Chemistry Regents Practice Test 2025 – 400 Free Practice Questions to Pass the Exam

Isotopes are specifically defined as atoms of the same element that have different numbers of neutrons. This distinction is critical because the number of protons in an atom determines its elemental identity, while variations in neutron count lead to different isotopes of that element.

For example, carbon has isotopes such as carbon-12 and carbon-14. Both isotopes have six protons, which makes them carbon, but carbon-12 has six neutrons, while carbon-14 has eight neutrons. This variance in neutron count can affect the stability and some chemical properties of the isotopes, but the element remains the same due to the consistent proton count.

The other options do not accurately describe isotopes. Atoms of different elements cannot be isotopes of one another; thus, the first option is incorrect. The second option mislabels protons as the differing particle, which is not correct since isotopes always involve neutrons. Lastly, the definition of mass number encompasses both protons and neutrons, and while isotopes possess different mass numbers due to varying neutron counts, stating that they have the same mass number would not classify them as isotopes.