Which one of the following is the most characteristic fundamental property that uniquely identifies a chemical element?

Introduction / Context:
This question tests your understanding of atomic structure and how elements are defined in modern chemistry. While many physical properties like density and boiling point are associated with specific elements, they can vary with conditions and do not uniquely define an element. The key property that does uniquely identify an element is the atomic number, which is central to the periodic table and chemical identity.

Given Data / Assumptions:

• The options list density, boiling point, mass number, atomic number and melting point.
• We are asked which is the most characteristic property of an element.
• We assume standard definitions where atomic number is the number of protons in the nucleus.

Concept / Approach:
Atomic number is defined as the number of protons in the nucleus of an atom. Every element has a unique atomic number. For example, hydrogen has atomic number 1, helium has 2, and so on. If an atom has a certain atomic number, it must be that element and no other. Mass number can vary among isotopes of the same element. Density, boiling point and melting point can change with pressure, temperature, and allotropic forms, and are not uniquely tied to a specific element in the same strict way. Therefore, atomic number is the most characteristic property of a chemical element.

Step-by-Step Solution:

Verification / Alternative check:
The periodic table is organised by increasing atomic number, not by mass number or density. When chemists discovered new elements, they assigned them places in the table using their atomic numbers. Isotopes of the same element share the same atomic number but differ in mass number, showing that mass number does not define the element. Physical properties such as density and boiling point also vary among isotopes and can overlap between different substances. This reinforces that atomic number is the primary identifier of a chemical element.

Why Other Options Are Wrong:

• Density: Depends on physical state, temperature and pressure, and can be similar for different substances.
• Boiling point: Varies with pressure and can be similar for different compounds or elements, so it does not uniquely identify an element.
• Mass number: Different isotopes of the same element have different mass numbers, so mass number is not unique to the element.
• Melting point: Like boiling point, this physical property can change with pressure and does not uniquely define the element.

Common Pitfalls:
Some learners may be misled by familiar tables of physical properties and think that properties like melting point always uniquely pick out an element. However, those properties can shift with conditions and may be similar among different substances. The key idea is that at the atomic level, the proton count is what makes one element different from another. Remembering that atomic number equals number of protons and defines the element will help you answer many questions about atomic structure and the periodic table accurately.

Final Answer:
The most characteristic property of an element is its Atomic number.