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INTERNATIONAL UNION OFPURE AND APPLIED CHEMISTRY
INORGANIC CHEMISTRY DIVISIONCOMMISSION ON ATOMIC WEIGHTS
ATOMIC WEIGHTSOF THE ELEMENTS
1973
LONDON
BUTTER WORTHS
INORGANIC CHEMISTRY DIVISION
COMMISSION ON ATOMIC WEIGHTS*
ATOMIC WEIGHTS OF THE ELEMENTS
CHANGES IN ATOMIC WEIGHT VALUES
On the basis of work published, accepted for publication, or re-evaluatedfrom earlier publications the Commission recommends only two changesin atomic weights: namely, for nickel 58.70 (instead of the former value of58.7k)' and for rhenium 186.207 (instead of the former value of 186.2). Thechange for nickel is well within the formerly implied uncertainty of 0.03 andthat uncertainty has been decreased by a factor of three. The new value forrhenium is consistent with the former less precise value but the uncertaintyhas been reduced by a factor of 100, a rare event in the history of atomicweight determinations. These changes in value were given to the technicalpress in a news release immediately following the XXVII IUPAC Conferenceheld in Munich, Germany, August 1973. The reasons for the two changesare set out below.
Nickel is an important and abundant element in nature but its atomicweight is among the least precisely known for reasons other than excessivevariability in 'normal' materials. The best chemical determinations of AXNi)are in a series of papers by Baxter using the Harvard titrimetric method.Baxter and Parsons2 analyzed nickel(n) oxide and obtained Ni/O = 3.66887for six determinations of terrestrial nickel. This gave AJNi) = 58.700 (thisand all other values here quoted were recalculated to conform to the scaleAj'2C) = 12 exactly). Baxter and Hilton3 analyzed nickelQi) chloride andobtained NiCI2/2Ag = 0.6007 for six determinations and NiCl2/2AgCl =0.452118 for two determinations of terrestrial nickel. This gave Ar(Ni) =58.692. Baxter and Ishimaru4 analyzed nickel0i) bromide and obtainedNiBr2/2Ag = 1.0 12829 for five determinations and NiBr2/2AgBr = 0.58 1817for three determinations of terrestrial nickel. This gave Ar(N1) = 58.695. Areasonable value from these results is A(Ni) = 58.695 ± 0.005. When Whiteand Cameron5 published their mass spectrometric data, which gave Ar(Ni) =58.7 10 after recalculation to AJ'2C) = 12, the Commission in 1955 preferredthat value to the chemical one. However, the still unexplained discrepancywith the chemical value led the Commission in 1969 to admit the possibilityof an error of 0.03 in that value thus amply encompassing both the physicaland the chemical values. It now appears that White and Cameron may have
* Titular Members: N. N. Greenwood. Chairman (UK): H. S. Peiser. Secretary (USA):P. Dc Biêvre (Belgium): S. Fuiiwara (Japan): N. F. Holden (USA): W. H. Johnson (USA):W. W. Meinke (USA): F. Roth (France): Associate Members: A. E. Cameron (USA): ci N.Flerov (USSR): R. U Martin (Australia): N. Saito (Japan): H. G. Thode (Canada) and A. H.Wapstra (The Netherlands).
591
TA
BLE
OF
AT
OM
IC W
EIG
HT
S
1973
Scal
ed t
o th
e re
lativ
e at
omic
mas
s. A
2C)
= 1
2
The
atom
ic w
eigh
ts o
f m
any
elem
ents
are
not
inva
rian
t hu
t dep
end
on th
e or
igin
and
tre
atm
ent o
f th
e m
ater
ial. T
he fo
otno
tes t
o th
is T
able
ela
bora
te th
e ty
pes
of v
aria
tion
to b
e ex
pect
ed
for
indi
vidu
al
elem
ents
. T
he v
alue
s of
A(E
) giv
en h
ere
appl
y to
ele
men
ts
as th
ey e
xist
nat
ural
ly o
n ea
rth
and
to c
erta
in a
rtif
icia
l el
emen
ts.
Whe
n us
ed w
ith d
ue re
gard
to th
e fo
otno
tes t
hey
are
cons
ider
ed r
elia
ble
to ±
I in
the
last
dig
it or
± 3
whe
n fo
llow
ed b
y an
ast
eris
k *,
Val
ues i
n pa
ren-
th
eses
are
use
d fo
r cer
tain
rad
ioac
tive e
lem
ents
w
hose
ato
mic
wei
ghts
can
not
be q
uote
d pr
ecis
ely
with
out k
now
ledg
e of
ori
gin:
the
valu
e gi
ven
is t
he a
tom
ic
mas
s num
ber o
f th
e is
otop
e of t
hat e
lem
ent o
f lon
gest
kno
wn
half
life
.
Nam
e
\ Ct in
ium
A
lum
iniu
m
Am
eric
ium
A
ntim
onv
.c
Arg
on
Ars
enic
A
stat
ine
Bar
ium
B
erke
lium
B
eryl
lium
B
ism
uth
Bor
on
Bro
min
e C
adm
ium
C
aesi
um
Cal
cium
C
alif
orni
um
Car
bon
Cer
ium
C
hlor
ine
Chr
omiu
m
Cob
alt
Cop
per
Cur
ium
D
yspr
osiu
m
Ato
mic
A
tom
ic
Sym
bol
num
ber
wei
ght
Ac
89
(227
) A
l 13
26
.981
54
Am
95
(2
43)
Sb
51
121.
75*
Ar
18
39,9
48*
As
33
74.9
216
At
85
(210
) B
a 56
13
7.34
* B
k 97
(2
47)
Be
4 9.
0121
8 B
i 83
20
8.98
04
B
5 10
.81
Br
35
79.9
04
Cd
48
112.
40
Cs
55
132.
9054
C
a 20
40
.08
Cf
98
(251
) C
6
12.0
11
Ce
58
140.
12
Cl
17
35.4
53
Cr
24
51.9
96
Co
27
58.9
332
Cu
29
63,5
46*
Cm
96
(2
47)
Dy
66
162.
50*
Mer
cury
M
olyb
denu
m
Neo
dym
ium
N
eon
Nep
tuni
um
Nic
kel
Nio
bium
N
itrog
en
Nob
eliu
m
Osm
ium
O
xyge
n Pa
lladi
um
Phos
phor
us
Plat
inum
Pl
uton
ium
Po
loni
um
Pota
ssiu
m
Pras
eody
miu
m
Prom
ethi
um
Prot
actin
ium
R
adiu
m
Rad
on
Rhe
nium
R
hodi
um
Rub
idiu
m
\tom
ic
Ato
mic
Sy
mbo
l nu
mbe
r w
eigh
t
Hg
80
200.
59*
Mo
42
9594
* N
d 60
14
4.24
* N
e 10
20
.179
* N
p 93
23
7.04
82
Ni
28
58.7
0 N
b 41
92
.906
4 N
7
14.0
067
No
It)2
(2
55)
Os
76
190.
2 O
8
15.9
994*
Pd
46
10
6.4
P 15
30
.973
76
Pt
78
195.
09*
Pu
94
(244
) Po
84
(2
09)
K
19
39.0
98*
Pr
59
140.
9077
Pm
61
(1
45)
Pa
91
231.
0359
R
a 88
22
6.02
54
Rn
86
(222
) R
e 75
18
6.20
7 R
h 45
10
2905
5 R
h 37
85
.467
8*
- C
Alp
habe
tical
Ord
er in
Eng
lish
Nam
e Fo
otno
tes
a b.c.
d. g
a a a c.
d. e
c a g b.
d
c c a c. d
a a f fg
C a c
Ele
men
t w
ith o
nly
one
stab
le nu
clid
e.
b E
lem
ent
with
one
pre
dom
inan
t iso
tope
ab
out 9
9 to
100
per
cen
t ab
unda
nce)
va
riat
ions
in
the
isot
opic
com
posi
tion
or e
rror
s in
its
det
erm
inat
ion
have
a
corr
espo
ndin
gly
smal
l ef
fect
on
the
valu
e of
A,(
E).
£
Ele
men
t for
hith
the
valu
e of
A,(
E)
deri
ves
its r
elia
bilit
y fr
om c
alib
rate
d m
easu
rem
ents
(i.e.
from
com
pari
sons
with
syn
thet
ic m
ixtu
res
of kn
own
isot
opic
co
mpo
sitio
n).
d E
lem
ent fo
r whi
ch k
now
n va
riat
ions
in
isot
opic
com
posi
tion
in t
erre
stri
al m
ater
ial
prev
ent
a m
ore
prec
ise
atom
ic w
eigh
t bei
ng g
iven
; A
,(E
) va
lues
sho
uld
be
appl
icab
le to
any
nor
maF
mat
eria
l. E
lem
ent
for w
hich
subs
tant
ial v
aria
tions
in A
, fr
om t
he v
alue
giv
en c
an o
ccur
in c
omm
erci
ally
ava
ilabl
e m
ater
ial b
ecau
se o
f ina
dver
tent
or
undi
sclo
sed
chan
ge
of is
otop
ic co
mpo
sitio
n.
Ele
men
t fo
r whi
ch t
he v
alue
of A
r is
that
of t
he m
ost
com
mon
ly a
vaila
ble
long
-liv
ed i
soto
pe.
Ele
men
t fo
r whi
ch g
eolo
gica
l sp
ecim
ens
are
know
n in
whi
ch t
he e
lem
ent
has
an a
nom
alou
s iso
topi
c co
mpo
sitio
n.
Ein
stei
nium
E
rbiu
m
Eur
opiu
m
Ferm
ium
fl
uori
ne
Fran
cium
G
adol
iniu
m
Gal
lium
G
erm
aniu
m
Gol
d H
afni
um
Hel
ium
H
olm
ium
H
ydro
gen
Indi
um
Iodi
ne
Irid
ium
Ir
on
Kry
pton
L
anth
anum
L
awre
nciu
m
Lea
d L
ithiu
m
Lut
etiu
m
Mag
nesi
um
Man
gane
se
Men
dele
vium
Es
99
(254
) E
r f8
16
7.26
* E
u 63
15
1.96
Fm
10
0 (2
57)
F 9
18.9
9840
Fr
87
(2
23)
Gd
64
157.
25*
Ga
31
69.7
2 G
e 32
72
.59*
A
u 79
19
6.96
65
Hf
72
178.
49*
He
2 4.
0026
0 H
o 67
16
4.93
04
H
1 1.
0079
In
49
11
4.82
1
53
126.
9045
Jr
77
19
2.22
* Fe
26
55
.847
* K
r 36
83
.80
l.a
57
138.
9055
* L
r 10
3 (2
60)
Pb
82
207.
2 l..
i 3
6.94
1*
Lu
71
174.
97
Mg
12
24.3
05
Mn
25
54.9
380
Md
101
(258
)
a a b, c
a b.
d
a e b d. g
c,
d.
c. g
a
Rut
heni
um
Ru
44
101.
07*
Sam
ariu
m
Sm
62
150.
4 Sc
andi
um
Sc
21
44.9
559
a Se
leni
um
Se
34
78.9
6*
Silic
on
Si
14
28.0
86*
d Si
lver
A
g 47
10
7.86
8 c
Sodi
um
Na
11
22.9
8977
a
Stro
ntiu
m
Sr
38
87.6
2 g
Sulf
ur
5 16
32
.06
d T
anta
lum
T
echn
etiu
m
Ta
Tc
73
43
180.
9479
* (9
7)
b H
Tel
luri
um
Te
52
127.
60*
Ter
bium
T
b 65
15
8.92
54
a T
halli
um
Tho
rium
T
huliu
m
Ti
Th
Tm
81
90
69
204.
37*
232.
0381
16
8.93
42
f. g
a
rn 6
Tin
.
Tita
nium
Sn
T
i 50
22
11
8.69
* 47
.90*
H
Tun
gste
n (W
olfr
am)
Ura
nium
V
anad
ium
X
enon
W
U
V
Xe
74
92
23
54
1838
5*
238.
029
50.9
4 14*
13
1.30
b. c
. e. g
b.
c
e
0 H
m
Ytte
rbiu
m
Yttr
ium
Y
b Y
70
39
17
304*
88
.905
9 a
r-
Zin
c Z
n 30
65
.38
Zir
coni
um
Zr
40
91.2
2 n H
e.g
TA
BL
E O
F A
TO
MIC
WE
IGH
TS
1973
Scal
ed to
the
rela
tive
atom
ic m
ass.
42C
) = 1
2
The
ato
mic
wei
ghts
of m
any
elem
ents
are
not
inva
rian
t but
dep
end
on th
e or
igin
and
trea
tmen
t of t
he m
ater
ial.
The
foo
tnot
es to
this
Tab
le e
labo
rate
the t
ypes
of
var
iatio
n to
be
expe
cted
for
indi
vidu
al e
lem
ents
. T
he v
alue
s of
Ar(
E g
iven
her
e ap
ply
to e
lem
ents
as
they
exi
st n
atur
ally
on
eart
h an
d to
cer
tain
art
ific
ial
elem
ents
. W
hen
used
with
due
reg
ard
to th
e fo
otno
tes
they
are
cons
ider
ed re
liabl
e to
± I
in th
e la
st d
igit
or ±
3 whe
n fo
llow
ed b
y an
ast
eris
k*.
Val
ues
in p
aren
- th
eses
are
use
d fo
r cer
tain
rad
ioac
tive e
lem
ents
w
hose
ato
mic
wei
ghts
can
not
he q
uote
d pr
ecis
ely
with
out k
now
ledg
e of
ori
gin
the
valu
e gi
ven
is th
e at
omic
m
ass n
umbe
r of
the
isot
ope o
f th
at e
lem
ent o
f lon
gest
kno
wn
half
life
.
Ord
er o
f Ato
mic
Num
ber
I H
ydro
gen
2 H
eliu
m
3 L
ithiu
m
4 B
eryl
lium
5
Bor
on
6 C
arbo
n 7
Nitr
ogen
8
Oxy
gen
9 F
luor
ine
10
Neo
n 11
So
dium
12
M
agne
sium
13
A
lum
iniu
m
14
Silic
on
15
Phos
phor
us
16
Sulf
ur
17
Chl
orin
e 18
Argon
19
Pota
ssiu
m
20
Cal
cium
21
Sc
andi
um
22
Tita
nium
23
V
anad
ium
24
C
hrom
ium
1-1
1.00
70
He
4.00
261)
L
i 69
41*
Be
9.t)
12l8
B
10
.81
C
12.0
11
N
14.0
067
0 15
9994
* F
l8.9
984(
) N
e 20
.179
* N
a 22
.989
77
Mg
24.305
Al
2608
154
Si
2808
6*
P 30
.973
76
S 32
.06
Cl
35.453
Ar
39.948*
K
39.0
98*
Ca
40,0
8 Sc
44
,955
0 T
i 47
,90*
V
50
.941
4*
Cr
51 996
b.d
53
b.c
54
c.d.
e.g
55
a 56
cd
.e
57
b.d
58
h,c
59
b.c.
d 60
a
61
c.e
62
a .
63
c.g
64
a 65
d
66
a i
67
d
68
C
69
b.c.
d.g
70
71
g
72
a 73
74
b.c
75
C
76
Iodi
ne
Xen
on
Cae
sium
B
ariu
m
Lan
than
um
Cer
ium
Pr
aseo
dym
ium
N
eody
miu
m
Prom
ethi
um
Sam
ariu
m
Eur
opiu
m
Gad
olin
ium
T
erbi
um
Dys
pros
ium
H
olm
ium
E
rbiu
m
Thu
lium
Y
tterb
ium
L
utet
ium
H
afni
um
Tan
talu
m
Wol
fram
(Tun
gste
n)
Rhe
nium
O
smiu
m
I 12
6.90
45
Xe
131.
30
Cs
132.
9054
B
a 13
7.34
* L
a 13
8.90
55*
Ce
140.
12
Pr
140.
9077
N
d 144.24*
Pm
(145
) Sm
15
0.4
Eu
151.
96
Gd
157.
25*
Tb
158.
9254
D
y 16
2.50
* H
o 16
4.93
04
Er
167.
26*
Tm
16
8.93
42
Yb
173.
04*
Lu
174.
97
Hf
178.
49*
Ta
180.
9479
* W
18
3.85
* R
e 18
6.20
7 O
s 19
0.2
Ato
mic
\to
mic
nu
mbe
r Sy
mbo
l w
eigh
t Fo
otno
tes
Ato
mic
A
tom
ic
num
ber
Nam
e Sy
mbo
l w
eigh
t Fo
otno
tes
a e a b a a a a b C 2
25
Man
gane
se
Mn
54.9
380
a 77
Ir
idiu
m
Jr
192.
22*
26
Iron
Fe
55
.847
* 78
Pl
atin
um
Pt
195.
09*
27
Cob
alt
Co
58.9
332
a 79
G
old
Au
196.
9665
a
28
Nic
kel
Ni
58.7
0 80
M
ercu
ry
Hg
200.
59*
29
Cop
per
Cu
63.5
46*
c. d
81
T
halli
um
Ti
204.
37*
30
Zin
c Z
n 65
.38
82
Lea
d Pb
20
7.2
d. g
31
G
alliu
m
Ga
69.7
2 83
B
ism
uth
Bi
208.
9804
a
32
Ger
man
ium
G
e 72
.59*
84
Po
loni
um
Po
(209
) 33
A
rsen
ic
As
74.9
216
a 85
A
stat
ine
At
(210
) 34
Se
leni
um
Se
78.9
6*
86
Rad
on
Rn
(222
) 0
35
Bro
min
e B
r 79
.904
c
87
Fran
cium
Fr
(2
23)
36
Kry
pton
K
r 83
.80
e 88
R
adiu
m
Ra
226.
0254
f.
g
C
37
Rub
idiu
m
Rb
85.4
678*
c
89
Act
iniu
m
Ac
(227
) 38
St
ront
ium
Sr
87
.62
g 90
T
hori
um
Tb
232.
0381
f.
g
39
Yttr
ium
Y
88
.905
9 a
91
Prot
actin
ium
Pa
23
1.03
59
f C
')
40
Zir
coni
um
Zr
91.2
2 92
U
rani
um
U
238.
029
b. c
. e. g
41
N
iobi
um
Nb
92.9
064
a 93
N
eptu
nium
N
p 23
7.04
82
f C
l)
42
Mol
ybde
num
M
o 95
94*
94
Plut
oniu
m
Pu
(244
) 0
43
Tec
hnet
ium
T
c (9
7)
95
Am
eric
ium
A
m
(243
) 44
R
uthe
nium
R
u 10
1.07
* 96
C
uriu
m
Cm
(2
47)
45
Rho
dium
R
h 10
2.90
55
a 97
B
erke
lium
B
k (2
47)
46
Palla
dium
Pd
10
6.4
98
Cal
ifor
nium
C
f (2
51)
47
Silv
er
Ag
107.
868
c 99
E
inst
eini
um
Es
(254
) nI
48
C
adm
ium
C
d 11
240
100
Ferm
ium
Fm
(2
57)
49
Indi
um
In
114.
82
101
Men
dele
vium
M
d (2
58)
50
Tin
Sn
11
8.69
* 10
2 N
obel
ium
N
o (2
55)
51
Ant
imon
y Sb
12
1.75
* 10
3 L
awre
nciu
m
Lr
(260
) 52
T
ellu
rium
T
e 12
7.60
*
a E
lem
ent w
ith o
nly
one
stab
le nu
clid
e.
Ele
men
t w
ith o
ne p
redo
min
ant i
soto
pe (a
bout
99
to 1
00 p
er c
ent
abun
danc
e);
vari
atio
ns in
the
isot
opic
com
posi
tion
or e
rror
s in
its d
eter
min
atio
n ha
ve a
co
rres
pond
ingl
y sm
all
effe
ct
on th
e va
lue
of A
1(E
).
Ele
men
t fo
r whi
ch t
he v
alue
of
AJE
) der
ives
its
rel
iabi
lity
from
cal
ibra
ted
mea
sure
men
ts (
i.e. f
rom
com
pari
sons
with
syn
thet
ic m
ixtu
res o
f kn
own
isot
opic
co
mpo
sitio
n).
d E
lem
ent
for w
hich
know
n va
riat
ions
in is
otop
ic co
mpo
sitio
n in
terr
estr
ial m
ater
ial
prev
ent a
mor
e pr
ecis
e at
omic
wei
ght b
eing
giv
en;
A(E
) val
ues
shou
ld b
e ap
plic
able
to a
ny 'n
orm
al' m
ater
ial.
C E
lem
ent f
or w
hich
sub
stan
tial v
aria
tions
in
A1
from
the
val
ue g
iven
can
occu
r in
com
mer
cial
ly a
vaila
ble
mat
eria
l bec
ause
of i
nadv
erte
nt or
und
iscl
osed
cha
nge
of is
otop
e co
mpo
sitio
n.
Ele
men
t fo
r whi
ch th
e va
lue
of A
1 is
that
of t
he m
ost
com
mon
ly a
vaila
ble
long
-liv
ed i
soto
pe.
Ele
men
t fo
r whi
ch ge
olog
ical
spe
cim
ens
are
know
n in
whi
ch th
e ele
men
t has
an
anom
alou
s iso
topi
c com
posi
tion.
ATOMIC WEIGHTS OF THE ELEMENTS
overestimated the abundance of 64Ni possibly by a small impurity of 64Znor 48Ti'60. Owing to the large difference between Ar(64Ni) and Ar(N) asmall overestimate of the 64Ni abundance has a relatively large effect on thevalue of A(Ni) determined. Other mass spectrometric values are also con-sistent with this decision: Ar(N) 58,700 Mattraw and Pachucki6 andj4r(1) = 58.700 Inghram and Hess7. The uncertainty is now estimated tobe much less as a result of the lowering of the mass spectroscopic value oflr(Nj) to that determined chemically. The Commission thus now feels
justified in lowering the value of Ar(N) to 58.70 and the implied uncertaintyto 0.01.
Rhenium has been the subject of a very careful study at the US NationalBureau of Standards8 and this permits a substantial improvement in thevalue of the atomic weight by the addition of two further significant figures.Before this work the atomic weight of rhenium was listed as 186.2 and thiswas one of the least precise values for any element (see Fiqure I of the 1971Report'). Natural rhenium consists of two isotopes '85Re and '87Re. thelatter being radioactive with a half-life of 3 x 1010 years or more: thisimplies that it would take at least 17 million years to alter the atomic weightof rhenium by 1 ppm. Two independent calibrated values of the isotopicabundance ratio, based on calibration by synthetic mixtures of the rheniumisotopes. have now been obtained on different instruments: the ratio is'85Re/'87Re = 0.59738 ± 0.00039 which yields atom percentages of '85Re
37.398 ± 0.016 and '87Re 62.602 ± 0.016. The atomic weight calcu-lated from this isotopic composition is 186.20679 ± 0.00031. The authorshave undertaken a very thorough analysis of possible sources of error andtheir full discussion of all experimental procedures gives confidence in thereliability of their results. A complete survey of the constancy in isotopiccomposition of this element in nature was not undertaken. Nevertheless, theCommission had considerable confidence in the experience at the NBS overa period of more than ten years and similar observations of the constancyof rhenium from some other laboratories where rhenium filaments are usedfor mass spectrometry. No major variation in the '85Re/'87Re ratio amongvarious lots of rhenium from different manufacturers have there been re-ported. On the basis of this work the Commission recommends the value of186.207 for the atomic weight of rhenium, with an implied uncertainty of0.001.
Although only these two changes in atomic weight values were made thedecision to make no change for quite a number of other elements was reachedonly after careful consideration of the evidence. The Commission is consciousof significant work being done currently especially at the US NationalBureau of Standards. Techniques which might yield significant new data inthe very near future are under surveillance. Quantitative chemical analysesof high precision are being carried out ever more widely. Their accuracy isbeing limited increasingly by the uncertainties in atomic weight values.
OTHER CHANGES IN THE TABLE
The Commission decided to amplify the heading of the Atomic WeightsTable. The new wording is incorporated in the Tables on pages 592 and 594.
596
ATOMIC WEIGHTS OF THE ELEMENTS
In the Tables of Atomic Weights for 1951 and 1955, the mass number ofthe isotope of longest half-life was listed instead of the atomic weight valueof certain radioactive elements. The Commission, however, in later years,decided to abandon this practice because it has a number of disadvantages.
(i) The use of the mass number is too imprecise to be useful for muchanalytical work.
(ii) Users generally know enough about the source of their material tohave the best useable knowledge of the applicable atomic weight.
(iii) New half-life determinations may change the listed isotope. Asrecently as in 1969, 255No and 256Lr would have been listed whereasnow we have to list 259No and 260Lr.
(iv) The isotope of longest half-life in several instances is not the onemost widely available, e.g. technetium, plutonium and californium.
Consequently in subsequent years the Commission did not indicate anyatomic weight value for these elements. However, the Commission hasbecome increasingly aware that many users were dissatisfied with theomission from the numerical column of any value for now well-knownelements, and, after reviewing possible alternatives, the members decided torevert to the former practice.
Another change indicated in the Table heading is the use of the asteriskinstead of subscript or small-size numbers to indicate uncertainties of threerather than one in the last digit. The Commission realized that these othermethods for indicating uncertainties have caused difficulties for printers andusers. The asterisk offers an additional advantage in being computer readablein well established computer languages.
Minor clarification in the wording of the footnotes has been made.Corresponding to their greater importance to chemists, the footnotes this yearare given greater prominence. They are set in full size print and the referenceto them is made from an additional column of the Table rather than fromsmall superscript letters.
The widespread use of separated stable isotopes of the rare gas elementsis leading to the isotopically depleted material reaching commerce. Whilenatural argon contains so little of the 36 and 38 isotopes that their depletiondoes not greatly affect the atomic weight, the problem is more serious withneon, krypton and xenon in which the atomic weight value may be seriouslyaffected by depleting the element of its rarer but stable isotopes. The footnote'e' has therefore been added to these elements, thereby warning users ofpossible substantial variation of atomic weight values in commercialmaterials.
The Commission wishes to emphasize that the greatly increased use offission-product materials and separated or enriched isotopic materialscarries with it the danger that laboratory samples of anomalous isotopiccomposition could inadvertently be used. For this reason, footnote 'e' isappended not only to Ne, Kr and Xe but also, as previously, to Li, B and U.At present the danger for other elements may seem remote, and the Com-mission does not yet wish to add footnote 'e' to H, He, C, N or 0, thoughthe increasing use of separated isotopes of these elements for a variety ofpurposes requires vigilance both in their use and their subsequent disposal.
Footnote 'g' has been given additionally to Os, Th and U. For osmium
597
P.A.C.—37 -4- Cl
ATOMIC WEIGHTS OF THE ELEMENTS
this is because of the occurrence of nearly pure '870s in rhenium ores fromthe decay of '87Re. For thorium footnote 'g' is added because of the rareoccurrence of almost pure 230Th (ionium) in certain minerals. Footnote 'g'has also been added to uranium. Very small variations in the relative abund-ance of 235U and relatively large variations in 234U have been previouslyclaimed. However, late in 1972. large variations in the abundance of 235Uwere reported9", and, as is important to note, assigned to a chain reactionthat took place in nature about 2 x lO years ago9. At the 0kb quarry inGabon it affected thousands of tons of ore, Along with the depletion of 235Uthrough neutron fission, fission products were produced. The discovery ofthese stable decay products and their anomalous isotopic composition hasconfirmed this natural phenomenon. Thus neodymium. samarium andeuropium have been described'0 showing isotopic compositions rangingfrom that of 'natural' elements to that of fission-produced elements. In thelatter case, some stable isotopes that can normally occur in nature. may betotally absent. e.g. Xe isotopes lighter than 130Xe. However, as the 0kbquarry is not a commercial source of these particular elements (exceptperhaps for uranium) the Commission decided not to add footnote 'g' tothe suite of elements of atomic number 32-66 but to warn against the possi-bility of meeting such anomalies here in the text of the Report.
TERMINOLOGY
The IUPAC Commission on Symbols. Terminology and Units in itsManual of Symbols and Terminology for Physicochemical Quantities andUnits defines atomic weight (relative atomic mass) of an element as 'theratio of the average mass per atom of a natural nuclidic composition of anelement to -' of the mass of an atom of nuclide '2C'.
Inherent in this definition is the notion that such a natural nuclidic com-position exists to define that atomic weight as a constant at least within theprecision achieved by practical chemical measurements. There was indeed atime when such an atomic weight was believed to exist as a universal constantfor every element. For all practical purposes such a constancy does indeedapply to all mononuclidic elements (with a single stable nuclide) and formany others for which the isotopic composition appears to be constantwithin the measured precision for all natural occurrences of 'normal'material. However, the existence of appreciable variations in isotopic com-position in some naturally occurring materials has forced the Commissionin its Atomic Weights Table to quote values (e.g. for H. Li, B. C. 0. Si. S. Ar.Cu. Pb) below the precision easily attainable by modern techniques (seefootnote 'd'). it has been possible in some instances to compromise and toavoid excessive loss of precision by a further footnote 'g' warning users ofsome unusual occurrences outside the normal implied variations (e.g. Li.Mg, Ar. Ca. Sr. Os, Pb. Ra. Th. U). However, the problem is rapidly becomingmore serious as the precision and application of meaningful analyticalmeasurements improve. The problem is further exacerbated by the pro-gressively increasing use of materials which have suffered changes in isotopiccomposition by artificial processes themselves ever more widely employedby industry. The view has also been expressed (for example by De Biêvre'2)
598
ATOMIC WEIGHTS OF THE ELEMENTS
that very precise atomic weights should not be regarded a property of theelement as such but that they should be held to apply only to a specifiedsample of material.
The Commission encourages widespread discussion by chemists not onlyof this viewpoint, but also of the inherent difficulty of refining the abovedefinition of atomic weight so that it is precise for elements with more thanone stable isotope. Should one average the mass per atom over all non-industrially processed non-meteoric terrestrial atoms of that element ormerely over those that could with available technology be described asaccessible to man? Perhaps we should use some special mean value becauseof the skew distribution of isotopic compositions. In any event such adefinition could frequently not be given an operational meaning to a pre-cision comparable with that attainable experimentally. In science this is aserious problem, and it is usually wise to sharpen the definition and at thesame time make it more accessible to experiment.
The Commission does not wish to propose a definitive solution to thesebasic problems at the present time. Rather, it would welcome suggestionswhich can be discussed with other interested IUPAC bodies at the 1975Conference in Spain. Some Commission members hope to publish a tenta-tive proposal to define 'atomic weight' in relation to material to be specified,plus 'atomic weight of an element' using one of the following criteria:
(a) a judiciously chosen number such as the mean between the largest andsmallest reliable, known atomic weights for all 'normal' materials
(b) a standard reference material(c) a mineral from a specified locality of proven homogeneity(d) a defined isotopic composition.These suggestions should be coupled with a precise convention linking
the tabulated value to the atomic weight of an element and the atomicweights commonly found in nature and in commerce.
DESIGNATION OF WELL CHARACTERIZED MATERIALS
In view of the large variety of materials in commerce which containelements having an isotopic composition other than 'normal' some manu-facturers and users may favor the introduction of precise statements onlabels so worded as to minimize misunderstandings and errors in the inter-pretation of analytical data or inadvertent use of valuable (isotopicallyenriched) materials for common purposes.
If such labelling is indeed desired on an increasing scale, it might well bethought proper for this Commission to make proposals for relevant wording.The Commission does not wish to put forward such wording at this stage.It does, however, hereby distribute some draft statements to the chemicalpublic and to reagent manufacturers for comment on usefulness, applicabilityand the tentative wording itself. If there appears to be support for thisCommission to disseminate such phrases for labelling, the topic will be morefully discussed during the 1975 Conference.
The draft phrases for labels of well characterized materials are as follows:(a) 'Atomic Weights Published by IUPAC Applicable'
599
ATOMIC WEIGHTS OF THE ELEMENTS
TABLE OF RELATIVE ATOMIC MASSES OF SELECTED NUCLIDES
Atomic Mass RelativeName Symbol Hall-Ide tnumber number atomic mass
Hydrogen H 1 1 1.007825
(Deuterium) ID) 1 2 2.014102
(Tritium) IT) 1 3 3.016049 12.33 a
Helium He 2 3 3.0160294 4.00260
Lithium Li 3 6 6.015127 7.01600
Boron B 5 10 10.0129411 11.00931
Carbon C 6 12 12 Exactly13 13.00335514 14.00324 5.73 x io a
Nitrogen N 7 14 14.00307415 15.00011
Oxygen 0 8 16 15.994915
17 16.999133
18 17.99916
Neon Ne 10 20 19.99244
21 20.99385
22 21.99138
Magnesium Mg 12 24 23.9850425 24.9858426 25.98259
Silicon Si 14 28 27.97693
29 28.97650
30 29.97377
Sulfur S 16 32 31.97207
33 32.97146
34 33.96787
36 35.96708
Argon Ar 18 36 35.9675538 37.9627340 39.96238
Calcium Ca 20 40 39.9625942 41.958643 42.9588
44 43.955546 45.9537
48 47.9525
Copper Cu 29 63 62.929665 64.9278
Krypton Kr 36 78 77.920480 79.916482 81.913583 82.9141
84 83.9115
86 85.9106
Strontium Sr 38 84 83.9134
86 85.909387 86.9089
88 87.9(156
Technetium Tc 43 97 96.9064 26 x 106 a99 98.9062 2.13 x io a
600
TABLE OF RELATIVE ATOMIC MASSES OF SELECTED NUCLIDES
Atomic Mass RelativeName Symbol 13 Half-Iife'4tnumber number atomic mass
Xenon Xc 54 124 123.9061126 125.9043128 127.9035129 128.9048130 129.9035
131 130.9051
132 131.9041
134 133.9054
136 135.9072
Promethium Pm 61 145 144.9128 18 a147 146.9152 2.6234 a
Osmium Os 76 184 183.9526186 185.9539187 186.9558188 187.9559189 188.9582
190 189,9585
192 191.9615
Lead Pb 82 204 203.9730
206 205.9745
207 206.9759
208 207.9766
Polonium Po 84 209 208.9824 102 a210 209.9829 138.38 d
Astatine At 85 210 209.987 8.1 hrRadon Rn 86 222 222.0176 3.824 dFrancium Fr 87 223 223.0197 22 mm
Radium Ra 88 226 226.0254 1.60 x 1O aActinium Ac 89 227 227.0278 21.77 aThorium Th 90 230 230.0331 7.7 x 10 a
232 232.0381 1.40 x lOb aProtactinium Pa 91 231 231.0359 3.25 x i0 aUranium U 92 233 233.0397 1.58 x i0 a
234 234.0409 2.44 x l0 a235 235.0439 7.04 x 108 a236 236.0456 2.34 x a
238 238.0508 4.47 x l0 aNeptunium Np 93 237 237.0482 2.14 x 106 aPlutonium Pu 94 238 238.0496 87.8 a
239 239.0522 2.439 x iO a240 240.0538 6.54 x i0 a241 241.0568 15 a
242 242.0587 3.87 x 10 a
244 244.0642 8.3 x a
Americium Am 95 241 2410568 433 a243 243.0614 7.37 x i0 a
Curium Cm 96 242 242.0588 163 d243 243.0614 28 a244 244.0627 18.1 a245 245.0655 8.5 x iO a246 246.0672 4.76 x a
247 247.0703 1.54 x iO a248 248.0723 3.5 x 10 a
250 250.0784 1.1 x iO a
a = year: d day: hr = hour. miii = minute. Continued overleaf
601
ATOMIC WEIGHTS OF THE ELEMENTS
TABLE OF RELATIVE ATOMIC MASSES OF SELECTED NUCLIDES (CONT.)
Name Symbol-Atomicnumber
Massnumber
Relative. iatomic mass Half-life 14 t
Berkelium
Californium
Einsteinium
Fermium
Bk
Cf
Es
Fm
97
98
99
100
247249251252254253254257
2470703249.0750251.0796252.0816254.0874253.0848254.0880257.0951
1.4 x 10311900
2.(36 x 10
20.47276100.5
adaadddd
ta = year: d = day.
This statement would be understood to mean that every element presentas major constituent has been either:
derived from a naturally occurring source by processes none of which issuspected of causing significant isotopic separation or nuclear reaction:and/orsubjected to atomic weight measurements to a stated precision and foundto agree with the latest IUPAC values for those elements.(b) Atomic Weight Published by IUPAC Inapplicable for Element(s).
for which 4rft) =This statement would be intended to convey the following meaning:In this material the element(s) specified differ in atomic weight(s) fromthose in the latest Table of Atomic Weights. and the measured values aregiven by:
4r(Emt1t) =
For the other elements (if any) present as major constituents the publishedatomic weights are applicable.
The labelling of separated stable isotopes or radioisotopes is held not toconcern this Commission. However, there may be cases in which a materialmay contain an element the specific atomic weight of which though withinthe range implied by the tabulated value-is known with greater accuracythan is implied by the value in the Table of Atomic Weights. This mightoccur especially when the tabulated value has to accommodate wide varia-tions in natural occurrences.
In such instances the label on the material might read:(c) The actual atomic weight of the element. ... in this particular sample
isThe meaning of this phrase would be self evident.Comments on these suggestions are invited and should be directed to the
Secretary of the Commission:
H. Steffen PeiserNational Bureau of StandardsWashington. DC 20234, USA
602
ATOMIC WEIGHTS THE ELEMENTS
RELATIVE ATOMIC MASSES OF SELECTED NUCLIDES
The principles by which the Table of Relative Atomic Masses of SelectedNuclides was prepared for the 1971 Report' have been found to be satis-factory and have not been changed for this 1973 Report. However, theadditional footnotes in the 1973 Table of Atomic Weights, namely 'e' forNe, Kr and Xe and 'g' for Os, Th and U have necessitated considerableexpansion of the 1973 Table of Relative Atomic Masses of Selected Nuclidesin order to accommodate the isotopes of these elements.
REFERENCES
TUPAC Commission on Atomic Weights. 'Atomic Weights of the Elements. 1971' PureApp!. Chem. 30. 639 (1972).
2 G. P. Baxter and L. W. Parsons. i. Amer. Chem. Soc. 43. 507 (1921).G. P. Baxter and F. A. Hilton. J. Amer. Chem. Soc. 45, 694 (1923).' G. P. Baxter and S. Ishimaru. J. Amer. Chem. Soc. 51. 1729 (1929)..1. P. White and A. F. Cameron. Phys. Rev. 74. 991 (1948).
6 H. C. Mattraw and C. F. Pachucki. AECU. 1903 (1952): see also Nuclear Science Abstracts.6.320(1952).M. C. Inghram and D. C. Hess. Argonne National Laboratory. 4120 (1948).J. W. Gramlich. T. J. Murphy. F. L. Garner and W. P. Shields. .J. Res. Not!. Bur. Standards.77A. 691 (1973).
R. Bodu. H. Bouzigues. N. Morin and J.-P. Pfiffelmann. CR Acad. Sd.. Paris. 275D. 1731(1972).
10 MIle M. Neuilly. J. Bussac. C. Fréjacques, C. Nief. G. Vendryes. and J. Yvon. CR Acad. Sc!..Paris. 27511. 1847 (1972).G. Baudin. C. Blain. R. Hagemann. M. Kremer. M. Lucas. L. Merlivat. R. Molina. G. Nief.F. Prost-Marechal. F. Regnaud and E. Roth. CR Acad. Sc!.. Paris. 27511. 2291 (1972).
12 P. de Biêvre, Z. Anal. Chem. 264. 365 (1973).' A. H. Wapstra. private communication.14 N. F. Holden and F. W. Walker. Chart cifNuc!ides. Educational Relations. General Electric
Company: Schenectady. NY 12305 (1972).
603
