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.

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