Power Plant Engineering

Fig. 10.1
As the mass of a neutron or a proton is nearly 1 amu, A is the integer nearest the mass of the
nucleus which in turn is approximately equal to the atomic mass of the atom. Isotopes of the same
element thus have the same atomic number but differ in mass number. Nucleus symbols are written
conventionally as
Z
X
A
where X is the usual chemical symbol. Thus the hydrogen nucleus is 
1
H
1
, deuterium is 
1
H
2
(and some-
times D), and ordinary helium is 
2
He
1
. For particles containing no protons, the subscript indicates the
magnitude and sign of the electric charge. The an electron is – e° (sometimes e or 
β
) and a neutron is 
0
n
1
.
Symbols are also often written in the form He-4, helium-4, etc. Another system of notation, written as 7.
will not be used in this text.
Fig. 10.2 shows, schematically, the structure of H
1
, He
4
and some heavier atoms and the distri-
bution of their electrons in various orbits. Two other particles of importance are the positron and the
neutrino. The positron is a positively charged electron having the symbols 
+1
e
0
, e
+
or 
β
+
. The neutrino
(little neutron) is a tiny electrically neutral particle that is difficult to observe experimentally. Initial
evidence of its existence was based on theoretical considerations, nuclear reactions where a/3 particle of
either kind is emitted or captured, the resulted energy (corresponding to the lost mass) was not all
accounted for by the energy the emitted 13 particle and the recoiling nucleus. It was first suggested by
Wolfgai Pauli in 1934 that the neutrino was simultaneously ejected in these reactions and the it carried
the balance of the energy, often larger than that carried by the 
β