232
10.
POSITIVE RAYS AND ISOTOPES
_
electr0ns.
The weight of these electrons in mass units is equal
to 000055 multiplied by Z, the atomic number. Thus, the nuclear
_
'
weight of the oxygen sixteen isotope is 160000 — 8 X 000055
=
159956 mass units.
10—9.
The Case of Hydrogen.—In 1927, Aston found the
atomic weight of hydrogen to be 100778, in good agreement with
the international value 100777.
But, in 1929, the 018 isotope
was
discovered.
With 018 as a standard, the mass spectrum
value for hydrogen would have been 100756, which is too small.
On the other hand, if the international atomic weight of oxygen
had been taken as slightly greater than 16, in order to allow for the
heavier isotope, then the hydrogen value would have been too
large to agree with that of Aston.
In 1931, Birge and Menzel
,
pointed out_that the discrepancy might be due to the existence of
,
heavier isotopes of hydrogen.
In 1932, Urey
18
sought for and established the existence of
an isotope of hydrogen whose mass was nearly twice that of
ordinary hydrogen. This isotope has been named deuterium and
‘
its symbol is H2 or D. The nucleus of the deuterium atom is
called the deuieron. It has about twice the mass of the proton but
carries the same electrical charge. H1 is about five thousand times
as abundant as DZ.
The atomic weight of H1 was remeasured by
-
Bainbridge and by Aston and found to be 100812.
.
“
Heavy water
”
or deuterium oxide (D20) may be prepared în
1
a
very nearly pure form by extended electrolysis of
;
The density of the heavy water is 1.1079 times that of ordinary
water.
number of possible chemical combinations has been
_
greatly extended by the discovery of deuterium. A third isotope
of hydrogen, T3, has also been found, but it is even rarer than D2..
10—10.‘
The Mass Defect.—The inc… numôer, Â, of an atom
-
is the nearest whole number to its isotopic weight, M. The màss
,
‘
def-ect (A) is the dierence‘ between the isotopic weight andthe
_
mass number. The defect is very small, never exceeding 0.1 mass
"
units, but is a real difference and not an experimental error.
…
_.
'
.'
_
A = M— Â.
(IO—3)
The measured masses (M) are greater than the mass numbers (Â) '
-
and for the very heavy elements, as indicated
_
10.
POSITIVE RAYS AND ISOTOPES
_
electr0ns.
The weight of these electrons in mass units is equal
to 000055 multiplied by Z, the atomic number. Thus, the nuclear
_
'
weight of the oxygen sixteen isotope is 160000 — 8 X 000055
=
159956 mass units.
10—9.
The Case of Hydrogen.—In 1927, Aston found the
atomic weight of hydrogen to be 100778, in good agreement with
the international value 100777.
But, in 1929, the 018 isotope
was
discovered.
With 018 as a standard, the mass spectrum
value for hydrogen would have been 100756, which is too small.
On the other hand, if the international atomic weight of oxygen
had been taken as slightly greater than 16, in order to allow for the
heavier isotope, then the hydrogen value would have been too
large to agree with that of Aston.
In 1931, Birge and Menzel
,
pointed out_that the discrepancy might be due to the existence of
,
heavier isotopes of hydrogen.
In 1932, Urey
18
sought for and established the existence of
an isotope of hydrogen whose mass was nearly twice that of
ordinary hydrogen. This isotope has been named deuterium and
‘
its symbol is H2 or D. The nucleus of the deuterium atom is
called the deuieron. It has about twice the mass of the proton but
carries the same electrical charge. H1 is about five thousand times
as abundant as DZ.
The atomic weight of H1 was remeasured by
-
Bainbridge and by Aston and found to be 100812.
.
“
Heavy water
”
or deuterium oxide (D20) may be prepared în
1
a
very nearly pure form by extended electrolysis of
;
The density of the heavy water is 1.1079 times that of ordinary
water.
number of possible chemical combinations has been
_
greatly extended by the discovery of deuterium. A third isotope
of hydrogen, T3, has also been found, but it is even rarer than D2..
10—10.‘
The Mass Defect.—The inc… numôer, Â, of an atom
-
is the nearest whole number to its isotopic weight, M. The màss
,
‘
def-ect (A) is the dierence‘ between the isotopic weight andthe
_
mass number. The defect is very small, never exceeding 0.1 mass
"
units, but is a real difference and not an experimental error.
…
_.
'
.'
_
A = M— Â.
(IO—3)
The measured masses (M) are greater than the mass numbers (Â) '
-
and for the very heavy elements, as indicated
_