Download Democritus (460

Democritus (460-371 B.C.)
One of the first "atomic theorists" we have any record of was a fifth-century BC Greek, Democritus. The basic
idea was that if you could look at matter on smaller and smaller scales (which they of course couldn't)
ultimately you would see individual atoms - objects that could not be divided further (that was the definition of
atom). Everything was made up of these atoms, which moved around in a void (a vacuum of empty space).
Atoms were impenetrably hard, meaning they could not be divided. In Greek, the prefix "a" means "not" and
the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable.
The different physical properties -- color, taste, and so on -- of materials came about because atoms in them had
different shapes and/or arrangements and orientations with respect to each other.
This was all pure guess, but the physical pictures they described sometimes seem incredibly accurate. These
Greek philosophers believed that atoms were in constant motion, and always had been. Basically, Democritus
and his followers had a very mechanical picture of the universe. They thought all natural phenomena could be
understood in terms of interacting amongst moving atoms.
Democritus also said that all atoms looked the same, meaning they were homogeneous. He assumed they had no
internal structure because they did not have the equipment at the time to look at atoms. Democritus also
speculated that atoms were solid all the way through.
Aristotle criticized Democritus’s theory as ridiculous saying that there was no such thing as empty space. He
convinced may others that this early theory of the atom couldn’t possibly be true. The four key components of
matter had to be earth, air, wind and fire. Little conceptual progress in atomic theory was made over the next
two thousand years, mostly because Aristotle discredited it.
Dalton (1803)
John Dalton (1766-1844) was born into a poor family near Manchester, England. He
supported himself to some extent by teaching from the age of twelve, when he started his own
small Quaker school. Dalton wrote A New System of Chemical Philosophy.
Englishman John Dalton was one of the first scientists to decide that all matter is made up of small particles, or
atoms. He is also remembered for his weather observations, which he began recording in 1787 using
instruments he made himself. Through the instruments he created, John Dalton could study humidity,
temperature, atmospheric pressure, and wind. He maintained these records for 57 years, until his death.
Throughout those years, over 200,000 meteorological values were recorded. Dalton grew up poor and had to
teach himself. In 1793 Dalton published Meteorological Observations and Essays, one of the first books of its
kind.
His studies led him to develop theories about water vapor and mixed gases. From there, Dalton decided that all
matter, not only gases, must consist of small particles. He revived the ancient theory of atoms and prepared the
first table of atomic weights, and announced his notions publicly in 1803.
In 1794 Dalton became the first to describe colorblindness as he himself was colorblind. He said that it was
hereditary because he and his brother were colorblind since birth. He said that he saw two main colors.
Dalton’s Atomic Theory:
1.
2.
3.
4.
All matter consists of atoms that are indivisible (cannot be divided into anything smaller).
Atoms of the same element are identical in mass and properties.
Atoms combine chemically in small whole number ratios called compounds (like H2O)
Atoms cannot be created or destroyed. They can be rearranged, separated, or combined in chemical
reactions.
Thomson (1897)
In 1897, the British physicist J.J. Thomson (1856–1940) discovered the electron in a series
of experiments designed to study the nature of electric discharge in a high-vacuum
cathode-ray tube, an area being investigated by numerous scientists at the time.
A cathode ray was sent through the tube. Scientists later realized that it was a stream of charged particles that were attracted and deflected to magnets. This led scientists to believe that there were charged particles in an atom.  Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of
“bodies much smaller than atoms” that he calculated as having a very large value for the charge-to-mass ratio.
This means that they were REALLY small, but carried a charge.
In 1904, Thomson suggested a model of the atom as a sphere of positive matter in
which electrons are positioned by electrostatic forces. They originally called this
model the “plum pudding model.” They also now refer to this model as the
“chocolate chip cookie dough model.”
With the help of other scientists, Thomson was able to realize that electrons
were negatively charged particles within the atom. They also concluded that
the electron was much, much smaller than a hydrogen atom. Specifically,
electrons had a mass of 1/1836th of a hydrogen atom. The hydrogen atom was
the smallest particle that scientists thought existed at the time.
Why was Thomson’s discovery so important? It disproved Dalton’s theory
that atoms were indivisible. We now knew that there were smaller parts to an
atom.
J. J. Thomson (left) and Ernest Rutherford in the 1930s. From The Growth of Physical Science, by Sir James
Hopwood Jeans (Cambridge: Cambridge University Press, 1948).
Rutherford (1909)
In 1909, Rutherford conducted his famous gold foil experiment. As part of an experiment
with x-rays, Rutherford was shooting a beam of alpha particles (emitted by the radioactive
element radium) at a sheet of gold foil only 1/3000 of an inch thick, and tracing the
particles' paths. In the experiment, Rutherford and his colleague Hans Geiger bombarded a
piece of gold foil with positively charged alpha particles, expecting particles to travel
straight through the foil like a cannonball tearing through tissue paper. Instead, many alpha
particles ricocheted off of the foil, suggesting that there was something positive inside the
atom that these particles were colliding with.
Rutherford concluded that since most alpha particles travelled straight through the foil, that an atom is mostly
empty space. But what caused the few particles to bounce back toward the source? He and his colleagues
concluded that there was something small and positively charged in the center of these atoms blocking the path
of the alpha particle. Rutherford named this positive force the nucleus. The nucleus was tiny – 10,000 times
smaller than the atom, VERY dense/compact and contained most of the mass of the entire atom. The
Rutherford Model was created based on this new data. In 1920, he later named the proton as the large
positively charged particle making up about ½ of the mass of the nucleus.
The diagram above depicts the expected and the actual results of the gold foil experiment. The diagram on the left
shows particles passing through the positively charged matrix of the plum pudding model. The diagram on the right
shows a particle ricocheting off of the nucleus in the center of the atom.
Bohr (1913)
In 1911, Niels Bohr earned his PhD in Denmark on the electron theory of metals. Right
afterwards, he went to England to study with J.J. Thomson, who had discovered the
electron in 1897. Few concerned themselves much with the work of Max Planck or Albert
Einstein. Thomson wasn't that interested in these new ideas, but Bohr had an open mind.
Bohr soon went to visit Ernest Rutherford (a former student of Thomson's) in another part
of England, where Rutherford had made a brand-new discovery about the atom.
In 1912 Bohr joined Rutherford. He realized that Rutherford's
model wasn't quite right. Through the study of Rutherford’s
atom, it became known that all of the negative charge was held
in the electrons, which must orbit the dense nucleus like planets
around the sun. By all rules of classical physics, it should be
very unstable. For one thing, the orbiting electrons should give
off energy and eventually spiral down into the nucleus, making
the atom collapse. Or the electrons could be knocked out of
position if a charged particle passed by.
Bohr turned to
Planck's quantum theory to explain the stability of most atoms. Bohr
suggested the revolutionary idea that electrons "jump" between energy
levels (orbits) without ever existing in an in-between state. Thus when an
atom absorbs or gives off energy (as in light or heat), the electron jumps to
higher or lower orbits.
Bohr's theory that electrons existed in set orbits around the nucleus was
the key to the periodic repetition of properties of the elements. The shells
in which electrons orbit have different quantum numbers and hold only
certain numbers of electrons -- the first shell holds no more than 2, the
second shell up to 8 electrons, the third 18 electrons, etc.
Chadwick (1932)
For four years, James Chadwick was a prisoner of war in Germany. When World War I
ended, he returned to his native England to rejoin the mentor of his undergraduate
days, Ernest Rutherford. Chadwick's own research focused on radioactivity. In 1920,
Rutherford had discovered the proton, a positively charged particle within the atom's
nucleus. But they and other researchers were finding that the proton did not seem to be the
only particle in the nucleus.
As they studied atoms, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to
the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a
helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have
almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass.
Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron,
and imagined it as a paired proton and electron. There was no evidence for any of these ideas.
Chadwick repeated the experiments of the Curie’s but with the goal of looking for a neutral particle -- one with
the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine
that the neutron did exist and that its mass was about 0.1 percent more than the proton's. The discovery of the
neutron eventually contributed to the study of atomic bombs and are currently used as the driving force of
nuclear bombs and weaponry.
Schrodinger (1926)
In 1926, Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further.
Schrödinger used mathematical equations to describe the likelihood of finding an electron in a
certain position. This atomic model is known as the quantum mechanical model of the atom.
Unlike the Bohr model, the quantum mechanical model does not define the exact path of an
electron. In other words, electrons do not orbit the nucleus in a circular path.
Instead Schrodinger predicts the odds of the location of the electron. This model can be pictured as a nucleus
surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is
greatest, and conversely, the electron is less likely to be in a less dense area of the cloud.
The probability function basically describes a cloud-like region where the electron is likely to be found. It
cannot say with any certainty, where the electron actually is at any point in time, yet can describe where it
should be.
The cloud model represents a sort of history of where the electron has
probably been and where it is likely to be going. The red dot in the middle
represents the nucleus while the red dot around the outside represents an
instance of the electron. Imagine, as the electron moves it leaves a trace of
where it was – kind of like a time lapse photo. This collection of traces
quickly begins to resemble a cloud. The probable locations of the electron
predicted by Schrödinger's equation happen to coincide with the locations
specified in Bohr's model.
He also built off of Bohr's model of the atom with the Electron Cloud Model. This model depicts the floating
motion of the electrons, rather than them having a set path of travel. He determined the probability location of
electrons in atoms. According to Schrodinger, electrons stuck in their orbits would set up "standing waves". He
said that you could describe only the probability of where an electron could be, it was not definite. The
distributions of these probabilities formed areas of space about the nucleus were called orbitals. An orbital is a
wave function describing the state of a single electron in an atom.