University in England, in the forty-five years centered on 1910, the
nature of the atom was first understood--partly
by shooting pieces of atoms at atoms and watching how they bounce off.
A typical atom has a kind of cloud of electrons on the
outside. Electrons are electrically charged, as their name suggests.
The charge is arbitrarily called negative. Electrons determine the
chemical properties of the atom--the glitter of gold, the cold feel of
iron, the crystal structure of the carbon diamond. Deep inside
the atom, hidden far beneath the electron cloud, is the nucleus,
generally composed of positively charged protons and electrically
neutral neutrons. Atoms are very small--one hundred million of them end
to end would be as large as the tip of your little finger. But
the nucleus is a hundred thousand times smaller still, which is part of
the reason it took so long to be discovered. Nevertheless, most
of the mass of an atom is in its nucleus; the electrons are by
comparison just clouds of moving fluff. Atoms are mainly empty space.
Matter is composed chiefly of nothing.