Experiments by J.J. Thomson in 1897 led to the
discovery of a fundamental building block of
matter one hundred years ago, the British
physicist J.J. Thomson was venturing into the
interior of the atom.
At the Cavendish Laboratory
at Cambridge University, Thomson was experimenting
with currents of electricity inside empty glass
tubes. He was investigating a long-standing puzzle
known as "cathode rays." His experiments prompted
him to make a bold proposal: these mysterious rays
are streams of particles much smaller than atoms,
they are in fact minuscule pieces of atoms. He
called these particles "corpuscles," and suggested
that they might make up all of the matter in
atoms. It was startling to imagine a particle
residing inside the atom--most people thought that
the atom was indivisible, the most fundamental
unit of matter.
It took more experimental work
by Thomson and others to sort out the confusion.
He found out that the rays are made up of
electrons: very small, negatively charged
particles that are indeed fundamental parts of
In 1911 Ernest Rutherford who
performed many experiments to explore
radioactivity did an experiment in which he
discovered that the atom must have a concentrated
positive center charge that contains most of the
atom's mass. He suggested that the nucleus
contained a particle with a positive charge the
proton. Atoms of different elements have different
numbers of protons giving their nuclei different
charges. That meant the hydrogen nucleus (it
has one proton) was an elementary particle.
Rutherford named it the proton, from the Greek
word "protos," meaning "first."
James Chadwick, an English physicist who had
worked with Rutherford, detected neutrons and
measured their mass in an invisible game of
billiards. He fired the neutrons at a block of
paraffin wax, which has a high concentration of
hydrogen and is therefore rich in protons. Some of
the neutrons collided with protons in the wax and
knocked them out. Chadwick could then detect these
protons and measure their energy. Using his
knowledge of energy and momentum, he was able to
work out the mass of the neutrons from the range
of energies of the protons that they knocked out.
He found that its mass was slightly more than that
of a proton. Chadwick, like Rutherford, used an
ingenious method to probe into what cannot be
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