All atoms of the element uranium share which identical fundamental property, regardless of isotope or ionisation state?

Introduction / Context:
This atomic structure question addresses the difference between atomic number, atomic mass, mass number, and electron count. An element such as uranium can exist in different isotopic and ionic forms, but there is one fundamental property that remains the same for all atoms of that element. Understanding which property is constant is essential for correctly interpreting the periodic table and for distinguishing isotopes from one another.

Given Data / Assumptions:
- The element under discussion is uranium.
- Options include atomic mass, number of electrons, atomic number, and mass number.
- We assume standard definitions where atomic number is the number of protons and defines the element.

Concept / Approach:
Atomic number is defined as the number of protons in the nucleus of an atom. All atoms of the same element have the same atomic number and therefore the same number of protons. Isotopes differ in the number of neutrons, which changes the mass number and atomic mass. Ions differ in the number of electrons. Therefore, atomic number is the unique property that is the same for all atoms of a given element, including uranium.

Step-by-Step Solution:
Step 1: Recall that atomic number (Z) is the number of protons in the nucleus and determines the identity of the element. Uranium has atomic number 92, so every uranium atom has 92 protons.
Step 2: Atomic mass and mass number involve both protons and neutrons. Different isotopes of uranium, such as uranium 235 and uranium 238, have different numbers of neutrons, so their mass numbers and atomic masses differ.
Step 3: The number of electrons can vary if the uranium atom gains or loses electrons to form ions, such as U4 plus or U6 plus, so electron count is not constant for all uranium atoms.
Step 4: Since only the proton count is fixed for all atoms of uranium, atomic number is the property that remains the same.

Verification / Alternative check:
Periodic tables list each element with a specific atomic number. Uranium is always listed with atomic number 92. Isotope notation such as 235U or 238U indicates different mass numbers (235 and 238) but always includes the element symbol U, which implies 92 protons. Ion notation may add charges, but the underlying element remains uranium as long as the proton count is 92. This consistent use of atomic number to define identity confirms that all uranium atoms share the same atomic number.

Why Other Options Are Wrong:
- Atomic mass: This is often a weighted average of the masses of all naturally occurring isotopes, and individual isotopes of uranium have different atomic masses.

Why Other Options Are Wrong (continued):
- Number of electrons: In neutral uranium atoms the electron count equals the proton count, but ions of uranium can lose or gain electrons, so this number can change.
- Mass number: Mass number varies between isotopes because it is the sum of protons and neutrons, so different isotopes of uranium have different mass numbers.

Common Pitfalls:
Students sometimes confuse atomic mass with atomic number because both are numbers printed near the element symbol. Another common error is to think that the number of electrons is always fixed, forgetting about ions. To avoid these mistakes, remember that atomic number is always an integer that defines the element, while atomic mass is often a decimal and mass number can change with isotopes. The element uranium is defined by having atomic number 92, regardless of its isotope or charge state.

Final Answer:
Atomic number is the property that is the same for all atoms of uranium.