Which of the following properties of an element is a whole number ?

Options

A. Atomic number
B. Atomic volume
C. Atomic radius
D. Mass number

Explanation

To determine which of the following properties of an element is a whole number, we will analyze each option provided:

1. **Atomic Number**:
- The atomic number of an element is defined as the number of protons in the nucleus of an atom.
- Since protons are discrete particles, the atomic number is always a whole number.
- For example, hydrogen has an atomic number of 1.

2. **Atomic Volume**:
- Atomic volume is defined as the volume occupied by one mole of an element in its solid state.
- This is typically measured in cubic centimeters per mole and can take on fractional values depending on the element and its state.

3. **Atomic Radius**:
- The atomic radius is a measure of the size of an atom, usually the distance from the nucleus to the outermost shell of electrons.
- This measurement can also be fractional and varies depending on the method of measurement and the element.

4. **Mass Number**:
- The mass number of an element is the total number of protons and neutrons in the nucleus.
- While the mass number is typically a whole number for stable isotopes, it can be represented as a decimal in certain contexts (like isotopic abundance), but it is generally considered a whole number when referring to a specific isotope.

Based on this analysis, the only property that is consistently a whole number is the **Atomic Number**.

### Final Answer:
The property of an element that is a whole number is **Atomic Number**.