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History of the Atom

model of atom

Matt Christopher

on 29 August 2016

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Transcript of History of the Atom

500 B.C.
History of the Atom
(the model through history)
Democritus-Not until around 460 B.C., did a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. (Primates have great mimicking ability.)
600 B.C. Thales of Miletus discovered that a piece of amber, after rubbing it with fur, attracts bits of hair and feathers and other light objects. He suggested that this mysterious force came from the amber. Thales, however, did not connect this force with any atomic particle.
John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental.
Thomsons 'Plum Pudding' model of the atom

In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.
In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, you can measure the energy only in discrete units. He called these energy packets, quanta.

Physicists at the time thought that light consisted of waves but, according to Albert Einstein, the quanta behaved like discrete particles. Physicists call Einstein's discrete light particle, a "photon*."

Photoelectric effect

Atoms not only emit photons, but they can also absorb them. In 1905, Albert Einstein wrote a ground-breaking paper that explained that light absorption can release electrons from atoms, a phenomenon called the "photoelectric effect." Einstein received his only Nobel Prize for physics in 1921 for his work on the photoelectric effect.
Other particles got discovered around this time called alpha rays. These particles had a positive charge and physicists thought that they consisted of the positive parts of the Thompson atom (now known as the nucleus of atoms).

In 1911 Ernest Rutherford thought it would prove interesting to bombard atoms with these alpha rays, figuring that this experiment could investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and shinned them onto the atoms in gold foil. Behind the foil sat a fluorescent screen for which he could observe the alpha particles impact.

The results of the experiments came unexpected. Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil! Rutherford reasoned that they must get scattered by tiny bits of positively charged matter. Most of the space around these positive centers had nothing in them. He thought that the electrons must exist somewhere within this empty space. Rutherford thought that the negative electrons orbited a positive center in a manner like the solar system where the planets orbit the sun.

Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.

Not until 1919 did Rutherford finally identify the particles of the nucleus as discrete positive charges of matter. Using alpha particles as bullets, Rutherford knocked hydrogen nuclei out of atoms of six elements: boron, fluorine, sodium, aluminum, phosphorus, an nitrogen. He named them protons, from the Greek for 'first', for they consisted of the first identified building blocks of the nuclei of all elements. He found the protons mass at 1,836 times as great as the mass of the electron.

But there appeared something terribly wrong with Rutherford's model of the atom. The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward. Moreover, physicists reasoned that the atoms should give off a rainbow of colors as they do so. But no experiment could verify this rainbow.
In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. (This began a new approach to science because for the first time rules had to fit the observation regardless of how they conflicted with the theories of the time.)

Bohr said, "Here's some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them." Bohr came up with two rules which agreed with experiment:

RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.

RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit

The electron can exist in only one of the orbits. (The diagram shows only five orbits, but any number of orbits can theoretically exist.)

Light (photons) emit whenever an electron jumps from one orbit to another. The jumps seem to happen instantaneously without moving through a trajectory. By the 1920s, further experiments showed that Bohr's model of the atom had some troubles. Bohr's atom seemed too simple to describe the heavier elements. In fact it only worked roughly in these cases.
In 1926 the Austrian physicist, Erwin Schrödinger had an interesting idea: Why not go all the way with particle waves and try to form a model of the atom on that basis? His theory worked kind of like harmonic theory for a violin string except that the vibrations traveled in circles.

The world of the atom, indeed, began to appear very strange. It proved difficult to form an accurate picture of an atom because nothing in our world really compares with it.

Schrödinger's wave mechanics did not question the makeup of the waves but he had to call it something so he gave it a symbol:

The "psi" symbol of Schrödinger's wave came from the Greek lettering system.
J.J. Thomson
John Dalton
A mystery of the nature of the nucleus remained unsolved. The nucleus contains most of the atom's mass as well as the positive charge. The protons supposedly accounted for this mass. However, a nucleus with twice the charge of another should have twice the number of protons and twice the mass. But this did not prove correct. Rutherford speculated in 1920 that there existed electrically neutral particles with the protons that make up the missing mass but no one accepted his idea at the time.

Not until 1932 did the English physicist James Chadwick finally discover the neutron. He found it to measure slightly heavier than the proton with a mass of 1840 electrons and with no charge (neutral). The proton-neutron together, received the name, "nucleon."
Aristotle- "You're wrong"
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.
Overview Video
Aristotle, which viewed the entire known universe as being made up of five distinct “elements” (earth, fire, air, water, and ether) which mixed and matched to form anything of substance. In his theory was no need for everything to be made of tiny little atoms, so the view of Democritus was largely ignored.
One may be quick to judge Aristotle and the rest of the “scientific” community of the time for so quickly dismissing the views of Democritus (which are now known to be, to a certain extent, more correct), but this would be a bit near-sighted. The truth is that in the world of the ancient Greeks, there truly was much more evidence for the views of Aristotle than that of Democritus.
Because of the logic inherent in Aristotle’s views, as well as the fact that as the foremost scholar of his day many of his views went almost entirely unchallenged, the atomic view of Democritus faded away and Aristotle’s view increased in popularity to the point where it was nearly heretical to question it.
And so things would remain for more than two thousand years. Even as other principles of Aristotle (the geocentric view of the universe, the law of gravity, the concept of light) finally began to be questioned by the likes of Galileo, Copernicus, Kepler and Newton, his theory on matter remained It was not until the nineteenth century(!) in fact, that Scientists finally began to give the ideas of Democritus a second look.

Aristotle not only studied almost every subject possible at the time, but made significant contributions to most of them. In physical science, Aristotle studied anatomy, astronomy, embryology, geography, geology, meteorology, physics and zoology. In philosophy, he wrote on aesthetics, ethics, government, metaphysics, politics, economics, psychology, rhetoric and theology. He also studied education, foreign customs, literature and poetry. His combined works constitute a virtual encyclopedia of Greek knowledge. It has been suggested that Aristotle was probably the last person to know everything there was to be known in his own time.
Popularly known as the Laughing Philosopher (for laughing at human follies), the terms Abderitan laughter, which means scoffing, incessant laughter, and Abderite, which means a scoffer, are derived from Democritus. To his fellow citizens he was also known as "The Mocker".
Alchemy was a protoscientific mix of chemistry, astrology, mysticism, metallurgy, physics, and religion. It had its origins in Egypt, India, and China, and has largely been associated with metallurgy and pharmacology. Its practitioners have been depicted as both cosmic clowns and demonic dabblers.
Alchemists searched for the Philosopher's Stone, which supposedly had the ability to transform base materials like copper or lead, into valuable substances, like gold. They also searched for the Elixir of Life, which when drunk by a particular person, would grant him immortality. Thank you Harry Potter.
But on a practical level, these alchemists attempted to understand and explain some of the early chemical reactions which had been discovered largely by accident. They involved such things as primitive metallurgy and dyeing of cloth. They were often involved in brass making, gold smithing, and assessing the noble metal content of ore, jewelry, or coins. It's a forerunner of chemistry.
Key Contributions of Alchemists
Many of the techniques used in chemistry began with the alchemists: refinement of distillation, sublimation, and other techniques still important in modern laboratories were developed and modified by them. They developed instruments and containers to carry out these processes, and though they were crude by modern standards they served as models.
They recognized that the presence of heat, such as fire often is necessary for chemical reactions to happen
Greeks believe all matter made up of earth, fire, air and water. Alchemists moved away from this and gradually managed to isolate more compounds and elements such as mercury, sulfur, phosphorus, and antimony.
It was an alchemist who discovered the process to make European porcelain as well as Cymbal making (alloy of copper and tin)
The alchemist Paracelsus helped transform medicine
They thought they could change one element into another…………lead into gold.
Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.
His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.
He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.
Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.
Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.
For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.
As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.
He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.
Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.
Extra Videos about Rutherford's Experiment
Interactive Explanation of Bohr Model.
Full transcript