Stories About Chemistry
21. Why Two Very Different Things?
A jester once remarked that two basic qualities distinguish humans from animals: their sense of humour and their sense of historical experience. A human being is capable of laughing over his own misfortune, and will not be caught a second time where he has stumbled once. We might add another quality, that of asking “why” and trying to find the answer.
And now let us use this little word “why.”
For instance, why are, the nonmetals not distributed evenly over the storeys and sections of the Big House, but grouped in definite part of it. Metals are metals and nonmetals are nonmetals, but what is the difference between them? Now that is a good question to start with.
When two elements (no matter which) react, the outermost electron shells of their atoms are rearranged. The atoms of one of the elements gives away electrons, and those of the other accepts them.
Now the difference between metals and nonmetals lies in this most important chemical law.
Nonmetals are capable of two opposite actions: as a rule, they acquire electrons. but they are also capable of giving them away. Their behaviour is pliable and they can change their aspect depending on the circumstances. If they find it more profitable to accept electrons, nonmetals appear as negative ions. If not, they form positive ions. Only fluorine and oxygen know no compromise: they only accept electrons and never give them away.
Metals are much less “diplomatic” and more consistent in their habits. Their motto is invariably “give electrons away and never accept them.” They form positively charged ions. The gain of extra electrons is not in their line. Such is the rigid rule of behaviour of the metallic elements.
This is the basic difference between metals and nonmetals.
However, meticulous chemists have found exceptions even to this very strict rule. There are inconsistent characters even among the metals. Two (so far!), only two metals have displayed “unmetallic” features. Astatine and rhenium (the inhabitants of the 85th and 75th boxes of the Mendeleyev Table) are known to form negative univalent ions. This fact is like a black mark for the surprisingly purposeful family of metals.
Now, generally speaking, which atoms part with their electrons more easily and which accept them more readily? Atoms which have few electrons on their outermost shell find it more convenient to give them away, and those which have many of them find it more profitable to complete their electron octets by acquiring them.
The alkali metal shave only a single electron on their outsides. These metals think nothing of parting with it. Once they have done so they find the stable electron shell of the nearest inert gas on their outside. That is why the alkali metals are chemically the most active of all known metals. And the “very most active” among them is francium (box 87). The heavier an element is in its group, the larger its atom and the weaker the hold the nucleus has on its only outside electron.
The most furious in the kingdom of nonmetals is fluorine. It has seven electrons in its “outer sphere.” All it needs for complete bliss is an eighth electron. And it grabs it greedily from almost any other element of the Periodic System; nothing can resist fluorine’s mad onslaught.
The other nonmetals also accept electrons, some of them more, others less easily. And now we can understand why they are grouped mainly in the upper right-hand corner of the table: they have plenty of electrons on their outsides, and this is possible only in atoms near the ends of the periods.