The Atom

Rachel, Elliot, and Zoe

Leucippus and Democritus

Thompson

Rutherford

Neils Bohr

Alfred Lauck Parson

De Broglie

Schrodinger

The Standard Model

Lucas

Notes: What the atom is

Leucippus and Democritus

Leucippus and Democritus were Greek Philosophers from the late 5^th century BC who were the first to propose that the universe was composed as atoms, which they describes as the smallest unit of matter, unbreakable, of infinite numbers, and of different sizes, shapes and weights.

The idea of atoms came up for them from ponderings on how many times you could break a piece of gold in half, and have it still be gold (they concluded that it would still be gold until you came to the atoms, and when you split that in half it would cease to be gold).[1]

Atoms were describe just as indivisible, solid spheres.

Aristotle was the most influential philosopher of the time, and lived during the same period as Leucippus and Democritus. He agreed that there was some type of indivisible “smallest part” of all matter, but not with the model that Leucippus and Democritus proposed. Aristotle thought that there were different atoms specific to the four elements (fire, earth, wind and water) and discredited the Democritus model, and set science back 2,000 years as far as atomic theory goes. [2]     

J.J. Thompson

In 1897 Thomson decided via his “famous experiment” that atoms were positively charged balls that also contain negative charges; the positive and negative charges thus equaled each other out making the atom neutral.[1]

He decided this based  on his research on cathode rays (cathode ray: “a flow of electrons emanating from a cathode in a vacuum tube and focused into a narrow beam)(cathode: “the electrode by which current leaves electrolytic cells, voltaic cells, batteries, etc.).[2]

Thompson conducted an experiment using Cathode Ray Tubes which were tubes that had all the air sucked out and which contained electrodes. Electrical currents were passed between two electrodes in the form of rays. Thompson so designed a tube in which the rays could visibly impact on the surface of a Zinc Sulfide coated screen. Each time a ray impacted on the screen it emitted a spark of light (caused by the zinc) so that the (otherwise invisible) path of the ray could be momentarily observed. Thompson then brought an electrical field (of a positive and negative charge) near the rays, which caused the ray’s path to deflect away from the plate causing the negative currents in the electrical field and turn instead toward the plate causing the positive currents. This indicated that the rays had a negative charge.  

The rays were later identified as having mass (because it was decided that because they had momentum, and momentum = mass x velocity) and identified as electrons. /

In 1886 Professor Goldstein performed a similar experiment producing particles that had a positive charge, which were later named protons.

It was then concluded that “Since both electrons and protons have come from the surface of matter, it is logical to conclude that all matter is composed of these particles within the atoms of matter.” [3]    

Rutherford

Rutherford was the first to popularize the idea that the positive charge in the atom was concentrated in the nucleus, and that the negative charges (electrons) were scattered around outside the nucleus

Rutherford came to his conclusion via an experiment involving firing atoms at thin gold foil coated with zinc sulfide. The majority of the particles passed through the foil, while approximately in every 8000 were deflected; meaning everyone 1 in 8000 had mass, and atoms were made mostly of empty space (filled with electrons).[1]   

Neils Bohr

The Bohr Model depicts the atom to be a bit like the solar system. Basically, Bohr says the model is a "small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus". Except, in the solar system, attraction of objects is caused by gravity, and in the model it is.

• "That electrons move in circular orbits around the atomic nucleus.
• Only certain orbits are permitted.
• That in these permitted orbits, the electrons would not radiate (would not create radio waves).
• That light of certain colors (and wavelengths) would be created when the electron (of its own power) changed orbits."

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Alfred Lauck Parson

Parson disagreed with Bohr's model. To him electron's weren't just point sized particles, they were shells that surrounded the the nucleus.

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De Broglie

“About 1924, Louis de Broglie proposed that all particles of matter (from single atoms to large objects) moving at some velocity would have the properties of a wave. Today, most physicists take this farther and say that all material objects actually are waves until they are measured or observed in some way. When this takes place, the wave is said to collapse and turn into an object. An example of this notion of reality is given by the famous Cornell physicist N. David Mermin who says "We now know that moon is demonstrably not there when nobody looks."

* * *

Schrodinger

Wave Model of the Atom

“"In 1925, soon after de Broglie had put forward his ideas, Schrodinger used them [to write] a wave equation to describe this new mechanics of particles." Schrodinger's model of the atom is not a physical model (where an object has size, shape and boundaries) but is a mathematical model (an equation where objects are point particles.) The equation is useful to predict some properties of objects (or atoms), but is not able to describe the object (or atom) itself

* * *

Lucas

The Lucas model has several advantages over previous models of the atom :

• The model gives reasons for the so-called "magic-numbers" (2, 8, 18, 32 - the maximum number of electrons that can fit in a shell) .
• The model explains why there are only seven rows in the Periodic Table of Elements.
• The model explains why "radiation death" does not occur in atoms.
• For the first time, the model correctly predicts the spins of all 1,500 nuclides that have been measured (other models are wrong for hundreds of nuclide spins).

Sources: *parts of paragraphs paraphrased or directly quoted from sources below.

http://www.commonsensescience.org/atom_models.html

http://en.wikipedia.org/wiki/Bohr_Model

http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/atommodels.com

http://csep10.phys.utk.edu/astr162/lect/light/bohr.html

http://theory.uwinnipeg.ca/physics/quant/node6.html

Notes

Notes on the Atom: What it is

• Atoms make up everything
• An atom would be the size of a soccer ball, and the soccer ball would be 4008 miles high
• Different types of atoms called elements. Some of the different elements are: oxygen, hydrogen, copper, helium, and iron.
• Normally, atoms will stick to each other to create other stuff. If atoms of different types join together, like in water (H2O), then scientists call the resulting materials ‘compounds’. Water is a compound.
• Atoms bond by sharing their electrons.
• Atoms won’t bond in a nuclear fusing because it is ‘ionized’
• Ionization occurs when an electron gets very hot (heat from a reactor, etc) and moves away from the nucleus of the atom. Without the electrons, the atoms won’t bond.
• Atoms are ionized in stars
• 99.9% of the atom is empty space, but…
• …In the atom there is an even smaller part called the nucleus, which is a “congregation of particles”
• Particles are called Protons and Neutrons. Neutrons, which are neutral, have no electric charge. Protons carry an electric charge of 1.
• In a carbon atom, the atom has an electric charge of six (six protons in nucleus). A normal atom is neutral.
• Stuff swirling around in nucleus = electron cloud
• “The electrons in the electron cloud counteract the positive charges of the protons in the atomic nucleus with their negative electrical charges”
• Same number of protons and electron in one atom (Proton:lectron = 1:1 ratio)

SOURCE: http://library.thinkquest.org/17940/texts/atom/atom.html

Things in quotes are directly from this site