262 



SCIENCE 



[N. S. Vol. LIV. No. 1395 



of that varying power of cliemical combina- 

 tion in an element we term valency. TMs 

 explanation I give substantially in his own 

 words. The number of electrons in an atom 

 of the different elements has now been deter- 

 mined, and has been found to be equal to the 

 atomic number of the element, that is to the 

 position which the element occupies in the 

 series when the elements are arranged in the 

 order of their atomic weights. We know now 

 the nature and quantity of the materials of 

 which the atoms are made up. The proper- 

 ties of the atom will depend not only upon 

 these factors but also upon the way in which 

 the electrons are arranged in the atom. This 

 arrangement will depend on the forces be- 

 tween the electrons themselves and also on 

 those between the electrons and the positive 

 charges or protons. One arrangement which 

 naturally suggested itself is that the positive 

 charges should be at the center with the nega- 

 tive electrons around it on the surface of a 

 sphere. Mathematical investigation shows 

 that this is a possible arrangement if the elec- 

 trons on the sphere are not too crowded. The 

 mutual repulsion of the electrons resents 

 overcrowding, and Sir J. J. Thomson has 

 shown that when there are more than a certain 

 number of electrons on the sphere, the at- 

 traction of a iKJsitive charge, limited as in 

 the case of the atom in magnitude to the sum 

 of the charges on the electrons, is not able 

 to keep the electrons in stable equilibrium on 

 the sphere, the layer of electrons explodes and 

 a new arrangement is formed. The number 

 of electrons which can be accommodated on 

 the outer layer will depend upon the law of 

 force between the positive charge and the 

 electrons. Sir J. J. Thomson has shown that 

 this number will be eight with a law of force 

 of a simple type. 



To show the bearing of this result as af- 

 fording an explanation of the Periodic Law, 

 let us, to begin with, take the case of the 

 atom of lithium, which is supposed to have 

 one electron in the outer layer. As each ele- 

 ment has one more free electron in its atom 

 than its predecessor, glucinum, the element 

 next in succession to lithium, will have two 



electrons in the outer layer of its atom, boron 

 will have three, carbon four, nitrogen five, 

 oxygen six, fluorine seven and neon eight. 

 As there can not be more than eight electrons 

 in the outer layer, the additional electron in 

 the atom of the next element, sodium, can not 

 find room in the same layer as the other elec- 

 trons, but will go outside, and thus the atom 

 of sodium, like that of lithium, will have one 

 electron in its outer layer. The additional 

 electron, in the atom of the next element, 

 magnesium, will join this, and the atom of 

 magnesium, like that of glucinum, will have 

 two electrons in the outer layer. Again, 

 aluminium, like boron, will have three; sili- 

 con, like carbon, four; phosphorus, like nitro- 

 gen, five; sulphur, like oxygen, six; chlorine, 

 like fluorine, seven ; and argon, like neon, 

 eight. The sequence will then begin again. 

 Thus the number of electrons, one, two, three, 

 up to eight in the outer layer of the atom, 

 will recur periodically as we proceed from 

 one element to another in the order of their 

 atomic weights, so that any property of an 

 elenient which depends on the number of 

 electrons in the outer layer of its atom will 

 also recur periodically, which is precisely 

 that remarkable property of the elements 

 which is expressed by the periodic law of 

 Mendeleeff, or the law of octaves of N'ew- 

 lands. 



The valency of the elements, like their 

 periodicity, is a consequence of the principle 

 that equilibrium becomes unstable when 

 there are more than eight electrons in the 

 outer layer of the atom. For on this view 

 the chemical combination between two atoms, 

 A and B, consists in the electrons of A 

 getting linked up with those of B. Consider 

 an atom like that of neon, which has already 

 eight electrons in its outer layer; it can not 

 find room for any more, so that no atoms 

 can be linked to it, and thus it can not form 

 any compounds. Now take an atom of flour- 

 ine, which has seven electrons in its outer 

 layer; it can find room for one, but only one, 

 electron, so that it can unite with one, but 

 not with more than one, atom of an element 

 like hydrogen, which has one electron in the 



