The Foundations of Chemistry

96

42 Mo

2

1 H 88n

97

43 Tc

1

0 n

209

083 Bi

4

2 88n

210

085 At^3

1

0 n

230

090 Th

1

1 p 88n

223

087 Fr^2

4

2

Many hitherto unknown, unstable, artificial isotopes of known elements have also been

synthesized so that their nuclear structures and behavior could be studied.

Neutron Bombardment

Neutrons bear no charge, so they are not repelled by nuclei as positively charged projec-

tiles are. They do not need to be accelerated to produce bombardment reactions. Neutron

beams can be generated in several ways. A frequently used method involves bombardment

of beryllium-9 with alpha particles.

(^94) Be (^42) 88n (^1206) C (^10) n
Nuclear reactors (Section 26-15) are also used as neutron sources. Neutrons ejected in
nuclear reactions usually possess high kinetic energies and are called fast neutrons.When
they are used as projectiles they cause reactions, such as (n, p) or (n,^ ) reactions, in which
subsidiary particles are ejected. The fourth “missing” element, number 61 (promethium),
was synthesized by fast neutron bombardment of neodymium-142.
(^142060) Nd (^10) n88n (^143061) Pm (^01)
Slow neutrons(“thermal” neutrons) are produced when fast neutrons collide with
moderatorssuch as hydrogen, deuterium, oxygen, or the carbon atoms in paraffin. These
neutrons are more likely to be captured by target nuclei. Bombardments with slow
neutrons can cause neutron-capture (n,) reactions.
(^200080) Hg (^10) n88n (^201080) Hg (^00) 
Slow neutron bombardment also produces the^3 H isotope (tritium).
(^63) Li (^10) n88n (^31) H (^42) (n, ) reaction
E. M. McMillan (1907–1991) discovered the first transuranium element, neptunium, in
1940 by bombarding uranium-238 with slow neutrons.
(^238092) U (^10) n88n (^239092) U (^00) 
(^239092) U88n (^239093) Np (^01)
Several additional elements have been prepared by neutron bombardment or by bombard-
ment of the nuclei so produced with positively charged particles. Some examples are
(^238092) U (^10) n88n (^239092) U (^00) 
(^239092) U88n (^239093) Np (^01)

plutonium
(^239093) Np88n (^239094) Pu (^01)
(^239094) Pu (^42) 88n (^242096) Cm (^10) n curium
(^246096) Cm (^1206) C88n (^254102) No 4 10 n nobelium
(^252098) Cf (^1005) B88n (^257103) Lr 5 10 n lawrencium
1024 CHAPTER 26: Nuclear Chemistry
Fast neutrons move so rapidly that
they are likely to pass right through
a target nucleus without reacting.
Hence, the probability of a reaction is
low, even though the neutrons may be
very energetic.