History of Atomic Theory from Democritus to Rutherford Atomic Structure History of Atomic Theory from Democritus to Rutherford

Democritus (approx. 400BC) Was the first person to come up with the idea of atom Believed that all matter was composed of indivisible particles he called “ATOMS” Which is derived from the Greek word “Atomos” – meaning indivisible Other philosophers of that time did not agree with his theories. Democritus model Not the model of today

John Dalton (early 1800’s) Dalton’s Atomic Theory Dalton’s ideas were so brilliant that they have remained essentially intact up to the present time and has only been slightly corrected.

Dalton’s Atomic Theory aka: 6 Postulates All matter is composed of extremely small particles called atoms. (I agree with Democritus!) 2. All atoms of a given element are identical, having the same: a) size b) mass c) chemical properties. 3. All atoms of different elements are different. Atoms are indivisible and indestructible 5. Different atoms combine in simple/definite whole number ratios to make compounds (you can’t have a ½ of a Carbon bonding with Oxygen; it’s a whole atom or no atom) 6. In a chemical reaction, atoms are separated, combined, or rearranged

A visual of some of Dalton’s postulates A visual of Dalton’s postulates

Dalton’s Atomic Model Based on Dalton’s Atomic Theory (6 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. aka billiard ball model Not the model of today Merits of Dalton's atomic theory The atomic theory explains the laws of chemical combination (the Law of Constant Composition and the Law of Multiple Proportions). Dalton was the first person to recognize a workable distinction between the fundamental particle of an element (atom) and that of a compound (molecule).

JJ Thomson (1856-1940) Cathode Ray Tubes Used cathode rays to prove that Dalton’s Solid-ball model could be broken into smaller particles Cathode Ray Tubes Cathode rays had been used for some time before Thompson’s experiments. A cathode ray is a tube that has a piece of metal, called an electrode, at each end. Each electrode is connected to a power source (battery). When the power is turned on, the electrodes become charged and produce a stream of charged particles. They travel from cathode, across the tube to the anode.

A diagram of another cathode ray set up

----------------------------- Cathode Ray Tubes Thomson put the tube in a magnetic field. He predicted that the stream would travel in a straight path. negative plate ++++++++++++++ ----------------------------- Predicted pathway Actual pathway Positive plate Instead, he found that the path curved away from a negatively charged plate and toward a positively charged plate Why? Like charges repel each other, and objects with unlike charges attract each other, Thomson concluded that the stream of charged particles THAT WERE NEGATIVELY CHARGED these particles are called electrons.

Cathode Ray Tube Experiment Thompson Concluded: Cathode rays are made up of invisible, negatively charged particles called Electrons. These electrons had to come from the matter (atoms) of the negative electrode. Since the electrodes could be made from a variety of metals, then all atoms must contain electrons!

Here are some websites regarding the cathode ray Experiment some go beyond what you are accountable for You do not need to visit these websites http://chemmovies.unl.edu/ChemAnime/CATHRAYD/CATHRAYD.html https://www.youtube.com/watch?v=GzMh4q-2HjM

Thomson’s ATOMIC MODEL known as the Plum Pudding Model Thomson’s Plum Pudding model is a + charge sphere that has (- )charged electrons scattered inside, like “raisins” in “plum pudding”. Helps if you know what plum pudding is It is also known as the Chocolate Chip Cookie or Blueberry Muffin Model. Ah now you can relate It was previously known that atoms are neutral, therefore, in Thomson’s model Since, the atom is neutral Thomson’s negative charges aka electrons Offset the positive charge of the sphere From Thomson’s experiments, scientists concluded that atoms were not just neutral spheres, but somehow were composed of electrically charged particles. The balance of positive and negative charge supports the neutral atom. But Not the model of today

Rutherford (1871-1937) Took Thomson’s Plum Pudding Model and added to it IT’s all about the GOLD FOIL ( experiment that is) Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. Rutherford’s experiment set up is below

Gold Foil Experiment Rutherford directed a narrow beam of alpha particles (+ charges) at a thin piece of gold foil. Based on observations from other experiments involving alpha particles, he predicted that the (+) charges would go through the foil

Results from Gold Foil Experiment Rutherford found that every once and a while, a + particle was deflected bounced back. (about 1% of the time) Why? Because the + charge hit a central mass of positive charge and was repelled.

Goil foil experiment results again

The Gold Foil Experiment Set Up visuals

Conclusions from Rutherford’s Gold Foil Experiment (memorize this!) The atom contains a positively charged “nucleus” This nucleus contains almost all of the mass of the atom, but occupies a very small volume of the atom. The negatively charged electrons occupied most of the volume of the atom.  The atom is mostly empty space.

To explain his observations, Rutherford developed a new model called the nuclear model by some and sometimes it is called the planetary model The electrons orbit the nucleus like the planets revolve around the sun. At this point in time ALL he knew was that there was a POSITIVE CENTER CORE By 1920 Rutherford had refined the concept of the nucleus and concluded that the nucleus contained positively charged particles called protons so they can be added to the model

1935 ish Chadwick shows the nucleus has another subatomic particle THAT HAS NO ELECTRICAL CHARGE Aka NEUTRAL and has a mass nearly equal to that of a proton THIS PARTICLE IS CALLED THE NEUTRON BELOW IS A Rutherford model containing ALL three subatomic particles discovered at this time

What is wrong with the Rutherford model SINCE POSITIVE( the nucleus) ATTRACTS NEGATIVE( the electrons) and moving objects tend to lose energy Why didn’t this happen –