The Foundations of Chemistry

A positron has the mass of an electron but a positive charge. Positrons are emitted
when protons are converted to neutrons.

1

1 p88n

1

0 n

 0

 1

Thus, positron emission results in a decreaseby one in atomic number and an increaseby
one in the number of neutrons, with no changein mass number.

38

19 K88n

38

18 Ar

 0

 1

 and

15

08 O88n

15

07 N

 0
 1

The same effect can be accomplished by electron capture (Kcapture), in which an elec-
tron from the Kshell (n1) is captured by the nucleus.

106

047 Ag

 0

 1 e88n

106

046 Pd and

37

18 Ar

 0

 1 e88n

37

17 Cl

Some nuclides, such as^2211 Na, undergo both electron capture and positron emission.

22

11 Na

 0

 1 e88n

22

10 Ne (3%) and

22

11 Na88n

22

10 Ne

 0

 1

 (97%)

Some of the neutron-poor nuclei, especially the heavier ones, increasetheir neutron-to-
proton ratios by undergoing alpha emission.Alpha particles are helium nuclei,^42 He,
consisting of two protons and two neutrons. Alpha emission also results in an increase of
the neutron-to-proton ratio. An example is the alpha emission of lead-204.

204

082 Pb88n

200

080 Hg

4

2

NUCLEI WITH ATOMIC NUMBER GREATER THAN 83

Allnuclides with atomic number greater than 83 are beyond the band of stability and are
radioactive. Many of these decay by alpha emission.

(^226088) Ra88n (^222086) Rn (^42) and (^210084) Po88n (^206082) Pb (^42)
The decay of radium-226 was originally reported in 1902 by Rutherford and Soddy. It
was the first transmutation of an element ever observed. A few heavy nuclides also decay
by beta emission, positron emission, and electron capture.
Some isotopes of uranium (Z92) and elements of higher atomic number, the
transuranium elements,also decay by spontaneous nuclear fission. In this process a heavy
nuclide splits into nuclides of intermediate mass and neutrons.
(^252098) Cf88n (^142056) Ba (^106042) Mo 4 10 n
DETECTION OF RADIATION
Photographic Detection
Emanations from radioactive substances affect photographic plates just as ordinary visible
light does. Becquerel’s discovery of radioactivity resulted from the unexpected exposure
of such a plate, wrapped in black paper, by a nearby enclosed sample of a uranium-
containing compound, potassium uranyl sulfate. After a photographic plate has been
developed and fixed, the intensity of the exposed spot is related to the amount of radia-
tion that struck the plate. Quantitative detection of radiation by this method is difficult
and tedious.
26-9
26-8
26-9 Detection of Radiation 1011
Electron capture by the nucleus differs
from an atom gaining an electron to
form an ion.
-particles carry a double positive
charge, but charge is usually not
shown in nuclear reactions.
The only stable nuclide with atomic
number 83 is^209083 Bi.