MYP Chemistry sample question: Atoms and isotopes

Chlorine has two main isotopes, chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl). Chlorine has atomic number 17.

1. (a)State the number of protons, neutrons and electrons in a neutral atom of ³⁵Cl.[3]
   Show the mark scheme for (a)

   • • 17 protons (1)
   • • 18 neutrons (1)
   • • 17 electrons (1)
2. (b)State what is meant by isotopes.[1]
   Show the mark scheme for (b)

   • • Atoms of the same element with the same number of protons but different numbers of neutrons (1)
3. (c)The relative atomic mass of chlorine is 35.5. Explain why it is not a whole number.[2]
   Show the mark scheme for (c)

   • • Chlorine is a mixture of isotopes with different masses (1)
   • • The relative atomic mass is a weighted average, closer to 35 because ³⁵Cl is more abundant (1)

Neutrons = mass number − atomic number. Isotopes differ only in neutrons, so their chemical properties are the same.