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Question 647: [Radioactive decay]

Nucleus of bohrium ^x_yBh decays to mendelevium ²⁵⁵₁₀₁Md by a sequence of three α-particle emissions.

How many neutrons are there in a nucleus of ^x_yBh?

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,

^x_yBh     - - - >   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.