Atomic number: The most characteristic property of an element

Introduction:
The characteristic properties of an element are the unique qualities that distinguish it from other elements. These properties are essential for identifying and categorizing elements in the periodic table. Among the given options, the most characteristic property of an element is its atomic number.

Explanation:

1. Atomic number:
- The atomic number of an element is the number of protons present in the nucleus of its atom.
- It is denoted by the symbol 'Z' and is specific to each element.
- The atomic number determines the element's position in the periodic table and its chemical identity.
- For example, hydrogen has an atomic number of 1, which means it has one proton in its nucleus.

2. Density:
- Density is a physical property of matter that describes the mass of a substance per unit volume.
- While density can vary among different elements, it is not a unique characteristic property.
- Elements can have similar densities, and density can also be influenced by factors such as temperature and pressure.

3. Boiling point:
- The boiling point of an element is the temperature at which it changes from a liquid to a gas.
- Boiling points can vary significantly among elements, and they are influenced by various factors such as intermolecular forces and atomic structure.
- While boiling point can provide some information about an element, it is not a unique characteristic property.

4. Mass number:
- The mass number of an element is the sum of its protons and neutrons.
- It is denoted by the symbol 'A' and can vary among different isotopes of the same element.
- Mass number alone does not provide enough information to identify or categorize an element uniquely.

Conclusion:
Among the given options, the most characteristic property of an element is its atomic number. The atomic number determines an element's position in the periodic table, its chemical identity, and the number of protons in its nucleus. While other properties such as density, boiling point, and mass number can provide useful information, they are not as specific and unique as the atomic number.