The atoms of the same element that have the same atomic number but different mass numbers, due to different numbers of neutrons, are known by what name?

Introduction / Context:
A single chemical element can exist in several nuclear forms that share the same number of protons but differ in the number of neutrons in the nucleus. These different forms have the same chemical behaviour but may have different physical properties, such as mass and nuclear stability. Understanding the terminology for these variants is essential in both chemistry and physics, especially in topics like radioactivity, nuclear medicine, and isotopic analysis. This question asks you to identify the term used for atoms that share an atomic number but differ in mass number.

Given Data / Assumptions:

• The atoms belong to the same element, so their atomic number Z is the same.
• They have different mass numbers A, which means different numbers of neutrons.
• Their electronic configurations in the neutral state are identical, leading to similar chemical behaviour.
• Available terms include isobars, isotopes, isoelectronic species, isotones, and radioisomers.

Concept / Approach:
Isotopes are defined as atoms of the same element that have the same atomic number but different mass numbers. This implies that the number of protons is constant while the number of neutrons varies. Isobars have the same mass number but different atomic numbers. Isotones have the same number of neutrons but different atomic and mass numbers. Isoelectronic species share the same number of electrons but may be different elements or ions. Radioisomers refer to different nuclear energy states. The scenario described in the question clearly matches the definition of isotopes.

Step-by-Step Solution:
Step 1: Recognise that atomic number Z equals the number of protons and defines the element identity. Step 2: Mass number A equals the sum of protons and neutrons in the nucleus. Step 3: If Z is the same but A is different, the difference must come from neutrons, so the nuclei have varying neutron counts. Step 4: Recall that such nuclei belonging to the same element but having different neutron numbers are called isotopes. Step 5: Confirm that none of the other listed terms better describe same Z but different A for the same element.

Verification / Alternative check:
Classic examples of isotopes include hydrogen 1, hydrogen 2, and hydrogen 3, often known as protium, deuterium, and tritium. All three have atomic number 1 because they contain one proton each, but their mass numbers are 1, 2, and 3 respectively due to different neutron counts. Similarly, carbon 12 and carbon 14 are isotopes of carbon; each has six protons, but carbon 12 has six neutrons while carbon 14 has eight. These examples match the description given in the question and are universally referred to as isotopes in textbooks.

Why Other Options Are Wrong:
Isobars are nuclides that have the same mass number but different atomic numbers, such as argon 40 and calcium 40, so they do not satisfy the condition of same atomic number. Isoelectronic species have the same number of electrons but can involve different elements or ions, and their atomic numbers may differ. Isotones have equal neutron numbers but different proton numbers. Radioisomers refer to different energy states of the same nucleus rather than simply differences in neutron count. Therefore, none of these options fits the simple relationship of same atomic number but different mass numbers as directly as isotopes.

Common Pitfalls:
Students sometimes confuse isotopes and isobars because both involve mass numbers. A common mistake is to think that any pair with the same mass number must be isotopes, even if they are different elements. Another confusion arises between isotopes and isotones when working with nuclear equations. To avoid these problems, remember the root meanings: "iso" means same, "topes" refers to place in the periodic table (same element and atomic number), "bars" hints at equal mass, and "tones" reminds you of neutrons. With this mental map, you can quickly identify isotopes as same atomic number but different mass numbers.

Final Answer:
Atoms of the same element that have the same atomic number but different mass numbers are called isotopes.