The R O W of INCREASING ATOMIC WEIGHTS and the PERIODIC LAW ARISTID V. GROSSE University of Chicago. Chicago, Illinois
A
S IS well known, Mendelkti placed all elements in a row of their increasing atomic weights and discovered the periodicity of their chemical and physical properties. In order to fulfill the requirements of the periodic law he had to reverse their sequence in three wel-known cases, namely: m4 (39.9) c m K (39.1) d.20 (58.9) t*sN~ (58.7) mTe (127.5) t d (126.9),
to which recently the fourth' SOT^ (232.1) +Pa
(231)
should be added.
-
1 A. V. Gnosss, J . Am. Chm. Soc.. 56, 2501 (1934); Proc. Royal Soc., London, A150, 363 (1935).
In the past this fact has always been brought out (practically in all textbooks), and properly so, as the main inconsistency in the periodic system. Although since Moseley's proof in 1913, that the exact sequence of the elements in the periodic system is given by their nuclear charges, a reference to atomic weights is not compelling, it is important to point out that: the row of increasing atomic weights is dentical with the sequence of increasing nuclear charges, if one frees oneself from the outgrown assumption that the so-called practical wlues of atomic weights must be used. The practical values have actually ceased to be representative constants of the elements ever since F. Soddy's introduction of the concept of isotopes and
F. W. Aston's discovery of isotopy among the common elf:merits in 1913. This because from the standpoint of the periodic law any isotope of an element is as good
a representative of it as another, disregarding whether i t is abundant or rare, stable or radioactive, since all of its properties described by the periodic law are identical. For this reason a mathematical function giving
equal weights to all isotopes of an element, the simplest being the arithmetic mean of atomic weights of all atomic species of an element, is a much more
logical constant of an element for the purposes of the
that the practical atomic weight takes into consideration the abundance of each particular isotope. However, this point is not valid, since the abundance and
periodic system than the practical atomic weight of the analytical chemist. Against this, no doubt, the argument will be made
also stability of isotopes, being: nuclear . properl.ies, . have nothing to do with the periodic law. Even the
much more important point of the abundance or stability of elements among themselves is not brought out in the periodic system. For particulars we refer the reader t o Noddacks' abundance Here we would like toquote only their figures for three sets of neighboring elements showing their relative abundance in the earth's crust (in fractions of the total mass, multiplied by lo6): 7 N 300 20 Ca 34000 81 TI 0.00085 Se 0.75 82 Pb 8.0 8 0 490000 21 Ti Bi 0.034 83 270 22 58000 9 F Furthermore in some cases of radioactive elements, the abundance changes with time. For instance, now protoactinium (91) is rarer than radium (as), but in past geological periods it was more abundant than the latter;' similarly actinium (89) is now about as abundant aspolonium(84), but in the future will become rarer and rarer, awing to the more rapid decay of actinouranium, the mother substance of the actinium series, as compared to uranium I. c w e s of the elements.'
-
I. NODDAM.N a t w w i ~ ~ e n ~ ~ h a j18, t e n755 , (1930). = A . V. Gnosse, Phys. Rev.,42, 565 (esp. p. 569) (1932): 3. Phys. C h m . , 38, 487 (1934). W.
AND
In the years following Aston's original discovery the number of established isotopes has continuously grown, especially recently since the discovery of artificial radioactive elements by the Joliot-Curies (which we had anticipated in 1932,4 immediately after Cockroft and Walton's first successful experiments on transmutation). During the present year the isotopic composition of the last remaining elements has been revealed, and although new rarer isotopes will no doubt be discovered in the years to come, particularly among the radioactive ones, it may be now safe to assume that the little over 400 known isotopes represent the largest bulk of possible atomic species,* so that new ones will not jimlly affect our conclusion.
-
'A. V. Gnoss~,Chem . Bulktin of the Chicago Section of Amer. Chem. Soc., 20, 15' (1933). * See also the discussion below on the natural limit of the number of isotopes of a given element.
TABLE 1 IsoroPEs OF m E ELEXENTI* ( A s or DBCEMBER, 1936)
6-1
Neutron Hydrogen
H'
D T
99.98 0.02 lo-'?
.. .
Helium
100 Ra-He
l,,.
Stable Stable Art Art. Stable
^-1 s
Lithium
Ra-Li Beryllium
Nitrogen
Unstable
?Lrt.
Stable Unstable
20.6 79.4
Boron
Carbon
Art.
100
Ra-B
Art.
Ra-C
Art.
Ra-N
99.3 10.7 Art.
Stable Stable 0.02 r
21 m Stable Stable
10.0 m
99.62 0.38
Stable Stable
99.76 0.04
Sfablc Stable Stable
Ra-N
oxygen
Ra-0
0 20
Fluorine
Ra-F
Nee"
Sodium
Art.
100 Ra-F
1.2 m Stable
Art.
12 r
90.0 0.27 9.73
Stable Stable Stable
40 s
Rn-Nc
Art.
RP-Na
Art.
>'ha
Art.
Stable 14.8 h
Ra-Ns
so0
Note 1
TABLE 1 (Conlinued)
Porkirg Flarliolis X 10' Magnesium Ra-Mg Aluminium
Ra-A1 Ra-A1 Ra-A1
Silicon
Ra-Si
Ra-Si
Phosphorus
Ra-P Ra-P
Sulfur Ra-S Chlorine
77.4 11.6 11.1 Art.
Stable Stable Stable 10 m
Art.
7s Stable 2.3 m 11 m
Art. 89.6 6.2 4.2 Art.
6m Stable Stable Stable 2.5 h?
Art. 100 Art.
Art. 100 Art.
4 7 . 0 5 0.8 ^-. 2 . 2 Art.
3m Stable 14.5 d Stable Stable Stable "80 d
Ra-CI
Art. 76 24 Art.
40 m Stable Stable 37 m
Rn-A
0.330 0.05 89.62 A*.
Stablc Stable Stable 110 m
Ra-K
93.4 0.01 6.6 Art.
Stablc 1V n Stable 16 b
Ra-Ca
96.76 0.77 0.17 2.30 Art.
Stablc Stable Stable Stable 4b
Ra-C1
Argon
Pota~sivm
cnkivm
Scandium
Isdadir Abl,ndo.rc in Half-life. 7t Nolvrc of of Tolo1 Slnble?," 1.41. Po"*iclr
Ra-sc Ra-Se Ra-sc
Art. Art. 100 Art.
8.5 7.8 71.3 5.5 6.9
Titanium
100
Vanadium RP-Y
4.9 81.6 10.4 3.1
Chromium
Manganese Ra-Mn
100 Art.
3h 4.4 h Stable >I a Stnble Stable Stable Stable Stable Stablc 3.7 m Stable Stable Stablc Stable Stnble 2.56
Iron
6.5 90.2 2.8 0.5
Stable Sfable Stable Stable
Cobalt
0.16 99.84 Art.
Stnble Sfablc 20 rn
67.5 27.1 1.7 3.8 Art. 0.9 Art.
Stable Stable Stable Stable 100 m Stable
Ra-Co Nickel
Ra-Ni
3h
Remarks
lndividuol Physical Symbols Moss Atomic Wcighls Syrnbnlr of lralop~r Numbers 0' = 16.0000 Cu Ra-Cu Ra-Cu
Gallium
Ga Rs-Ga Ra-Ga
32
x
10.
63 64 65 66 64 66 67 68 70
31
Pocking Fronions
63.937
- 9 9
69 70 71 72
0 -
0
-
9.8 9.8
Germanium
/=-
7.7
1 As Ra-As
75 76
74.934
-8.8
* 1.6
IrotoOic Abundoncc in Bolf-life. Tt %of T o l d StnbleP,=lM. Stable 12.8h Stable 6m
#+and#...
50.4 27.2 4.2 17.8 0.4
Stable Stable Stable Stable Stable
... ... ... ... ...
61.5 Art. 38.5 ~rt.
Stable 23 h Stable 20 m
21.2 27.3 7.9 37.1 6.3
Stable Stable Stable Stable Stable
100 Art.
Stable 26 h
Rn-Br
Ra-Br
-
40
Zirconium
41
Niobium
Nb
42
Molybdenum
Ma
Ra-Mo
6-
... ... ... ...
... .. ,
8'
Stable 4 2h Stable 18 m 3; h
0.42 2.45 11.79 11.79 56.85 16.70
Stable Stable Stable Stable Stable Stable
... 8...
... ... ... ...
8.2
72.7
.--
8.2
27.3
Stable 10' s Stable
\=-
8.2
0.5 9.6 7.5 82.4
Stable Stable Stable
100 Art.
Stable 70 b
48 11.5 22 Art. 17 1.5
Stable Stable Stable 40 h Stable Stable
100
Ra-Mo
... ...
Bt
50.6 Art. 49.4 Art. Art.
-
Strontium
Yttrium
B-
Stable Stable Stable Stable Stable Stable 33 rn
Ra-Br
39
...
68 Art. 32 Art.
Ra-Se
38
Norum of Emitted Porliclcs
14.2 10.0 15.5 17.8 9.6 23.0 Art. 9.8 M.
Stable Stable Stable Stable Stable Stable Stable 36 h Stable 30 rn
8-
Remarks
TABLE 1 (Contirucd)
Indinidual
Phrsical
Parking Prortiar X 101
-
Symbols Moss Atomic Weigh1 Symbols of IsolaP~s Numbers 0" 16.0000
Irota9ic Nature d A b m d n n u in Kolf-life. Tt EmilUd % of T o l d Slobicl.= IM. Pnrliclct 5 12 14 22 30 Art. 17 Art.
Rh
101 103 Ra-Rb
Pd
Ra-Pd
Ra-Pd
ion
Ra-Pd
110 111
Ra-Ag
Indium
Tin
Rn-Sn Ra-So
Tellurium
102 103 1M 105 106 107 108
Ra-Pd
Ra-Ag
Antimony
104
1
"-5.7
-39.9 0.1
Stable Stable Stable Stable Stable 40 5 100 s Stable 11 h 75 h
... ... ... ... ...
8.7 '
... 8-7
Art.
Stable Stable 44 s 3.9 m
0.8 ~ r f . 9.3 22.6 27.2 ~ r t . 26.8 Art. 13.5 ~ r f .
Stable 60 h Stable Stnbk Stable 12 h Stable 15 m Stable 3m
52.5 An. 47.5 Art.
Stable 2.3 m Stable 22 r
1.4 1.0 12.8 13.0 24.2 12.3 28.0 ~ r t . 7.3
Stable Stable Stable Stable Stable Stable Stable 3.5 b Stable
... ... ... .. .. .. ... ... 8...
4.5 Art. 95.5 Art.
Stable 13 s Stable 54 m
8" ... 8-
1.1 0.8 0.4 lK5 9.1 22.5 9.8 28.5 5.5 Art. 6.8 Art.
Stable Stable Stable Stable Stable Stable Stable Stable Stable 18 m Stable 8m
56 44 Art.
Stable Stable 2.54
0-
Very rare 2.9 1.6 4.5 6.0 19.0 Art. 32.8 33.1
Stable Stable Stable Stable Stable Stable 45 m Stable St~ble
... ... ... ... ... ... 0... ...
8.79
Stable
Iodine
xenon
... 88.. . 8'7 ... ...
... ... 8-1 .,. 8-7
8-7
...
E-
.. .
8-
...
... ... ... ... ... ,.. ... ...
8-7 8-I
...
...
TABLE 1 (Continued)
Atomic Numbers
Elcmcnlr
Symbols
lndioidunl Physical S ~ n b o l r Moss Atomic Wcighlr 16.0000 of Isotopes Numbws 0"
-
Pading Frnrtions
X 10'
Isolooic Narurc of Abundance i n Half-lifc, T f Emillrd 9 . of T o l d B o b l c f , 11 M. Porliclcr
Rcnovh
Stable
1.5 b
Ra-Cs
Stable Sfablc Stable Stable Stable Stable Stable Stable 80 m
Rs-Ba
Ra-Bn
Stable
Lanthanum Ra-1.1 cerium Praseodymium Rn-Pr Ra-Pr
1.9d 89 11
Stable Stable
100
Stable 19 h 5m
Art. Art.
Stable Stable Stsble Stable Stable Ib
Neodymium
Rn-Nd
Ra-Sa Ra-Sa
17 14 15 5
Art. 26 Art.
20
Ra-Eu very rare?
21 23 I7 23 Ra-Gd Terbium Ra-Tb Dysprorium
7h
100
Stable
Art.
3.9 h
25 25 28
Stable Stable Stsble Stable Stable
Art.
2.5h
Art. 36 v e r y rare?
24 Ra-Er
30 Art.
Ra-Er
Art.
10 100 Ra-Tm
.-
Stable Stable Stsble Stable Stable Stable
100
Thulium
one of them active
Stable Stable Stable Stable 2d Stable 40 m Stable
16
22
Ra-Ho
Stable
Art.
very rare?
Ra-Dy
Product of Nd*a Note 3
Stable Sfablc 9.2 b
Europium
Gadolinium
Stable
Art. very rare?
Stable
35 h Stable Stable? Stable Stable 12 b Stable 7m Stable
1120 d Stable
Product of He-
TABLE 1 (Continued)
I~~dioiduol
Symbols Syrbola oflrolopcr
Mars Numbcn
*
1
Ytterbium
Vcry rare? 9
'D a
. 24
I h
Rn-Yb
17 38 Alf. 12
4;;100 ; Art. 8 2 Art. ts
1.utceium
Re-1." Rn-Lu
Stable B-7 d 4h
5 19 28 18 30
Stable Stnblc Stable Stable Stnble
Ra-HI
Art.
Long
Rn-W
22.6 17.3 30.2 29.9 Art.
Stable Stable Stable Stable Id
38.2
Rn-Re
61.8 An.
stable 85 h Stable 20 b
Ra-0s
1.0 0.6 13.4 17.4 25.1 42.5 ~ r t .
Stable stable Stable Stable Stable Stable 40 b
38.5 Art. 61.5 ~,t.
Steblc 68 d Stable 19 h
0.8 Art. 30.2 35.3 26.6 Art. 7.2
Stnblc 49 m Stable Stable Stable 14.5 h Stable
Hnlnium
in'
"-
gs
3
aa
... ...
Stable Stable Stable Sfable Stable 3.5 h Steble
Tantalum Tvngrteo
"0 Rhenium Ra-Re
~,t.
*
-1
2
Osmium
-
-1 -1
1-
2 2
Iridium Rn-lr Ra-lr Platinum
Rn-Pf
100
Gold
Mercury +0.8
-
2
Ra-Hg Thsllium
+1.8
*2
+1.8
t
Ra-TI Ra-TI A&" ThC" Rs-C" Lead RaC AcD ThD RaD AcB
ThB RaB
206.00 208.00
-
2
0.10 9.89 16.45 23.77 18.67 -~~~ 29.27 *.W6 6.85 Art.
Stable Stable Stable Stable Stahle -~ Stable Stable Stable 40 b
29.4 Art. 70.6 ~ r t . ~ = trad. . N*L rad. Nat. rad.
Stable 97 m Stable 4m 4.76 rn 3.1 m 1.32 m
~
0.01
* 0.01
0.0*0.5 0.0
* 0.5
~
... ... ...
V?
... ...
n-
0-
... ... ... . .. ... ... 6-?
...
B-? . ..
n-
... ... ... ... B... ... B
Stable 2.76
Art.
Ra-Au
...
+
8-
... ... ... ... . .. ... ... ...
6-1
.
..
. 0-7
...
a-? ana-
~-
28.03 20.40 50.05 Naf. rad. ~ ~rad. t Naf. rad. Nat. rad.
Stable Stable Stable 19 n . 36.0 ?n 10.6 h 26.8 m
...
...
B-
a@-
8-
Product of Tmm
All reliably known atomic species, both stable and naturally or artificially radioactive are listed in Table 1 and Figures 1 4 , and from them we derive the values of the mean atomic weights (or mean mass numbers). Listing these in ascending order (see Table 2) we see immediately that the mean atomic weight of an element always increases with its nuclear charge and that, tkerefore, the row of increasing mean atomic weights i s identical with the sequence of increasing nuclear charges, so that both may be used for the exact formulation of the periodic law. The four exceptions, mentioned a t the beginning, now realign themselves as follows:
of an element is set by radioactivity. The data of Figure 1 show (a particular example being the classical case of Al, Si, and P) that if for a given number of protons in the nucleus (Z), the number of neutrons increases beyond certain limits, a neutron changes into a proton by electron emission, leading to an element of higher atomic number (Z 1). On the other hand, if they decrease beyond certain limits a proton changes into a neutron, emitting a positron, thereby decreasing the atomic number by one, as illustrated in Table 3. (An equivalent change can also take place, particularly among the heaviest nuclei, through the emission of an m-particle.)
In the present language of nuclear physics this empirical rule means that the sum of the number of protons and the mean number of neutrons that combine with them to form all possible isotopes of an element always increases with the number of protons in its nucleus. A natural limit to the number of possible isotopes
A rigorous theoretical explanation of these facts should be ~ossiblewhen the laws of interaction between nuclear particles are more definitely known.
+
