X、Y、Z三种主族元素,已知X和Y的原子核外电子层数相同,Y和Z的原子最外层电子...

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X、Y、Z三种主族元素,已知X和Y的原子核外电子层数相同,Y和Z的原子最外层电子数相同,又知三种元素的原子最外层电子数总和为质子数总和为28,符合此条件的元素组的数目为( )

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