The Foundations of Chemistry

From an electrostatic point of view, it is amazing that positively charged protons can

be packed so closely together. Yet many nuclei do not spontaneously decompose, so they

must be stable. In the early twentieth century when Rutherford postulated the nuclear

model of the atom, scientists were puzzled by such a situation. Physicists have since

detected many very short-lived subatomic particles (in addition to protons, neutrons, and

electrons) as products of nuclear reactions. Well over 100 have been identified. A discus-

sion of these many particles is beyond the scope of a chemistry text. Furthermore their

functions are not entirely understood, but it is now thought that they help to overcome

the proton–proton repulsions and to bind nuclear particles (nucleons)together. The

attractive forces among nucleons appear to be important over only extremely small

distances, about 10^13 cm.

NEUTRON–PROTON RATIO AND NUCLEAR STABILITY

The term “nuclide”is used to refer to different atomic forms of all elements. The term

“isotope” applies only to different forms of the same element. Most naturally occurring

nuclides have even numbers of protons and even numbers of neutrons; 157 nuclides fall

into this category. Nuclides with odd numbers of both protons and neutrons are least

common (there are only four), and those with odd–even combinations are intermediate

in abundance (Table 26-2). Furthermore, nuclides with certain “magic numbers” of protons

and neutrons seem to be especially stable. Nuclides with a number of protons ora number

of neutrons ora sum of the two equal to 2, 8, 20, 28, 50, 82, or 126 have unusual stability.

Examples are^42 He,^1608 O,^4220 Ca,^8838 Sr, and^208082 Pb. This suggests an energy level (shell) model

for the nucleus similar to the shell model of electron configurations.

Figure 26-1 is a plot of the number of neutrons (N) versus number of protons (Z) for

the stable nuclides (the band of stability). For low atomic numbers, the most stable

nuclides have equal numbers of protons and neutrons (NZ). Above atomic number 20,

the most stable nuclides have more neutrons than protons. Careful examination reveals

an approximately stepwise shape to the plot due to the stability of nuclides with even

numbers of nucleons.

26-2

1004 CHAPTER 26: Nuclear Chemistry

The nuclide symbol for an element
(Section 5-7) is
A
ZE
where Eis the chemical symbol for the
element, Zis its atomic number, and A
is its mass number.

TABLE 26-1 Fundamental Particles of Matter

Particle Mass Charge

Electron (e) 0.00054858 amu 1 

Proton (por p) 1.0073 amu 1 

Neutron (nor n^0 ) 1.0087 amu none

TABLE 26-2 Abundance of Naturally Occurring Nuclides

Number of protons even even odd odd

Number of neutrons even odd even odd

Number of such nuclides 157 52 50 4