Saturday, May 2, 2015

Physics 9702 Doubts | Help Page 129

  • Physics 9702 Doubts | Help Page 129

Question 647: [Radioactive decay]
Nucleus of bohrium xyBh decays to mendelevium 255101Md by a sequence of three α-particle emissions.

How many neutrons are there in a nucleus of xyBh?
A 267                          B 261                          C 160                          D 154

Reference: Past Exam Paper – June 2002 Paper 1 Q40

Solution 647:
Answer: C.
An α-particle is a helium nucleus and thus, is 42α.

Since 3 α-particles are emitted,
xyBh     - - - >   255101Md          +          3 42α
Value of x = 255 + 3(4) = 267
Value of y = 101 + 3(2) = 107

Number of neutrons = x – y = 267 – 107 = 160

Question 648: [Nuclear Physics]
A thin gold foil is bombarded with α-particles as shown.

Results of this experiment provide information about the
A binding energy of a gold nucleus.
B energy levels of electrons in gold atoms.
C size of a gold nucleus.
D structure of a gold nucleus.

Reference: Past Exam Paper – June 2004 Paper 1 Q39

Solution 648:
Answer: C.
This experiment (bombardment of a thin gold foil with α-particles) tells us about the size of the nucleus, not its structure.

Most of the α-particles pass through the foil. This indicates that the gold atom is mostly empty spaced. That is, the size of the nucleus is much smaller than that of the atom.

A few of the α-particles are deflected at very large angles – this tells that something massive is present in the atom and this is the nucleus as we know.

A few of the α-particles are reflected back. This tells us that the nucleus is positively charged as it repels the α-particles which are themselves positively charged.

But none of these deductions tells us about the structure of the gold nucleus – that is, that the gold nucleus contains protons and neutrons.

Question 649: [Radioactive decay]
Student conducts an experiment using an α-particle source.
When considering safety precautions, what can be assumed to be maximum range of
α-particles in air?
A between 0 and 5 mm
B between 5 mm and 200 mm
C between 200 mm and 500 mm
D between 500 mm and 1000 mm

Reference: Past Exam Paper – June 2005 Paper 1 Q39

Solution 649:
Answer: B.
In this question, we need to be careful of the unit used for the range. The values are given in mm.

The range of α-particles in air is about 0 – 5cm.

Choice A is in mm, so it is incorrect.
Choice D is almost completely out of the range while choice C neglects a significant portion of the range (0 – 199mm is neglected).

Question 650: [Nuclear Physics]
Which conclusion can be drawn from the results of the experiment showing scattering of α-particles by gold foil?
A Electrons orbit the atomic nucleus in well-defined paths.
B Nuclei of different isotopes contain different numbers of neutrons.
C The atomic nucleus contains protons and neutrons.
D The nucleus is very small compared with the size of the atom.

Reference: Past Exam Paper – November 2008 Paper 1 Q38

Solution 650:
Answer: D.
Most of the alpha particles pass through the gold foil. This indicates that the nucleus is very small compared with the size of the atom.


  1. Here are the remaining questions for now:

    22/O/N/10 Q.4(b)(ii)

    21/O/N/11 Q.5(b)

    23/O/N/11 Q.6(c)

    23/M/J/12 Q.2(c)

    21/O/N/12 Q.6(c)

    22/O/N/12 Q.3(b)

    23/O/N/12 Q.1(e)

    21/M/J/13 Q.6(c)(d)

    22/O/N/13 Q.3(c)(ii)

    23/O/N/13 Q.5(c)

    1. For 22/O/N/10 Q.4(b)(ii)


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