THE QUANTUM PHYSICS OF SOLIDS 

 The Periodic Table 



661 



The elements from lithium to neon constitute the first short period 

 of the periodic table, Fig. 5. The second short period, running from 

 sodium to argon, is built up in a similar way by filling the 3s and 3p 



I H j He ] 



TlL Be|B 



1-1 Ate 



I s 



Na Mgj Al SI 



O f^Ne] 



2S,2P 



3S,3p 



/ 



/ 



/' 



K ca I sc Ti V 



19 20 ] 21 22 23 

 I '-' -n -n 



CO 



<o 



Cr Mn Fe Co NliCu Zn|Ga Ge As Se B 



24 25 26 27 28 I 29 30 I 31 32 33 34 35 



lO >fi <o >"- CO o^ "2^^ ! o fy fO ^ ^ 



-o -o "D -D -o-^J^I,, Q. Q. a. Q. 



in <^ <r) in \ m '^ \ '" <n in <n <n 

 Fig. 5 — First part of the periodic table. 



r1 Kr 



I 36 



4S,3d,4p 



levels. Some of the chemical properties of the elements of these 

 periods are quite easily understood in terms of the electron con- 

 figurations of the atoms. An atom of lithium or of sodium has one 

 easily removed electron and thus can become a positively charged ion ; 

 only one electron, however, can be so easily removed, and for this 

 reason sodium and lithium do not have doubly charged positive ions 

 in chemistry and are, therefore, monovalent positive elements. Simi- 

 larly beryllium and magnesium have two easily removable electrons, 

 and are divalent; however, their electrons are harder to remove than 

 those of the alkali metals; hence the alkaline earth metals, beryllium and 

 magnesium, are not so electropositive as the alkalis.* The halogens, 

 fluorine and chlorine, present a contrasting picture. Instead of having 

 one loosely bound electron, they have one low-lying empty state. 

 They can therefore hold tightly an extra electron, and thus be negative 

 ions. The rare or noble gas elements helium, neon and argon consist 

 only of closed shells. They can neither gain nor lose electrons in 

 chemical compounds and are, therefore, generally aloof to chemical 

 urges. 



* For brevity we shall refer to the alkali metals and alkaline earth metals simply as 

 alkalis and alkaline earths. 



