Which one of the following is the most characteristic property of an element?

The atomic number is the most characteristic and critical property defining a chemical element. It represents the number of protons in the nucleus, which determines the element's unique identity and its position on the periodic table [1]. While physical properties like density and boiling point can vary based on the state or isotope of an element, and the mass number varies among isotopes of the same element, the atomic number remains constant for all atoms of a specific element [2]. This number also dictates the electron configuration in a neutral atom, which fundamentally governs the element's chemical behavior and reactivity. Consequently, the atomic number is the distinguishing feature that makes an element physically and chemically distinct from others.

Sources

1. [1] https://www.energy.gov/science/doe-explainsisotopes
2. [2] https://isotopes.gov/isotope-basics