Isotope

Isotopes are atoms of the same element that have the same proton number but different nucleon numbers (that is, different numbers of neutrons). Malay: isotop · Chinese: 同位素. This is one of the most reliably examined definitions in the chapter, so learn the exact wording.

Because isotopes have the same proton number, they are the same element and have the same number of electrons, so they share the same electron arrangement and the same chemical properties. What differs is the number of neutrons, which changes the nucleon number and gives each isotope a slightly different mass and physical properties such as density.

Example. Chlorine has two common isotopes: chlorine-35, with 17 protons and 18 neutrons, and chlorine-37, with 17 protons and 20 neutrons. Both react in exactly the same way because both have 17 electrons arranged 2.8.7, but chlorine-37 is heavier. Carbon-12 and carbon-14 are another pair, same 6 protons, different neutrons.

Confusion to avoid. The single most common error is to reverse the definition. Isotopes have the same proton number and different nucleon numbers, not the other way round. Writing “same nucleon number” or “different proton number” turns a correct answer into a wrong one, so check the wording every time. Isotopes are also not different elements; they are variants of the same element.

In the exam, isotope questions often show two atoms with the same proton number but different nucleon numbers and ask you to explain why they are isotopes, or to state how their subatomic particles differ. The safe answer names the same number of protons and electrons and a different number of neutrons. You may also be asked why isotopes have identical chemical properties, because they have the same electron arrangement.

Isotopes matter beyond this chapter because relative atomic mass, met later in the Mole Concept, is an average that takes the different isotopes and their abundances into account. Getting the definition precisely right here pays off directly in that calculation.