A standard sheet of office paper measures 8.5 inches
wide and 11 inches long. If one took that sheet of
paper, tore it in half across its width, and threw away
the other half, one would be left with a piece of paper 5.5
inches wide and 8.5 inches long. After six tears, the sheet
of paper would be a little more than an inch on a side.
After seven more tears, that small chit of paper would be
about 0.1 inch on a side. It would be difficult to tear that
minute piece of paper easily, but perhaps it could be done.
However, one would eventually reach the limit of one’s
manual dexterity and be left with a small speck of paper.
Though one could imagine some much smaller creature
like an ant continuing the process, there would have to be
some point at which the paper could not be torn anymore.
In the 4th century bce, the Greek philosopher
Democritus followed such a speculation to postulate the
existence of some indivisible piece of matter. He called
these pieces “atomos” (Greek for indivisible), and from
atomos comes the modern word, atom. Democritus and
other Greek philosophers did not tear scrolls of papyrus
into smaller and smaller pieces to find the size of the atom.
In ancient times, the atom was a thought experiment, an
idea about matter and its properties that was never tested
as a scientific theory. But how far would one have to tear
the sheet of paper to get down to the atom?
It was not until 1865 that German chemist Joseph
Loschmidt, who studied the motion and diffusion of gases,
answered this question. The answer was a few billionths of
an inch (or one-hundred millionth of a centimetre). Such
a minuscule distance would have staggered the ancient
Greek philosophers.
This book delves inside the atom to reveal many
facets of nuclear physics. The first subject is the history
of how humanity began to understand the atom, beginning
with the first speculations of Democritus, Leucippus, and Epicurus. In the 18th century, scientists such as John
Dalton and Joseph-Louis Gay-Lussac studied how substances
combined in chemical reactions and deduced that
these substances must be made up of atoms. It was soon
realized that atoms differed from each other by weight.
For example, hydrogen was much lighter than a metal like
cesium. The Russian chemist Dmitry Mendeleyev realized
that the elements were arranged in tabular form according
to the weight of their individual atoms. This arrangement
is known worldwide as the periodic table of the elements.
From it, Mendeleyev was able to predict the properties of
the elements gallium, scandium, and germanium, which
had not yet been discovered.
The 19th century saw tremendous advances in the
study of the atom. Atoms were discovered to have electrical
properties. The spectral lines of hydrogen were seen to
fit a numerical pattern. One of the greatest advances was
the discovery that the atom itself contained even smaller
particles. In 1897 the English physicist J.J. Thomson discovered
the electron. Not only was the electron the carrier
of electric charge, but it was also 1/1,836ths as heavy as a
hydrogen ion. (The fact that the electron only existed in
certain energy levels that corresponded to something like
an orbit around the nucleus explained the numerical pattern
of the hydrogen spectral lines.)
Another significant advance was the discovery of radioactivity
in 1896 by the French physicist Henri Becquerel.
This showed that not only did the atom contain small particles,
but also that when the atom experienced radioactive
decay, it became that of another element. The medieval
alchemists had sought the transmutation of elements, in
particular, the remunerative changing of lead into gold.
Here was such a transmutation, but one that occurred naturally.
The discovery of radioactivity led to the discoverythat the atom was mostly empty space. Nearly all the mass
of an atom save the pittance found in the electrons was
concentrated in a central nucleus. These discoveries in
turn spurred the development of quantum mechanics,
which overthrew commonsense notions of the universe by
explaining physical phenomena in terms of probabilities.
The development of quantum mechanics led to an
accurate description of the atom and its properties. The
atom is made up of a nucleus of positively charged protons
and the neutrons, which have no electric charge. These two
particles are much heavier than the electron. Every atom
has an atomic number, which is the number of protons in
the nucleus. Hydrogen, the lightest element, has an atomic
number of 1. The heaviest element thus far discovered, ununoctium
(of which only a few atoms have been produced),
has an atomic number of 118.
No comments:
Post a Comment