Answers and Explanations

REVIEW QUESTIONS

1 . B

Emission    of  three   alpha   particles   by  the (as yet unknown)    parent  results in  the following

changes:

Mass    number: decreases   by  3   ×   4   or  12  units

Atomic  number: decreases   by  3   ×   2   or  6   units

Emission    of  two negative    betas   results in  the following   changes:

Mass    number: no  change

Atomic  number: increases   by  2   ×    1   or 2   units

So  the net change  is: Mass    number  decreases   by  12  units;  atomic  number  decreases   by  4   units.

Therefore,  the mass    number  of  the parent  is  12  greater than    102,    or  114;    the atomic  number  of

the parent  is  4   greater than    20, or  24. The only    choice  given   with    these   numbers is  B.

2 .  12

Since   the half-life   of  element X   is  4   days,   then    50% of  an  original    sample  remains after   4   days,

25% of  an  original    sample  remains after   8   days,   and 12.5%   of  an  original    sample  remains after

12  days.   Thus    N   =   12  days.   A   different   approach    is  to  set (1/2)n  =   0.125   where   n   is  the number  of

half-lives  that    have    elapsed.    Solving for n,  gives   n   =   3.  Thus    3   half-lives  have    elapsed,    so  given

the half-life   is  4   days,   N   =   (3)(4   days),  or  12  days.

3 .  3

Given   that    the half-life   of  ^131 I  is  8.05    days,   2   half-lives  have    elapsed after   16.1    days,   which

means   that    25% of  the original    amount  of  ^131 I  is  still   present.    Thus,   only    25% of  the original