Development of the Atom:



Development of the Atom:

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I. Democritus (400BC)

a. Did not believe the prevailing idea that all matter could be broken down into elements of Earth, Wind, Fire, and Water. He also did not believe that matter could be endlessly divided into smaller pieces.

b. Believed that all matter could be broken down into tiny indivisible spheres that he called “atomos”.

c. He believed these atoms could not be created or destroyed or subdivided.

d. Problem: he could not answer “What holds the atoms together?”

e. His idea really never caught on because Aristotle came along 100 years later and told everyone that Democritus was wrong. People went back to believing in the Earth, Wind, Fire, Water model

f. Important to remember the Greeks did not actually experiment, they only thought!

g. For the next 2000 years nobody except the ALCHEMISTS thought much about matter. These nomadic people were on a quest for the elixir of life and trying to turn lead into gold. They discovered elements, made equipment, developed procedures, and studied elements. They recorded their works, although they were not published or shared.

II. John Dalton (early 1800’s)

a. Dalton was a schoolmaster who studied a great deal about the Alchemists and Democritus. He thought their ideas and studies deserved further investigation

b. Dalton performs experiments in his school’s physics lab

c. As a result of his studies he states:

i. All elements are composed of tiny indivisible particles called atoms (he uses Democritus’ term as a tribute)

ii. Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

iii. Atoms of different elements combine in simple whole-number ratios to form chemical compounds

iv. In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

v. Law of definite proportion - This law is readily explained by Dalton's atomic theory. Suppose a compound is made of element A and element B, giving the formula AB. Since the weight of A is constant and the weight of B is constant, the A:B weight ratio will always be the same. Hence, the Law of Definite Proportions.

d. For the next 80 years other scientist are working with Dalton’s Atomic Theory and trying to learn more about the atom.

III. JJ Thomson (1897)

a. Thompson is a physicist trying to learn more about the atom

b. Does studies with hydrogen gas in a Cathode Ray Tube (CRT)

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c. Passing an electric current and applying a magnetic field made a beam of light deflect away from positive charges toward negative ones.

d. He proved that atoms of any element can be made to emit tiny negative particles.

e. From this he concluded that ALL atoms must contain these negative particles.

f. He knew that atoms did not have a net negative charge and so there must be something balancing the negative charge.

g. From his studies he developed the “plum pudding” model of the atom.

h. Calls the negatively charge particles electrons

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IV. Ernest Rutherford (1911)

a. Studied physics under JJ Thomson

b. Performs the famous gold foil experiment

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c. Rutherford notices that when an alpha-particle (type of radiation) goes through air sometimes the particles get deflected. He wondered why.

d. When he put a piece of gold foil in front of a beam of alpha particles and put a piece of detecting film around it, he observes that MOST particles go straight through the gold foil, some get deflected a little bit, and very few get reflected.

e. The gold foil was only 0.00004 centimeter thick, meaning on a few hundreds of atoms thick.

f. He concluded that since most alpha-particles went straight through, the atoms of gold on the foil must contain mostly empty space. Also, form the experiment he was able to conclude that the nucleus was very small, very dense, and positively charged.

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V. Neil Bohr (1913)

a. Studying at the same time as Rutherford

b. While studying how light is emitted from heated atoms he comes up with an improved model based on Rutherford’s

c. It is known as the “planetary model” where electrons orbit the nucleus like planets orbit the sun.

[pic]Bohr’s model of the atom…electrons traveling in fixed orbits

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VI. Thomas Millikan (1916) and James Chadwick (1920)

a. Millikan performs the “oil drop” experiment

b. Through a bunch of calculations he is able to accurately determine the mass and the charge of an electron.

c. He determined that an electron in hydrogen has about 1/2000 the mass of the hydrogen nucleus.

d. Chadwick discovers the neutron in the nucleus which is great, because the mass of the electrons and the mass of protons did not add up to the mass of the atom. When you add the masses of the protons, neutrons, and electrons you do get the mass of the atom.

e. At this point we knew a lot about subatomic particles, but further studies revealed some mistakes…

VII. Erwin Schrodinger, Werner Heisenberg, and Antonio DeBroglie (1924-1940)

a. Through many fancy-smchancy calculations and observations these brilliant gentlemen improved Bohr’s model of the atom.

b. We know now that electrons do not travel in specific orbits like planets around the sun.

c. We know that electrons exist, traveling at very high speeds, in certain 3D areas around the nucleus. We call those areas orbitals or energy levels.

d. The sum total of the various paths of electrons, traveling at very high speeds, is described as the electron cloud.

e. We know a lot about how electrons behave based on the works of these scientists.

f. The current day model is called the quantum mechanical model or the charge-cloud model

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