Isotopes of the atoms of the same element have which identical feature?

Introduction / Context:
Isotopes are a key concept in atomic structure. Atoms of the same element can have slightly different masses but still behave almost identically in chemical reactions. This question checks whether you know what remains the same in all isotopes of a given element and which properties can change from one isotope to another.

Given Data / Assumptions:

• We deal with isotopes, which are variants of the same chemical element.
• Options mention numbers of neutrons, protons, atomic mass, chemical properties, and atomic number.
• We assume a basic model of the atom with protons, neutrons, and electrons.
• We focus on how isotopes are defined in chemistry.

Concept / Approach:
Isotopes are defined as atoms of the same element that have the same atomic number but different mass numbers. Atomic number is the number of protons in the nucleus, while mass number is the sum of protons and neutrons. Therefore, isotopes have the same number of protons but different numbers of neutrons. Because chemical properties depend largely on the number of protons and electrons, isotopes of an element have very similar chemical behaviour. Their atomic masses differ, and so do their neutron counts.

Step-by-Step Solution:
Step 1: Recall that the atomic number (Z) of an element is the number of protons in the nucleus. Step 2: The mass number (A) of a given atom is equal to the sum of protons and neutrons. Step 3: Isotopes have the same Z value but different A values, meaning they share the same number of protons but differ in neutron count. Step 4: Because the number of protons is the same, isotopes have the same position in the periodic table and belong to the same element. Step 5: Their chemical properties are very similar because electron configuration is determined by nuclear charge (proton number). Step 6: Therefore, the defining identical feature of isotopes of the same element is the same number of protons.

Verification / Alternative check:
Consider carbon 12 and carbon 14. Both have 6 protons, so their atomic number is 6 and they are both carbon. Carbon 12 has 6 neutrons, while carbon 14 has 8, so they differ in neutron count and mass number. Yet both behave as carbon in chemical reactions. If the number of protons changed, the element would change completely. This simple example confirms that the key common feature of isotopes of one element is the same number of protons in the nucleus.

Why Other Options Are Wrong:
Same number of neutrons: By definition, isotopes differ in neutron number, so this cannot be the common feature.
Same atomic mass: Isotopes have different mass numbers and therefore different atomic masses.
Different chemical properties: Chemical properties of isotopes of an element are very similar, not fundamentally different, although small kinetic differences may exist.
Different atomic numbers: A different atomic number would mean a different element, so this cannot apply to isotopes of the same element.

Common Pitfalls:
Students sometimes confuse isotopes with isobars. Isobars have the same mass number but different atomic numbers, whereas isotopes have the same atomic number and different mass numbers. Another confusion arises from statements that isotopes have different physical properties; this is true but does not define isotopes. Always focus on the number of protons as the key to element identity and remember that isotopes share this number.

Final Answer:
Isotopes of the same element have the same number of protons in the nucleus.