Humber Admissions Practice Test

What defines isotopes of an element?

1. Same number of neutrons

2. Same number of protons

3. Different element symbols

4. Different mass numbers

The correct answer is: Same number of protons

Isotopes of an element are defined by their number of protons, which is the same for all isotopes of a given element. This consistent number of protons determines the identity of the element itself, meaning that regardless of how many neutrons are present, the element remains the same. For example, carbon always has six protons, and thus remains carbon, whether it has six, seven, or eight neutrons (which correspond to its different isotopes: carbon-12, carbon-13, and carbon-14). In contrast, the number of neutrons can vary, which results in different isotopes—this also leads to varying mass numbers, but the element remains the same due to the unchanged proton count. This underlying principle illustrates why the choice indicating the same number of protons characterizes isotopes. Other options, such as different element symbols or different mass numbers, do not reflect the consistent identity of isotopes.