Fundamental particles, mass number and isotopesAQA A-Level Chemistry: Subtopic test

1. 1
   In 1909 Geiger and Marsden, working with Rutherford, fired a beam of positively charged alpha particles at a very thin sheet of gold foil and recorded where each particle landed using a detector screen. At the time, the accepted 'plum pudding' model pictured an atom as a sphere of positive charge with electrons embedded throughout it.

   (a)

   The plum pudding model predicted that the alpha particles would pass through the foil with only very small deflections. What was observed for the great majority of the alpha particles?

   [1 mark]

   • AThey passed straight through the foil with no deflection
   • BThey were reflected straight back towards the source
   • CThey were absorbed by the gold atoms
   • DThey were all deflected through about 90°

   (b)

   A very small fraction of the alpha particles were deflected through large angles. Which conclusion is best supported by this observation?

   [1 mark]

   • AElectrons are concentrated in a small central region
   • BAtoms contain neutrons that repel alpha particles
   • CThe positive charge and most of the mass of an atom are concentrated in a tiny nucleus
   • DAn atom is a uniform sphere of positive charge

   (c)

   Explain how the results of this experiment led scientists to conclude that the nucleus is positively charged and that it contains most of the mass of the atom.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A forensic laboratory identifies an iron compound in a paint sample. The iron is present as the ion ⁵⁶Fe³⁺, and the analyst's data sheet gives the atomic number of iron as 26.

   (a)

   How many neutrons are there in one ⁵⁶Fe³⁺ ion?

   [1 mark]

   • A26
   • B30
   • C56
   • D23

   (b)

   How many electrons are there in one ⁵⁶Fe³⁺ ion?

   [1 mark]

   • A26
   • B29
   • C30
   • D23

   (c)

   A second sample contains the ion ⁵⁴Fe²⁺. State the number of protons, the number of neutrons and the number of electrons in one ⁵⁴Fe²⁺ ion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Magnesium has three naturally occurring isotopes, ²⁴Mg, ²⁵Mg and ²⁶Mg. A chemist is comparing atoms of these isotopes. The atomic number of magnesium is 12.

   (a)

   Explain what is meant by the term isotopes, and state the number of protons, neutrons and electrons in an atom of ²⁵Mg.

   [3 marks]

   (b)

   Explain why the three isotopes of magnesium have the same chemical properties, but a sample of ²⁶Mg has a greater mass and density than the same number of ²⁴Mg atoms.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A science museum is preparing a gallery on atomic structure. One panel is a timeline showing how the model of the atom changed. Another panel displays four species, ¹⁸O²⁻, ¹⁹F⁻, ²⁰Ne and ²³Na⁺ (atomic numbers 8, 9, 10 and 11), beneath the caption 'These four are all isotopes of each other because they all have the same number of electrons.'

   (a)

   The timeline panel must explain how the plum pudding model was replaced by the nuclear model and why the neutron was later added. Describe how the alpha particle scattering experiment led to the nuclear model, and explain why the discovery of the neutron was needed to account for the masses of atoms.

   [6 marks]

   (b)

   Evaluate the claim in the caption on the second panel. Use the numbers of protons, neutrons and electrons in each of the four species in your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions