Question
Fig. 6.1 shows alpha particles W, X, Y and Z moving towards a gold nucleus that is in thin foil.
(a) (i) The paths of particles X and Z are shown.
Complete Fig. 6.1 to show two possible paths for particles W and Y. (3 marks)
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(ii) Describe what may be inferred about the structure of an atom from the path of particle X. (1 mark)
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(iii) When a beam containing many alpha particles is incident on thin gold foil, nearly all of the alpha particles follow paths that are similar to the path of particle Z.
Describe what may be inferred from this about the structure of an atom. (1 mark)
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(b) State the mass and the charge, in terms of the atomic mass unit \(u\) and the elementary charge \(e\), of an alpha particle. (2 marks)
mass = __________________________ \(u\)
charge = __________________________ \(e\)
(c) There are two types of hadron.
The hadrons that are in alpha particles are each composed of three quarks.
(i) State the name of this type of hadron. (1 mark)
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(ii) Show, by reference to their constituent quarks, that the hadrons in an alpha particle have charges of either zero or \(+1e\). (2 marks)
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Syllabus Topic Codes (Cambridge International AS & A Level Physics 9702):
• 11.2: Fundamental particles — parts (c)(i) and (c)(ii)
▶️ Answer/Explanation
(a)(i) Paths of alpha particles W and Y [3 marks]
Alpha particles are positively charged, so they are repelled by the positively charged gold nucleus.
Particle W should therefore be deflected upwards. Its deflection is the mirror image of the path of X.
Particle Y passes further from the nucleus than X, so it experiences a smaller repulsive force and is deflected downwards by a smaller angle.
Answer: \( \boxed{\text{W deflects upwards; Y deflects downwards by less than X}} \)
(a)(ii) Structure of the atom from particle X [1 mark]
The large deflection of particle X shows that the atom contains a small, positively charged nucleus.
Answer: \( \boxed{\text{the nucleus is charged and contains most of the mass of the atom}} \)
(a)(iii) Structure of the atom from particle Z [1 mark]
Nearly all alpha particles pass through the foil with little or no deflection.
Therefore, the atom is mostly empty space, with the nucleus occupying a very small proportion of the atom.
Answer: \( \boxed{\text{the atom is mostly empty space}} \)
(b) Mass and charge of an alpha particle [2 marks]
An alpha particle is a helium nucleus containing two protons and two neutrons.
Its mass is approximately \(4u\).
Its charge is \(+2e\).
Answer:
\( \boxed{\text{mass}=4u} \)
\( \boxed{\text{charge}=+2e} \)
(c)(i) Type of hadron [1 mark]
A hadron composed of three quarks is called a baryon.
Answer: \( \boxed{\text{baryon}} \)
(c)(ii) Charges of the hadrons in an alpha particle [2 marks]
The charge of an up quark is \(+\dfrac{2}{3}e\), while the charge of a down quark is \(-\dfrac{1}{3}e\).
A proton has quark composition \(uud\), so its charge is
\(\left(2\times\dfrac{2}{3}e\right)-\left(\dfrac{1}{3}e\right)=+1e\)
A neutron has quark composition \(udd\), so its charge is
\(\left(\dfrac{2}{3}e\right)-\left(2\times\dfrac{1}{3}e\right)=0\)
An alpha particle contains two protons and two neutrons. Therefore, the hadrons in an alpha particle have charges of either \(+1e\) or \(0\).
Answer: \( \boxed{\text{proton}=+1e,\quad \text{neutron}=0} \)