Monday 27, September

To start off class, Mr. Henderson clarified any information that we may have missed concerning the TLC and the tutors there. He told us that if we were struggling in Chemistry, we could visit the TLC for help from actual science teachers or AP students as well as visit Mr. H early in the morning. We then moved on to the subject of the day; The history of the atomic model and the various scientists that helped create each one in their respective times.

On Pg. 15 in your chemistry Unit2 packet, there is a very general history concerning the atomic model starting with Dalton. However, Mr. Henderson told us that the first ever recorded theory concerning the atom dates all the way back to Ancient Greece and a philosopher by the name of Democritus. Democritus argued that he could chop a piece of papyrus into halves and halves and halves until he would reach some sort of building block that could not be broken down any further into a smaller or simpler form. He called these building blocks atomos and he believed they made up all matter. Democritus had no idea how accurate he actually was, but for nothing more than an educated guess, this was a remarkable theory. It is important to note that Democritus never actually tested this theory using science and therefore, he is regarded as a philosopher rather than a scientist. It was not until the late 1700's and early 1800's that the scientific revolution ushered in a new wave of interest in the field of atomic structure.

The next man to take a stab at an accurate atomic model was John Dalton. This scientist used the scientific method of developing a hypothesis and testing it to form a theory. He called these theories postulates, or basic rules. The five postulates that Dalton formed are as follows.
#1) All matter consists of atoms

#2) Atoms are indivisible (cannot be broken down any further)

#3) In a chemical change or reaction, atoms simply rearrange themselves but never turn into any other atoms. This is demonstrated in the picture below in which Hydrogen and Oxygen combine to form H20.



#3 (continued) Also, atoms never gain or lose mass in any way during any chemical or physical change.

#4) Compounds have a determinable amount of atoms and always have the same amount of each atom.

#5) Law of Constant Composition. All Compounds contain the same amount of each atom no matter where the compound is found or under what circumstances.

These 5 postulates are still seen today as a basic level in understanding the structure of atoms and compounds. Dalton never did create a finite model that he believed was what an atom might look like. He did however represent an atom as a circle with a symbol on it, such as H for Hydrogen.

Proceeding John Dalton, J.J. Thomson was at the forefront of the atomic model issue. J. J. Thomson believed that atoms did not only consist of alpha particles, but also possessed some other feature. By separating the alpha particles from a beam of light and firing this beam, known as a cathode ray, through a glass tube with positive or negative plates on either side Thomson was able to test this. After seeing the cathode ray bend towards the positive plate, Thomson was able to determine that the are other particles that make up an atom and that they are negatively charged. Thomson called these negatively charged particles electrons. This lead Thomson to relate his atomic model to the favorite desert of England at the time, earning it the name The Plum Pudding Atom. Thomson believed that atoms consisted of moving negatively charged electrons that are immersed in a sea of positively charged stationary particles. The model looked something like this picture, with the dots being electrons and the red material being the positively charged "sea".





For many years, A scientist by the name of Ernest Rutherford studied this model and debated it's accuracy. To test the Thomson models validity, Rutherford set up an experiment in which he took a thin (only a few hundred atoms thick) piece of gold foil and placed it in a container which had sensors on either side while shooting a stream of atoms in its direction. If Thomson's model was correct, the atoms should have gone through the gold foil with no problem and triggered only the sensors on the other side. What Rutherford found was that although most of the atoms did behave this way, 1 out of every 10/20,000 deflected off of the foil and reflected back at an acute angle. Rutherford was quoted saying "Its as if you shot 20,000 bullets at a tissue with most of them going through the tissue with no problem, but 1 out of every 20,00 of those bullets would reflect back at you!" Rutherford was amazed at this and determined that atoms were not full of positive particles, but were rather made up mostly of empty space with 99% of its mass concentrated in its nucleus, earning his model the name The Empty Space model. Rutherford still did not know how the electrons were distributed across the atom, leaving his model still somewhat inaccurate

Finally, one of Rutherford's colleagues, Niels Bohr, measured light energy to more accurately determine where the electrons would be relative to the nucleus of the atom. By measuring each layer of electrons of an atom and observing the light they emitted, Bohr was able to establish the general amount of electrons and their area relative to the nucleus. The downfall to Bohr's method was that once he began getting into the atoms with more electrons, such as Gold, his estimates were off by more than 20-40% at times. However, Niels Bohr was able to develop an atomic model, known as The Planetary Model.

Neils Bohr called it this because he thought that the electrons orbited the nucleus much like moons orbit a planet. This is the most accurate of all the depictions discussed so far.

To wrap up the class, Mr. Henderson gave us the answers to page 15 which are as follows:

1: b
2: d
3: c
4: a
5: c, b, a

We also drew sketches of what each scientist, (Dalton, Thomson, Rutherford, Bohr) believed
that atom looked like and those can be seen by asking me or any other student before or after class. Tomorrow, we will incorporate verbal explanations with these visuals models to describe each scientist views on concerning the structure of an atom.