100 anos do "Átomo de Rutherford"

EDITORIAL
A Nucleated Century
Published: March 22, 2011

If you asked someone to draw an atom, he or she would probably draw something like a cockeyed solar system. The sun — the nucleus — is at the center, and the planets — the electrons — orbit in several different planes.

The critical discovery in this atomic model emerged a century ago in a talk before the Manchester Literary and Philosophical Society in March 1911 and a paper published soon after in the Philosophical Magazine. Both were by Ernest Rutherford, who had won the 1908 Nobel Prize in Chemistry in part for his discovery of the alpha particle, which he later proved was the nucleus of a helium atom.
By 1911, scientists had already measured the charge and mass of an electron. But no one was sure how the atom was structured. Among his endless contributions to atomic theory, Rutherford explained a curious phenomenon. When fired at an extremely thin sheet of gold foil, some alpha particles scattered at surprising angles.
A few even bounced straight back at the observer, which Rutherford said was as unexpected as firing a cannon shell at tissue paper and having it come back and hit you.
The answer, Rutherford wrote, is that “the atom consists of a central charge” that is “concentrated at a point” — a point soon called the nucleus. A near collision with the nucleus caused the alpha particle to deflect. A direct collision caused it to bounce straight back. Compared to the whole atom, Rutherford said, the nucleus was like “a fly in a cathedral.”
As the scientist Freeman Dyson later wrote, finding the nucleus was the start of nuclear physics, which has transformed our picture of the atom. Now it looks something like a composite of quarks surrounded by clouds of uncertainty. More accurate. Much harder to draw.
A version of this editorial appeared in print on March 23, 2011, on page A26 of the New York edition.