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ALCHEMY
79
1,000),5 and several attempts to explain this remarkable fact have been put forward.
Modern scientific speculations concerning the constitution of atoms tend towards a
modified form of Prout's hypothesis, or to the view that the atoms of other elements are,
in a manner, polymerides of hydrogen and helium atoms. As has been pointed out, it is
possible, according to modern views, for elements of different atomic weight to have
identical chemical properties, since these latter depend only upon the number of free
electrons in the atom and not at all upon the massive central nucleus. By a method
somewhat similar to that used for determining the mass of kathode particles (see 79), but
applied to positively charged particles, Sir Joseph Thomson and Dr. F. W. Aston
discovered that the element neon was a mixture of two isotopic elements in unequal
proportions, one having an atomic mass of 20, the other (present only to a slight extent)
having an atomic mass of 22. Dr. Aston has perfected this method of analysing mixtures
of isotopes and determining their atomic masses.6 The results are of great interest. The
atomic weight of hydrogen, 1.008, is confirmed. The elements helium, carbon, nitrogen,
oxygen, fluorine, phosphorus, sulphur, arsenic, iodine and sodium are found to be simple
bodies with whole-number atomic weights. On the other hand, boron, neon, silicon,
chlorine, bromine, krypton, xenon,
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mercury, lithium, potassium and rubidium are found to be mixtures. What is specially of
interest is that the indicated atomic mass of each of the constituents is a whole number.
Thus chlorine, whose atomic weight is 35.46, is found to be a mixture of two chemically-
identical elements whose atomic weights are 35 and 37. Some of the elements, e.g.,
xenon, are mixtures of more than two isotopes.
It is highly probable that what is true of the elements investigated by Dr. Aston is true
of the remainder. It appears, therefore, that the irregularities presented by the atomic
weights of the ordinary elements, which have so much puzzled men of science in the
past, are due to the fact that these elements are, in many cases, mixtures. As concerns
hydrogen, it is only reasonable to suppose that the close packing of electrically charged
particles should give rise to a slight decrease in their total mass, so that the atomic
weights of other elements referred to H = 1 should be slightly less than whole numbers,
or, what is the same thing, that the atomic weight of hydrogen referred to O = 16 should
be slightly more than unity.
78. The "Periodic Law".
A remarkable property of the atomic weights was discovered, in the sixties,
independently by Lothar Meyer and Mendeléeff. They found that the elements could be
arranged in rows in the order of their atomic weights so that similar elements would be
found in the same columns. A modernised form of the Periodic Table will be found on
pp. 106, 107. It will be noticed, for example, that the "alkali" metals, Lithium, Sodium,
Rubidium and Cæsium, which
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ALCHEMY
80
Page 106
THE
PERIODIC
TABLE OF
THE
1 2 3 4 5 6 7
CHEMICAL
ELEMENTS
[Hydrogen Hydrogen
H=1o008]a H=1o008
Helium Lithium Glucinum Boron Carbon Nitrogen Oxygen Flourine
He=4o00 Li=6o94 Gl=9o1 B=10o9 C=12o005 N=14.008 O=16o00 F=19o0
Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulphur Chlorine
Ne=20o2 Na=23o00 Mg=24o32 Al=27o1 Si=28o3 P=31o04 S=32o06 Cl=35o46
Argon Potassiumb Calcium Scandium Titanium Vanadium Chromium Manganese
A=39o9' K=39o10 Ca=40o07 Sc=45o1 Ti=48o1 V=51o0 Cr=52o0 Mn=54o93
Copper Zinc Gallium Germanium Arsenic Selenium Bromine
Cu=63o57 Zn=65o37 Ga=70o1 Ge=72o5 As=74o96 Se=79o2 Br=79o92
Krypton Rubidium Strontium Yttrium Zirconium Columbium Molybdenum
?
Kr=82o92 Rb=85o45 Sr=87o63 Y=89o33 Zr=90o6 Cb=93o1 Mo=96o0
Iodined
Silver Cadmium Indium Tin Antimony Tellurium
I (orJ)-
Ag=107o88 Cd=112o40 In=114o8 Sn=118o7 Sb=120o2 Te=127o5
126o92
Xenon Cæsium Barium Lanthanum Ceriume
? ? ?
Xe=130o2 Cs=132o81 Ba=137o37 La=139o0 Ce=140o25
? ? ? ? ? ? ?
Tantalum Tungsten
? ? ? ? ? ?
Ta=181o5 W=184o0
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ALCHEMY
81
Gold Mercury Thallium Lead Bismuth Polonium
?
Au=197o2 Hg=200o6 Tl=204o0 Pb=207o20 Bi=208o0
Emanation Radium Actinium Thorium Ekatantalum Uranium
? ?
(Niton)222o0 Ra=226o0 ? Th=232o15 ? U=238o2
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NOTES.
There are several somewhat different forms of this Periodic Table. This is one of the
simplest, but it lacks certain advantages of some of the more complicated forms. The
atomic weights given are those of the International Atomic Weights Committee for 1920-
1. They are calculated on the basis. Oxygen = 16. The number of decimal places given in
each case indicates the degree of accuracy with which each atomic weight has been
determined. The letter or letters underneath the name of each element is the symbol by
which it is invariably designated by chemists.
The number above each column indicates the valency which the elements of each group
exhibit towards oxygen. Many of the elements are exceptional in this respect.
[a] The exact position of Hydrogen is in dispute.
[b] The positions of Argon and Potassium have been inverted in order that these
elements may fall in the right columns with the elements they resemble; [d] 50 also have
the positions of Tellurium and Iodine.
[c] The whole of "Group 8" forms an exception to the Table. [ Pobierz całość w formacie PDF ]

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