碱金属元素原子的次外层电子数只能为2或8,不可能为18?为什么?

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对,Li、Na、K就不写了,肯定不是18.
Rb、Cs、Fr的原子结构示意图：
铷Rb：+37,2,8,18,8,1.
铯Cs：+55,2,8,18,18,8,1.
钫Fr：+87,2,8,18,32,18,8,1.
很明显,次外层电子数都是8.
附录：+21,2,8,9,2,钪,Sc
+22,2,8,10,2,钛,Ti
+23,2,8,11,2,钒,V
+24,2,8,13,1,铬,Cr
+25,2,8,13,2,锰,Mn
+26,2,8,14,2,铁,Fe
+27,2,8,15,2,钴,Co
+28,2,8,16,2,镍,Ni
+29,2,8,18,1,铜,Cu
+30,2,8,18,2,锌,Zn
+31,2,8,18,3,镓,Ga
+32,2,8,18,4,锗,Ge
+33,2,8,18,5,砷,As
+34,2,8,18,6,硒,Se
+35,2,8,18,7,溴,Br
+36,2,8,18,8,氪,Kr
+37,2,8,18,8,1,铷,Rb
+38,2,8,18,8,2,锶,Sr
+39,2,8,18,9,2,钇,Y
+40,2,8,18,10,2,锆,Zr
+41,2,8,18,12,1,铌,Nb
+42,2,8,18,13,1,钼,Mo
+43,2,8,18,13,2,锝,Tc
+44,2,8,18,15,1,钌,Ru
+45,2,8,18,16,1,铑,Rh
+46,2,8,18,18,钯,Pd
+47,2,8,18,18,1,银,Ag
+48,2,8,18,18,2,镉,Cd
+49,2,8,18,18,3,铟,In
+50,2,8,18,18,4,锡,Sn
+51,2,8,18,18,5,锑,Sb
+52,2,8,18,18,6,碲,Te
+53,2,8,18,18,7,碘,I
+54,2,8,18,18,8,氙,Xe
+55,2,8,18,18,8,1,铯,Cs
+56,2,8,18,18,8,2,钡,Ba
+57,2,8,18,18,9,2,镧,La
+58,2,8,18,19,9,2,铈,Ce
+59,2,8,18,21,8,2,镨,Pr+60,2,8,18,22,8,2,钕,Nd+61,2,8,18,23,8,2,钷,Pm+62,2,8,18,24,8,2,钐,Sm+63,2,8,18,25,8,2,铕,Eu+64,2,8,18,25,9,2,钆,Gd+65,2,8,18,27,8,2,铽,Td+66,2,8,18,28,8,2,镝,Dy+67,2,8,18,29,8,2,钬,Ho+68,2,8,18,30,8,2,铒,Er+69,2,8,18,31,8,2,铥,Tm+70,2,8,18,32,8,2,镱,Yb+71,2,8,18,32,9,2,镥,Lu+72,2,8,18,32,10,2,铪,Hf+73,2,8,18,32,11,2,钽,Ta+74,2,8,18,32,12,2,钨,W+75,2,8,18,32,13,2,铼,Re+7