bei48482_FM

Nuclear Structure 397

spins. Energy levels in nuclei are filled in sequence, just as energy levels in atoms are,

to achieve configurations of minimum energy and therefore maximum stability. Thus

the boron isotope^125 B has more energy than the carbon isotope^126 C because one of its

neutrons is in a higher energy level, and^125 B is accordingly unstable (Fig. 11.8). If

140

130

120

110

100

90

90

80

80

70

70

60

60

50

50

20

20

10

10

30

0 30

Proton number (Z)

Stable nuclei

Neutron number ( N = Z

N

)

40

40

Figure 11.7Neutron-proton diagram for stable nuclides. There are no stable nuclides with Z43 or

61, with N19, 35, 39, 45, 61, 89, 115, 126, or with AZN5 or 8. All nuclides with Z83,

N126, and A209 are unstable.

( )

Stable Stable Unstable Stable Stable

10

5 B

11

5 B

12

6 C

13

6 C

12

5 B

Energy

Neutron

Proton

Figure 11.8Simplified energy-level diagrams of some boron and carbon isotopes. The exclusion

principle limits the occupancy of each level to two neutrons of opposite spin and two protons of

opposite spin. Stable nuclei have configurations of minimum energy.

bei48482_ch11.qxd 1/23/02 3:14 AM Page 397 RKAUL-9 RKAUL-9:Desktop Folder: