107

Stories About Chemistry

INDEX

72. The Most Unusual Atom, the Most Unusual Chemistry

The symbol of this remarkable atom is Ps But in vain will you search Mendeleyev�s Table for it. It is not the atom of any chemical element

And its lifetime is only an instant, less than a ten-millionth of a second Still, it cannot be said to be radioactive.

Ps stands for positronium. Its structure is very simple.

Take a hydrogen atom, the simplest of all the atoms of chemical elements. One electron revolves about a single proton.

The positronium atom appears in certain types of radioactive transformations accompanied by the emission of a positron. For a very short time the positron forms a stable system with an electron.

In positronium the part of the proton is played by an elementary particle known as the positron. It is the antipode of the electron. The positron has the same size and the same mass as the electron, the difference being that its charge is of the opposite sign (positive).

A collision between a positron and an electron is the end of both of them. As the physicists put it, they annihilate each other. In other words, they turn into nothing, or to be more exact, into a radiation.

But just before disappearing these two unreconcilable enemies exist side by side for a short instant giving rise to the ghost positronium atom. It is an atom with no nucleus, for the electron and the positron revolve about a common centre of gravity.

Now who could be interested in positronium? Only theoretical physicists, it would seem; or maybe science-fiction writers searching for new types of fuel for their stellar spaceships.

But not long ago a thick book entitled �The Chemistry of Positronium� was published in the USA. This is no science fiction. The book was written by serious scientists and treats of how the investigators make this unusual atom serve their purposes.

During its brief lifetime, positronium is capable of entering into chemical reaction. It reacts especially readily with chemical compounds which have free valence bonds left. These unused vacancies are occupied by positronium atoms.

By means of special instruments chemists can trace the nature of decay of positronium which has got into the molecule of a substance. It is found to decay differently, depending on the structure of the molecule. This enables chemists to study the intricacies of molecular designs and to solve many sophisticated and controversial problems where other methods have failed.