Stories About Chemistry
22. Two More “Whys”
Why are there so many metals and so few nonmetals on Earth? And why do metals resemble each other much more than nonmetals? Indeed, one is not likely to confuse, say, sulphur and phosphorus or iodine and carbon by their appearance. But even the expert eye cannot always distinguish between niobium and tantalum, potassium and sodium, or molybdenum and tungsten.
Transposition of terms does not change the sum. This is probably one of the most “rigid” principles of arithmetic. But in chemistry, relative to the structure of atomic electron shells, this principle applies far from always.
All is good and well as long as we have to do with the elements of the second and third periods of the Periodic Table.
In each element of these periods the new electron goes into the outermost shell of the atom. An electron is added, and the properties of the new element are entirely different from those of its predecessor. Silicon does not resemble aluminium, sulphur has nothing in common with phosphorus. Metallic properties soon give way to nonmetallic, because the more electrons the atom has in its outer shell, the less readily it parts with them.
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 scandium-zinc “span.”
Why should they accept electrons in their outer shell when forming compounds if they have only two electrons in it? It is much easier for them to give away these two electrons to the elements they react with. Besides, they do not object to borrowing additional electrons from their incomplete second-last shell. As a result, they can display various positive valences. For instance, manganese may be positively di-, tri-, tetra-, hexa-, and even heptavalent. The same is observed in the subsequent periods of the Periodic Table.
That is why there are so many metals and why they are more like one another than the nonmetals.