Atomic structure and isotopesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A thyroid clinic uses iodine-131, a radioactive isotope, to treat an overactive thyroid gland. Naturally occurring iodine is entirely stable iodine-127. Iodine has atomic number 53.

   (a)

   How many neutrons are there in one atom of iodine-131?

   [1 mark]

   • A53
   • B78
   • C131
   • D184

   (b)

   Which statement correctly describes the relationship between iodine-131 and iodine-127?

   [1 mark]

   • AThey have the same number of neutrons but different numbers of protons
   • BThey have the same mass number but different atomic numbers
   • CThey have different numbers of electrons in their neutral atoms
   • DThey have the same number of protons but different numbers of neutrons

   (c)

   Explain why iodine-131 and iodine-127 have the same chemical properties but different masses.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A physics demonstration passes separate beams of protons, neutrons and electrons between two electrically charged metal plates. The beams are deflected by different amounts, or not at all.

   (a)

   Which particle has a relative charge of −1 and a relative mass of about 1/1836?

   [1 mark]

   • AElectron
   • BProton
   • CNeutron
   • DHydrogen atom

   (b)

   Which particles pass between the charged plates without being deflected?

   [1 mark]

   • AProtons only
   • BElectrons only
   • CNeutrons only
   • DProtons and electrons

   (c)

   Explain why a neutral atom has no overall charge, and why almost all of its mass is in the nucleus.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Magnesium (atomic number 12) occurs naturally as three isotopes: ²⁴Mg, ²⁵Mg and ²⁶Mg.

   (a)

   Explain what is meant by the term isotopes, using ²⁴Mg and ²⁶Mg as examples.

   [3 marks]

   (b)

   Deduce the number of protons, neutrons and electrons in a ²⁴Mg atom and in a ²⁵Mg²⁺ ion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrogen has three isotopes: protium (¹H), deuterium (²H) and tritium (³H). Water made from deuterium, ²H₂O, is called heavy water and is used to slow neutrons in some nuclear reactors. Assume all oxygen in this question is ¹⁶O (atomic number 8).

   (a)

   Compare the atomic structure of protium, deuterium and tritium, and explain why they react chemically in the same way but differ in some physical properties.

   [6 marks]

   (b)

   Deduce the numbers of protons, neutrons and electrons in one molecule of heavy water, ²H₂¹⁶O, and in one molecule of tritiated water, ³H₂¹⁶O. Explain why their relative molecular masses differ but their chemical properties are the same.

   [6 marks]

   Total for question 4: 12 marks

End of questions