| How did Ernest Rutherford discover the proton?
What was his experiment?
|Question Date: 2016-02-03
Ernest Rutherford bombarded nitrogen atoms with
helium nuclei (alpha particles), and hydrogen
atoms (protons) were
formed as a result. From this, he concluded that
nitrogen nuclei contained protons.
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Rutherford's experiment for the discovery of the
Great question! Rutherford's experiment was
incredibly powerful due to its simplicity, use of
cheap materials, and the information he
uncovered. Rutherford's main focus was on
studying the nucleus through radioactive decay.
After the electron was discovered by Thomson in
1897 and after Rutherford's work on discovering
the existence of the nucleus 1911, it was known
that there must exist particles of positive charge
to balance the negatively charged electrons to
create electrically neutral atoms.
The series of experiments, performed by Rutherford
and his student James Chadwick, consisted of
changing one element into another by hitting atoms
with high energy alpha particles.
Specifically, they noticed that nitrogen, oxygen,
and aluminum, when hit with an alpha particle,
disintegrated and emitted a fast particle of
positive charge. Or said more specifically,
hydrogen nuclei were always emitted in the
process. In a dark room, they were able to observe
flashes of light when alpha particles hit the
target. Alpha particles, which were also
discovered by Rutherford, are spontaneously
emitted by radioactive materials such as uranium.
It was realized that the positive charge of any
nucleus could be accounted for by a whole
(integer) number of positively charged hydrogen
nuclei, which were named protons by Rutherford in
They immediately wondered what was left behind
on the target after this process occurred. The
conclusion was that the target captured the alpha
particle (2+ charge) and emitted a proton (1+
charge), resulting in the target having a nuclear
charge different than before. The target was now
an isotope of another element. In the case of a
nitrogen target, the nitrogen had a nuclear charge
of 8 instead of seven, making it an isotope of
oxygen (an isotope is an atom with the same
charge but a different atomic weight.) This
describes how the proton itself was discovered.
Many key experiments by Rutherford, his
students, and others occurred before this and led
to the understanding of the atom that is still the
accepted model today.
Note from ScienceLine Moderator:
A Science Journalist wrote the following
information for ScienceLine in order to correct
the historical error about the first man-made
nuclear transmutation. We thank this person for
their time and interest in providing reliable
information to our audience.
From a Science Journalist
For nearly 70 years, most scholars have
incorrectly attributed the first man-made nuclear
transmutation to Rutherford, however, the credit
belongs to Patrick Blackett, a research
fellow working under Rutherford. Between 1921 and
1924, Blackett performed the experiments that
identified and proved the transmutation of
nitrogen to oxygen. He published his results in 1925.
In 2016, I published a forensic historical
examination of the early 20th century
transmutation research in my book Lost History. In
2017, I communicated my findings to
the U.S. Department of Energy, Office of History
and Heritage Resources; the American Institute of
Physics, Center for History of Physics; the
Imperial College London, Physics Department (Home
to Blackett's laboratory); and the Cambridge
University, Physics Department (Home to
Rutherford's laboratory). Each organization has
now completed its own independent analysis,
concurred, and corrected their respective Web
sites. Here are the respective URLS:
(U.S. Department of Energy)
history aip org
(Click on Nobel Prize Winners)
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