107 Stories About Chemistry
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But now we come to the fourth period. Potassium and calcium are first-rate metals. We expect them to be followed soon by nonmetals. Not so fast! We are in for a disappointment, because beginning with scandium each added electron prefers the second-last shell to the outer one "Transposition of terms." But this transposition changes the "sum" - the sum of the properties of the elements. The second-last shell is more conservative than the outer one, and it affects the chemical properties of the elements much less. Therefore, the difference between the elements is less pronounced. Scandium "recalls", as it were, that its third electron shell is incomplete. It should contain 18 electrons but has only 10 so far. Potassium and calcium must have "forgotten" about this, and arranged their newly-added electrons in their fourth shells. In scandium, justice is restored. The second-last shell is gradually completed over a series of ten elements. The outer shell remains unchanged, with only two electrons in it. Such a small number of electrons in the outer shell of an atom is peculiar to metals. And that is why there are only metals in the scandiumzinc "span."
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