Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Isotope
What is Isotope?
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Because the proton number is unchanged, they are the same element and, as neutral atoms, they have the same number of electrons and the same electronic configuration, which is why they have the same chemical properties. They differ in mass number, and therefore in properties that depend on mass.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Common mistakes with Isotope
- 4. “Change the number of neutrons and you get a different element.” Repair You get a different isotope of the same element. Identity is set by the proton number alone. Taught fully in Lesson 2.3A.
Questions students ask about Isotope
What is the difference between mass number and relative atomic mass?
Mass number (nucleon number) belongs to one atom: it is the total number of protons and neutrons in that nucleus, so it is always a whole number. Relative atomic mass belongs to the element as a whole: it is the mean mass of its atoms, weighted by the abundance of each isotope, so it is usually not a whole number. Chlorine has atoms of mass number 35 and 37, but a relative atomic mass of 35.5 because the two isotopes occur in different proportions.
What is an isotope, and why do isotopes have the same chemical properties?
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Changing the number of neutrons does not make a different element, because identity is set by the proton number alone. Isotopes have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration, and it is the outer electrons that decide how an atom bonds and reacts.