10—12. THE BINDING ENERGY
235
'
3 X 1.0090 = 3.0270, making a total of 6.0513 maSs units.
But this is 00346 mass units greater than the observédnuclear
.
weight of 6.0167. If the theory is correct, it must be con‘cluded
that the elementary particles have less mass, when bound together
in a nucleus, than the sum of their masses when they are free.
'
Now, the relativity theory states that mass and energy are interchangeable. Thus, using equation 2—27, we may Say that the
_
binding energy of the lithium atom is 0.0346 mass units, 51.3 micro’
ergs or 32.2 MeV, compared With free protons and neutrons. This _
is a measure of the sz‘aâilz‘z‘y of the lithium nucleus, or, in other
words, a measure of the
of breaking. down a lithium
nucleus into free neutrons and protons.
By comparison of the
sums of the masses of helium (He4) and deuterium (DZ) or of
helium and tritium (T3), it cari be shoWn that the lithium six
nucleus could be broken
into a helium nucleus (called
’
an
alpha particle)
deuteron than into an helium nucleus
and a tritium nucleus. Thus, the greater the_binding energy, the
more stable the nucleus.
»
[
It has been suggested by astro-physicists that the energy
ated from the sun and the stars cônsists of that which is lOst when
heavier atoms
from lighter particles. Only a small mass
is needed to give a comparatively large amount
of radiant energy.
dots and circles in gure 10—11 represent the known
is‘Otopes from Z = 25
in smâlæ: farm in Nature:
Z and N are, according to theory, the respective.
numbers
pro'
tons.and neutrons in the
the isotopes
,
lie
This means
f
that for a given elementWlthconstantZvalæ iN'has & limited
range;
formsanunstale_atûm which» does not
'
_eXist1£innaturalstate-‘? Slmllarly,f@raglven Nvalue, Z is restricted
_