Fig. 1 shows the path of alpha particles near a thin gold foil in a museum reconstruction of a scattering experiment. The source produces a narrow beam. The...
Question 1 Report
Fig. 1 shows the path of alpha particles near a thin gold foil in a museum reconstruction of a scattering experiment. The source produces a narrow beam. The detector can be moved around the foil to count particles at different angles. Students observe that the beam is not attracted towards the foil. Their teacher asks them to use the paths to infer the charge and position of particles within an atom.
(a) What is the charge on an alpha particle? [1]
(b) Describe why some alpha particles were deflected as they passed the foil. [2]
(c) Which part of the atom caused the deflection? [1]
(d) Give the relative mass of an electron. [1]
(e) Calculate the number of neutrons in a gold-197 atom with proton number 79. [1]
Answer Details
(a) An alpha particle has relative charge \(+2\). [1]
(b) Alpha particles are positively charged. When they pass close to the positively charged nucleus, they are repelled, so their paths are deflected. [2]
(c) The part of the atom that causes the deflection is the nucleus. [1]
(d) The relative mass of an electron is \(\frac{1}{1836}\), often treated as approximately zero compared with protons and neutrons. [1]
(e) Neutrons = mass number − proton number:
\[197-79=118\]
A gold-197 atom has 118 neutrons. [1]
Examination reminder: Repulsion shows that the nucleus is positive. Alpha particles are not attracted because they are also positive.
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