Download General Chemistry/Atomic Structure/History of Atomic Structure

General Chemistry/Atomic Structure/History of Atomic Structure
General Chemistry/Atomic Structure/History of
Atomic Structure
Why is the history of the atom so important?
It is fundamental to the understanding of science that science is understood to be a process of trial and improvement
and represents the best known at the time, not an unerring oracle of truth. Development of an idea and refinement
through testing is shown more in the understanding of atomic structure
The Greek Theorists
The earliest known proponent of anything resembling modern atomic
theory was the ancient Greek thinker Democritus. He proposed the
existence of indivisible atoms as a response to the arguments of
Parmenides, and the paradoxes of Zeno.
Parmenides argued against the possibility of movement, change, and
plurality on the premise that something cannot come from nothing.
Zeno attempted to prove Parmenides' point by a series of paradoxes
based on difficulties with infinite divisibility.
In response to these ideas, Democritus posited the existence of
indestructible atoms that exist in a void. Their indestructibility
provided a retort to Zeno, and the void allowed him to account for
plurality, change, and movement. It remained for him to account for
the properties of atoms, and how they related to our experiences of
objects in the world.
Democritus proposed that atoms possessed few actual properties, with
A bust of Democritus (or Democrites), who came
size, shape, and mass being the most important. All other properties, he
up with the idea of indivisible atoms.
argued, could be explained in terms of the three primary properties. A
smooth substance, for instance, might be composed of primarily
smooth atoms, while a sour substance is composed of rough or sharp ones. Solid substances might be composed of
atoms with numerous hooks, by which they connect to each other, while the atoms of liquid substances possess far
fewer points of connection.
Democritus proposed 6 points to his theory of atoms. [1] These were:
1. All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further
split into smaller portions.
2. There is a void, which is empty space between atoms.
3. Atoms are completely solid
4. Atoms are homogeneous, with no internal structure.
5. Atoms are different in: their sizes, their shapes, and their weights.
6. Atoms are the building blocks of life
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General Chemistry/Atomic Structure/History of Atomic Structure
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Alchemy
Empedocles proposed that there were four elements, air, earth, water, and fire and that
everything else was a mixture of these. This belief was very popular in the medieval ages
and introduced the science of alchemy. Alchemy was based on the belief that since
everything was made of only four elements, you could transmute a mixture into another
mixture of the same type. For example, it was believed that lead could be made into
gold.
Alchemy's problem was exposed by Antoine Lavoisier when he heated metallic tin in a
sealed flask. A grayish ash appeared on the surface of the melting tin, which Lavoisier
heated until no more ash formed. After the flask cooled, he inverted it and opened it
underwater. He discovered the water rose one-fifth of the way into the glass, leading
Lavoisier to conclude that air itself is a mixture, with one-fifth of it having combined
with the tin, yet the other four-fifths did not, showing that air was not an element.
A fire, shown by Lavoisier
to be a chemical reaction
and not an element.
Lavoisier repeated the experiment again, substituting mercury for tin, and found that the same happened. Yet after
heating gently, he found that the ash released the air, showing that the experiment could be reversed. He concluded
that the ash was a compound of the metal and oxygen, which he proved by weighing the metal and the ash, and
showing that their combined weight was greater than that of the original metal.
Lavoisier then stated that combustion was not an element, but instead was a chemical reaction of a fuel and oxygen.
John Dalton
Modern atomic theory was born with Dalton when he published his
theories in 1803. His theory consists of five important points, which
are considered to be mostly true today: (from Wikipedia)
• Elements are composed of tiny particles called atoms.
• All atoms of a given element are identical.
• The atoms of a given element are different from those of any other
element; the atoms of different elements can be distinguished from
one another by their respective relative weights.
Different elements, different atoms.
• Atoms of one element can combine with atoms of other elements to form chemical compounds; a given
compound always has the same relative numbers of types of atoms.
• Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical
reaction simply changes the way atoms are grouped together.
We now know that elements have different isotopes, which have slightly different weights. Also, nuclear reactions
can divide atoms into smaller parts (but nuclear reactions aren't really considered chemical reactions). Otherwise, his
theory still stands today.
Dmitri Mendeleev
While writing a series of textbooks, Mendeleev realized he was running out of space to treat each element
individually. He sorted the elements by weight, regularly "linewrapped" the elements onto the next line, and made
what is now called the periodic table of the elements. Using his table, he predicted the existence of "eka-aluminum"
and "eka-silicon" (gallium and selenium) successfully, confirming the usefulness of his table. Later theories, those of
the electrons around the atom, explain why elements in the same period, or group, have similar chemical properties.
General Chemistry/Atomic Structure/History of Atomic Structure
J.J. Thompson
Discovery of the Electron
In the year 1889 the British physicist J.J. Thomson discovered the
electron. Thomson conducted a number of experiments using cathode
rays. Cathode rays are constructed by sealing two electrodes in a glass
tube and removing the air from it. When the electrodes are attached to
high voltage, a beam of radiation is emitted from the negative
electrode. These beams are called cathode rays.
Thompson discovered that cathode rays travel in straight lines except
Cathode rays are actually made of electrons.
when they are bent by electric or magnetic fields. Because the cathode
rays bent away from a negatively charged plate, Thomson concluded
that these rays are made of negatively charged particles; today we call them electrons. Thompson found that he could
produce cathode rays using electrodes of various materials. He then concluded that electrons were found in all atoms
and are over a thousand times smaller than protons.
The "Plum Pudding" Atomic Model
Soon after the discovery of the electron, Thompson began speculating on the nature of the atom. He suggested a
"plum pudding" model. In this model the bits of "plum" were the electrons which were floating around in a
"pudding" of positive charge to match that of the electrons and make an electrically neutral atom. A modern
illustration of this idea would be a chocolate chip cookie, with the chips representing negatively charged electrons
and the dough representing positive charge.
Rutherford
Ernest Rutherford is known for his famous gold foil experiment in
1909. Alpha particles, which are heavy and positively charged
(actually, helium nuclei, but that's beside the point), were fired at a
very thin layer of gold. Most of the alpha particles passed straight
through, as expected. According to the plum pudding model all of the
particles should have slowed as they passed through the "pudding", but
none should have been deflected. Surprisingly, a few alpha particles
were deflected back the way they came. He stated that it was "as if you
fired a 15-inch shell at a piece of tissue paper and it came back and hit
you."
The result of the experiment allowed Rutherford to conclude that the
plum pudding model is wrong.
• Atoms have a nucleus, very small and dense, containing the
positive charge and most of the atom's mass.
• The atom consists of mostly empty space.
• The electrons are attracted to the nucleus, but remain far outside it.
The results of the gold foil experiment disproved
the "plum pudding" model: the alpha particles
should have passed through (top), but a few of
them deflected at large angles (bottom).
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General Chemistry/Atomic Structure/History of Atomic Structure
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Niels Bohr
Bohr created his own model of the atom, improving on Rutherford's.
Bohr created an equation that could predict the spectral lines of the
hydrogen atom. He then realized that electrons must orbit the nucleus
in "shells", each with a certain energy level. An atom will absorb and
release photons that have a specific amount of energy. The energy is
the result of an electron jumping to a different shell. The Bohr model
depicts the atom as a nucleus with electrons orbiting around it at
specific distances.
Millikan
The Bohr model of the atom has shells with
numbered spherical energy levels - the larger
numbers mean larger spheres and higher energy
levels. The wave exiting the picture on the left
has come from an electron jump, resulting in a
photon. (Level sizes not to scale.)
Robert Millikan is accredited for the "Oil Drop Experiment", in which
the value of the electron charge was determined. He created a
mechanism where he could spray oil drops that would settle into a
beam of X rays. The beam of X rays caused the oil drops to become
charged with electrons. The oil droplets were in between a positively
charged plate and a negatively charged plate which, when proper electric voltage was applied, caused the oil droplet
to remain still. Robert Millikan measured the diameter of each individual oil drop using a telescope.
Millikan was able to calculate the mass of each oil droplet because he knew the density of the oil (
). Using the mass of each oil droplet and the equation for the force of gravitational
attraction (which he rearranged from
individual oil droplet,
to
is the acceleration due to gravity, and
, where
is the mass of each
is the electrical force which equals force in the
first equation), Millikan was able to find the value of the charge of the electron, .
The X rays, however, did not always produce an oil drop with only one negative charge. Thus, the values Millikan
obtained may have looked like this:
•
•
coulomb
coulomb
•
•
coulomb
coulomb
Millikan found that these values all had a common divisor:
coulomb. He concluded that different
values occurred because the droplets acquired charges of -5, -4, -3, and -2, as in this example. Thus, he stated that the
common divisor,
coulomb, was the charge of the electron.
References
[1] http:/ / dbhs. wvusd. k12. ca. us/ webdocs/ AtomicStructure/ Greeks. html
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