energies of elements along a period do not vary in a regular manner with an increase in atomic number.
D. For transition elements the ionisation energies increase gradually with increase in atomic number.

Answer

To solve the question regarding which statement is not correct for the periodic classification of elements, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Identify the Statements**: We need to evaluate the statements provided in the question. The statements are related to the periodic classification of elements and their properties.

2. **Evaluate Statement A**:
- **Statement**: The property of an element is a periodic function of atomic number.
- **Analysis**: This statement is correct as per the modern periodic law, which states that the properties of elements are periodic functions of their atomic numbers.

3. **Evaluate Statement B**:
- **Statement**: Non-metallic elements are less in number than metallic elements.
- **Analysis**: This statement is also correct. In the periodic table, metals outnumber non-metals.

4. **Evaluate Statement C**:
- **Statement**: The first ionization energy of elements along a period does not vary in a regular manner with an increase in atomic number.
- **Analysis**: This statement is correct. While there is a general trend of increasing ionization energy across a period, there are exceptions (e.g., between nitrogen and oxygen) due to half-filled stability.

5. **Evaluate Statement D**:
- **Statement**: For transition elements, the ionization energy increases casually with an increase in atomic number.
- **Analysis**: This statement is incorrect. The ionization energy of transition metals does not follow a consistent trend; it can increase or decrease depending on the specific element.

6. **Conclusion**: After evaluating all statements, we find that the incorrect statement is **Statement D**.

### Final Answer:
The statement that is not correct for periodic classification of elements is **Statement D**.

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