1. 1
   A student is studying an atom of the isotope carbon-14, which is used in radiocarbon dating. The nucleus of this atom can be represented using the symbol notation $^{14}_{6}\text{C}$. The atom has 6 protons and is electrically neutral overall.

   (a)

   How many neutrons are there in a nucleus of carbon-14?

   [1 mark]

   • A6
   • B8
   • C14
   • D20

   (b)

   Why is the atom of carbon-14 electrically neutral overall?

   [1 mark]

   • AIt contains no electrons
   • BThe nucleus itself carries no charge
   • CThe number of neutrons equals the number of protons
   • DThe number of electrons equals the number of protons

   (c)

   State two ways in which the structure of a carbon-14 atom differs from the structure of a carbon-12 atom, in terms of subatomic particles.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In 1909, scientists fired a beam of alpha particles at a very thin sheet of gold foil, only a few atoms thick, and recorded where the particles landed using a fluorescent screen surrounding the foil. Before this experiment, the accepted model of the atom was the 'plum pudding' model, which described the atom as a sphere of positive charge with negatively charged electrons embedded throughout it, like plums in a pudding.

   (a)

   According to the plum pudding model, what result would scientists have expected when firing alpha particles at the gold foil?

   [1 mark]

   • AAll the alpha particles would pass straight through with little or no deflection
   • BMost alpha particles would bounce straight back towards the source
   • CAll the alpha particles would be absorbed by the foil
   • DThe alpha particles would all be deflected through exactly 90 degrees

   (b)

   What was the unexpected observation from the experiment that could not be explained by the plum pudding model?

   [1 mark]

   • AThe alpha particles changed into gamma rays after passing through the foil
   • BAll of the alpha particles were absorbed by the gold foil
   • CA very small number of alpha particles were deflected through large angles or bounced straight back
   • DThe gold foil itself became radioactive after the experiment

   (c)

   Explain what the unexpected observation described in part (b) revealed about the structure of the atom.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An electron in an atom can occupy only set energy levels (orbits) around the nucleus, at fixed distances from the nucleus. In one process, an electron in an atom absorbs a burst of electromagnetic radiation and moves to an orbit further from the nucleus. In a separate atom, the outermost electron instead gains enough energy to leave the atom completely.

   (a)

   Explain, in terms of electron orbits and electromagnetic radiation, what happens when the electron described above absorbs a burst of electromagnetic radiation.

   [3 marks]

   (b)

   Explain how the atom described above, which loses its outermost electron, becomes a positive ion, and state the effect this has on the overall charge of the atom.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Scientists' understanding of atomic structure changed considerably over time. J. J. Thomson proposed the plum pudding model in 1904, describing the atom as a sphere of positive charge with negative electrons embedded within it. Ernest Rutherford's alpha particle scattering experiment (1909) showed that although most alpha particles passed straight through a thin gold foil, a small number were deflected through large angles. Niels Bohr later proposed that electrons orbit the nucleus only at fixed, distinct distances (energy levels), rather than existing in a diffuse cloud around it.

   (a)

   Explain how the results of Rutherford's alpha particle scattering experiment led scientists to replace the plum pudding model with a nuclear model of the atom.

   [6 marks]

   (b)

   Explain why Bohr's proposal that electrons occupy only fixed orbits was an improvement on Rutherford's nuclear model, and describe the evidence that supports the existence of these fixed electron energy levels.

   [6 marks]

   Total for question 4: 12 marks

End of questions