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question you have about that subject.
Development of the Model of the Atom
The atom is defined as the smallest part of an element that retains the chemical properties of that
element. The existence of atoms was first guessed as early as 400 BC, when Greek philosophers debated
whether one could divide a substance into infinitely smaller pieces or if eventually a smallest, indivisible
particle would be reached. Around 450 BC, the Greek philosopher Democritus proposed that all matter
is made up of small, indivisible particles called atomos (meaning indivisible). His ideas were met with
much criticism, and it was not until the early 1800s that the atomic theory took complete hold in
scientific thought.
Early atomic theory
Atomic models have their beginnings in the early atomic theory. In 1808, John Dalton (1766-1844)
proposed his atomic theory, which served as an explanation for these phenomena. His theory states that
all matter is composed of atoms, which cannot be further divided, created, nor destroyed.
Not all of Dalton's atomic theory has stood the test of time. The evolution of the atomic model has led
to the discovery that atoms are divisible into smaller particles, and that not all atoms of a given element
are exactly the same. The discovery of these subatomic particles, along with Dalton's atomic theory, set
in motion the development of several atomic models
Discovery of the electron
The discovery of the first subatomic particle, the electron, resulted from experiments involving the
effects of electricity on matter. In the late 1800s, the cathode ray tube was developed and used in
several investigations. Early experiments showed that a magnet caused the cathode ray to bend toward
or away from the poles. It was concluded that the particles that compose a cathode ray not only had
mass but also a negative charge. J.J. Thomson concluded that all cathode rays are made up of the same
charged particles, which were later named electrons.
The American physicist Robert A. Millikan (1868-1953) of the University of Chicago performed
experiments that further confirmed Thomson's results. Through his "oil droplet" experiments, in 1909
he discovered that the mass of one electron is approximately one two-thousandth that of a hydrogen
atom. Using these data, he calculated that the charge of every oil droplet was a multiple of the same
number each time, and concluded that this must be the charge of a single electron.
The first atomic models
Thomson used these results to formulate his "plum pudding" model of the atom between the years
1903-1907. According to this model, the negatively charged electrons of an atom are found within a
positively charged material, much like plums embedded in plum pudding. Thomson's "plum pudding"
model was accepted for only a few years. In 1911, New Zealand scientist Ernest Rutherford (1871-1937)
proposed his own atomic model based on his famous gold foil experiments. Rutherford’s team aimed a
stream of alpha particles at a thin sheet of gold foil. Their hypothesis was that the alpha particles would
pass through the gold foil with minimal deflection, since mass and charge are distributed uniformly
throughout an atom, as proposed by Thomson. The data did not agree with this assumption.
Some of the alpha particles were deflected at large angles as they passed through the gold foil. He
proposed that the deflected alpha particles must have come in contact with a densely packed positive
charge. He called this region of positive charge the nucleus. They believed their data supported a
nucleus surrounded by empty space, through which the electrons circled like the planets circle the sun.
This model had it questions and problems. It was difficult to mathematically explain how electrons could
remain in orbit without its orbit decaying and crashing into its nucleus.
Discovery of the proton
The English scientist Henry Gwyn Jeffreys Moseley (1887-1915) soon solved the mystery of nuclear
charge determining the properties of the atom. Moseley discovered in 1913 that each element contains
a unique positive charge in its nucleus. The nucleus of the atom must contain positively charged
particles, called protons.
Discovery of the neutron
The discovery of the proton resulted in another mystery. The mass of the hydrogen atom was known to
be larger than the mass of a proton and an electron added together. Scientists searched for the source
of the missing mass by assuming that another particle that also contributes to the mass of the atom
must exist in the nucleus of the atom. Experiments showed that this particle must be neutral in charge,
but about the same mass as a neutron. British scientist James Chadwick (1891-1974) discovered this
particle we now know as the neutrons. Neutrons together with protons in the nucleus make up the mass
of an atom, since the mass of the electrons are insignificant. Electrons are located in the empty space
surrounding the atom, which makes up most of its volume.
The Bohr model of the atom
In 1913, the Danish theorist Niels Bohr (1885-1962) developed his Bohr Model to satisfy long time
observations about how excited atoms emit specific colors of light. His model describes electrons in
separate energy levels also called orbits outside the nucleus.
These orbits can be thought of as the rungs of a ladder. As a person climbs up a ladder, they step on one
rung or another, but not in between rungs, because a person cannot stand on air. Likewise, the
electrons of an atom can occupy one orbit or another, but cannot exist in between orbits. While in an
orbit, the electron has a fixed amount of energy. When an electron gains energy and becomes excited it
jumps to a new orbit of higher energy and further from the nucleus. When an electron falls from the
excited state to a lower energy state, light is emitted with a specific energy. The Bohr model is often
used today as a simplified representation of the atomic model. However, further experimental
observations would necessitate further changes to produce a working model.