International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

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What is an isotope?

  1. Atoms with the same number of neutrons

  2. Atoms with different numbers of protons

  3. Atoms of the same element with different numbers of neutrons

  4. Atoms of different elements with the same mass

The correct answer is: Atoms of the same element with different numbers of neutrons

An isotope refers to atoms of the same element that have the same number of protons but different numbers of neutrons. This characteristic means that while isotopes of an element exhibit the same chemical properties due to having identical electron configurations, they will have different mass numbers as a result of the varying neutron counts. For example, carbon-12 and carbon-14 are both isotopes of carbon. They each contain six protons, which defines them as carbon, but carbon-12 has six neutrons, while carbon-14 has eight neutrons. This difference in neutrons leads to variations in the stability and some physical properties of isotopes, which can influence behaviors in chemical reactions and nuclear processes. The other options are not accurate definitions of isotopes. Atoms with the same number of neutrons point to a specific scenario that does not hold true for isotopes, which are defined by their differing neutron counts. Atoms with different numbers of protons would actually represent different elements altogether, not isotopes. Lastly, the notion of different elements sharing the same mass does not align with the definition of isotopes, as isotopes must be variations of a single element rather than differing entities.