Atoms with the same atomic number but different mass numbers are called

• A. Isotopes
• B. Isomers
• C. Allotropes
• D. Ion types

Answer: (A)

Same atomic number but different mass numbers describes isotopes. The atomic number (the number of protons) fixes the element, while the mass number equals protons plus neutrons. If you keep the protons the same and vary the neutrons, you get different isotopes of that element. They share almost identical chemical behavior because chemistry is driven by electron structure set by the proton count, but their nuclear properties and masses differ, which can affect stability (some are radioactive) and physical properties. For example, carbon-12 and carbon-14 are both carbon with six protons, but have 6 and 8 neutrons, respectively. Isomers refer to different arrangements of atoms in molecules, allotropes are different forms of the same element, and ion types are charged species.