Advanced Placement (AP) Chemistry Practice Exam

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What defines an isotope of an element?

An element with a different number of protons
An element with a different number of neutrons
An element with a different atomic number
An element with different electron configurations

The correct answer is: An element with a different number of neutrons

An isotope of an element is defined by having the same number of protons but a different number of neutrons. This means that while isotopes belong to the same element and share chemical properties, they have different mass numbers due to the variation in neutrons. For example, carbon-12 and carbon-14 are both isotopes of carbon; they both have 6 protons but differ in their neutron counts, with carbon-12 having 6 neutrons and carbon-14 having 8. The number of protons determines the atomic identity of an element, hence different numbers of protons would indicate a different element altogether. This makes options discussing different numbers of protons or atomic numbers incorrect with respect to the definition of an isotope. Additionally, variations in electron configurations concern the element's ionization state or chemical bonding characteristics, which are not relevant to isotope classification.