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History of the Atom

atom n. A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, the entire structure having an approximate diameter of 10-8 centimeter and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

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History of the Atom

Models used by scientists do not provide an absolute understanding of the atom but only a way

of abstracting so that they can make useful predictions about them. The epistemological

methods that scientists use provide us with the best known way of arriving at useful science and

factual knowledge. No other method has yet proven as successful.

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History of the Atom

Epistemology or theory of knowledge is the branch of philosophy that studies the nature,

methods, limitations, and validity of knowledge and belief.

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History of the Atom

460 B.C., a Greek philosopher, Democritus:– develop the idea of atoms. – “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.

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History of the Atom

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.

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History of the Atom

1800's -

An English chemist, 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

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History of the Atom Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were. The core concepts of the theory, that chemical reactions can be explained by the union and separation of atoms, and that these atoms have characteristic properties, are foundations of modern physical science.

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1897: 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.

History of the Atom

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History of the Atom

Thomsons 'Rasin in the Pudding' model of the atom

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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.

History of the Atom

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Planck:

Electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit E = hν, where h is Planck's constant, also known as Planck's action quantum and ν is the frequency of the radiation.

History of the Atom

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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."

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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.

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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).

History of the Atom

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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.

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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.

History of the Atom

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Rutherford thought that the negative electrons orbited a positive center in a manner like the solar system where the planets orbit the sun.

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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 (10 ) 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.

History of the Atom

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1919 :

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.

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Rutheford 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.

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Problem :

The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward.

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Moreover, physicists reasoned that the atoms should give off a rainbow of colors as they do so.

But no experiment could verify this rainbow

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1912:

a Danish physicist, Niels Bohr:

TWO RULES:

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

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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.

History of the Atom

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History of the AtomNiels Bohr :

Bohr's atom for Hydrogen  The electron can exist in only one of the orbits. (The diagram shows only five orbits, but any number of orbits can theoretically exist.)

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History of the Atom

Light (photons) emit whenever an electron jumps from one orbit to another. The jumps seem to happen instantaneously without moving through a trajectory.

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Wolfgang Pauli : Electron Spin

Erwin Schrodinger: Wave Mechanics

Theory of Atoms

Max Born : Probability & ‘Waves of Chance

Werner Heisenberg: Uncertainty Principle

History of the Atom