Electron (e-)
John Dalton believed that the atom was the
smallest component of matter. Little did he know that all would change as
experiments on electricity were conducted. If you rub a balloon on your hair,
small pieces of paper get attracted to it. What
makes the pieces of paper stick to the balloon? John Dalton’s Atomic Theory
was not able to explain this phenomenon. There must be something inside the
atoms of the balloon that attracts the atoms of the pieces of paper.
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| Joseph John Thomson |
This question was answered by Joseph John
Thomson when he discovered the electrons, the negatively charged particle of an
atom. Thomson discovered the electrons when he conducted experiments using a
Crookes Tube, named after its inventor William Crookes, which was later called
Cathode Ray Tube (CRT).
This CRT is a vacuum tube that produces a
beam of light as residual air inside the tube gets ionized. This beams of light moves from cathode to
anode. Thomson noticed that the beam
of light was attracted to a positive plate in electric field. Since the rule in
electromagnetism is “like charges repel each other; unlike charges attract each other”, Thomson
concluded from his experiment that the beam of light is negatively charged, and
since matter also behave in the same manner as the beam of light, he also
concluded that matter is made up atoms with negatively charged particles called
electrons,
a term introduced for this charge by George Johnstone Stoney. After discovering electrons, Thomson proposed a
new atomic model which was called the Raisin Bread Model or the Plum Pudding Model
of the Atom, where the raisins or plums are the corpuscles or electrons
embedded on a sphere of uniform electrification.
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| The beam of light gets attracted to a positive plate in electric field |
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| Crooke's Tube |
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| Raisin Bread or Plum Pudding Model |
Another
scientist was awarded a Nobel Laureate in Physics for his measurement of the
charge of the electron. It was Robert Andrews Millikan, an American
experimental physicist. He was able to measure the charge of an electron by
observing the movement of tiny droplets of oil in an apparatus with
electrically charged plates and an x-ray source. Based on his experiment, the
charge of an electron is -1.602 x 10-19C
(coulomb). Further calculations were made and the mass of an electron was found
out to be 9.109 x 10-31 kg.
Studies on Radioactivity
Radioactivity
is another field of science that provided insights on the structure of the
atom. Scientists like Henri Becquerel and Marie Curie noticed that some
elements change in chemical properties as they emit radiation. Some of the
known types of radiation during those times are the alpha, beta and gamma radiation.
Nucleus
A New Zealand-born British chemist and physicist
named Ernest Rutherford conducted experiments using a newly discovered
radiation called alpha particles to
determine the composition of the atom. His experiment was called the Alpha
Scattering Experiment, sometimes called as the Gold Foil Experiment.
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| Ernest Rutherford |
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| Alpha-Scattering Experiment |
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| Gold Foil as Alpha particles pass through |
In his experiment, a thin gold foil
surrounded by a phosphor-coated detector was bombarded with positively charges
alpha particles. In his experiment, he observed that most of the alpha
particles just went through the gold foil, but to his surprise, some of the
particles got deflected while some even bounced back. He concluded from his
experiment that the atom is mostly empty space since most of the alpha
particles just went through the gold foil. For the deflected alpha particles,
he said that it must have hit something inside the atom that is of very small
volume and is positively charged, which he called the nucleus. The nucleus
covers the total mass of an atom.
From his discovery of the positively charged
nucleus of the atom, he proposed a new atomic model called Nuclear Model of the Atom (sometimes called the Planetary Model), where
the negatively charged electrons move around the positively charged nucleus of
the atom, like the planets moving around the sun.
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| Nuclear Model of the Atom |
Proton (p+)
Eugen Goldstein is sometimes credited for the
discovery of the proton, the positively charged particle of the atom with a
mass of 1.6726×10−27 kg and a charge of +1. He
conducted experiments using the Crookes Tube. He observed that as negatively
charged electrons move from cathode to anode, there were also particles moving
from anode to cathode, called the canal rays or anode rays. However,
Goldstein was not able to single out the proton from the gases he worked with,
leaving his search unfinished.
After
his discovery of the nucleus, Ernest Rutherford discovered that the hydrogen
nucleus is present in other nuclei. His experiments showed that nitrogen gas
gave signatures of hydrogen nuclei. This hydrogen nuclei could have only come
from nitrogen, thus, nitrogen must contain hydrogen nuclei. He named this
hydrogen nucleus as proton, which means first.
Neutron
The neutron is a subatomic particle located at
the nucleus that has almost the same mass as the proton but has no charge. It
was discovered by James Chadwick in 1932. The discovery of neutrons led to the
study of nuclear fission and was the particle used to split up the uranium
atoms in the first atomic bomb.
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