History of Atomic Theory Atomic Structure History of Atomic Theory

Democritus (460 - 370 BC) 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 He also believed that different atoms: Are different sizes Have different properties Other philosophers of that time did not agree with his theories.

Our “Story So Far…” We began the year Everything is made of matter – matter has mass and takes up space Matter moves faster when it has more energy Volume of matter that is occupied varies with the number of particles, temperature and pressure of the particles The energy into and out of the system changes the state of the particles or changes the temperature we measure We classify matter based on how the particles are attached / not attached to other particles We can count particles and use patterns and data to determine the number and kinds of atoms in a compound.

Continuing our Story We also learned That the simplest form of matter, the atom, is made of mobile, negative parts. Since the atom was identified as neutral, there is also a positive material present. The movement of the negative particles causes the atom to have a charge; charged atoms combine in specific patterns Chemical reactions can be outlined with equations and we can identify specific kinds of reactions, yet all observe the Law of Conservation of Matter We can predict the amounts (moles, grams, volume) of reactants and products needed in any reaction, whether the reaction is solid or solutions of matter.

Our timeline Our story takes us from Democritus to the mid 1800’s Important concepts were our focus, but as we now look at a much smaller time frame in our understanding of our chemical world, people, events, and concepts will be our focus So let’s go back and review what we learned with Dalton and Thomson

John Dalton (1766-1844) Dalton is the “Father of 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 (1803) aka: 5 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: - size - mass - chemical properties. 3. All atoms of different elements are different.

Dalton’s Atomic Theory (1803) aka: 5 Postulates Atoms cannot be created, divided into smaller particles, or destroyed. **In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element. We will learn more later** 5. Atoms combine in 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)

Dalton’s Atomic Model Based on Dalton’s Atomic Theory (5 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. Dalton (for the most part) was credited for the three Atomic Laws that were proven after his time.

Dalton’s Atomic Laws 1. Law of Conservation of Mass Matter cannot be created or destroyed in any physical or chemical process, just transferred. 2.     Law of Definite Composition When atoms combine to form molecules, the ratio of atoms is constant. Example – H2O will always have 2 times as many Hydrogen atoms as Oxygen.

Dalton’s Atomic Laws 3.     Law of Multiple Proportions – if two elements can combine to form more than one compound, then the ratio of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Example: CO vs. CO2 Formula Ratio of N:O

The Law of Conservation of Mass When a chemical reaction occurs, mass is neither created nor destroyed but only changed. https://youtu.be/QiiyvzZBKT8

JJ Thomson (1856-1940) Used cathode rays to prove that Dalton’s Solid-ball model could be broken into smaller particles Thomson is credited with discovering electrons

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.

Cathode Ray Tube Experiment Video Clip https://youtu.be/O9Goyscbazk

----------------------------- Cathode Ray Tubes Thomson put the tube in a magnetic field. He predicted that the stream would travel in a straight path. 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 had electrons in them.

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!

You’ve got to love Bill Nye and his enthusiasm for science You’ve got to love Bill Nye and his enthusiasm for science. This is a clip from his episode “100 greatest discoveries in science” http://science.howstuffworks.com/29292-100-greatest-discoveries-the-cathode-ray-tube-video.htm

Thomson’s Plum Pudding Model Thomson’s Plum Pudding model is a + charge sphere that has (- )charged electrons scattered inside, like “raisins” in “plum pudding”. Overall, the atom is neutral atom because the atom had the same number of positive and negative charges. 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.

You will need 1-3 sheets of clean paper and a pencil. Activity Time You will need 1-3 sheets of clean paper and a pencil. Mrs. Pieper will show you what to do with the boards. Be sure to NOT peek at the underside.

Rutherford (1871-1937) Took Thomson’s Plum Pudding Model and added to it Used the “Gold Foil Experiment” to discover the existence of: An atomic Nucleus Protons (in later experiments) You must be able to explain the Gold Foil Experiment…it will be on the CST

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. http://micro.magnet.fsu.edu/electromag/java/rutherford/

The Gold Foil Experiment

The Nuclear Model of the Atom To explain the results of the experiment, Rutherford’s team proposed a new model of the atom. http://sciblogs.co.nz/code-for-life/2011/04/22/ernest-rutherfords-gold-foil-experiment/

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.

Rutherford’s Planetary Model To explain his observations, Rutherford developed a new model The electrons orbit the nucleus like the planets revolve around the sun.

Are you curious about that gold foil Are you curious about that gold foil? Here is a video to show just how thin a thin piece of gold foil was for Rutherford. https://youtu.be/8B8ZZ4imNhU

Activity Time Beads… You will be provided a handout and instructions Purpose: Now that we know a nucleus exists, scientists examined further about the make up of that nucleus. This has you examine characteristics about what was learned

Bohr (1885-1962) Worked in Rutherford’s lab Wondered why – electrons are not attracted to the + nucleus and cluster around it Disproved Rutherford’s Planetary Model Experimented with light and its interaction with matter to develop a new model.

Demonstration We need to completely darken the room and block light from seeping in: Cover windows on doors Cover the cracks under the doors Close all computers, put away electronic devices I will take care of my office, my computer and classroom lights

Bohr’s Energy Level Model Energy Level Model: Electrons are arranged in circles around the nucleus. Each circle has a different energy. Electrons are in constant motion, traveling around the circle at the speed of light. Electrons can “jump” from one circle to the next But they can’t go to the nucleus they traveling too fast to be fully attracted. http://bigbang.physics.tamu.edu/Figures/StolenAnimations/bohr.swf

Let’s look at the emission spectrum Imagine the atom’s energy levels Looking at it as data

Bohr’s Energy Level Model ·  He proposed the following: 1. Protons and neutrons are in the nucleus 2. Electrons can only be certain distances from the nucleus. 3. The electrons orbit the nucleus at fixed energy levels. 4. The electrons must absorb or emit a fixed amount of energy to travel between these energy levels 5.http://www.mrbigler.com/moodle/pluginfile.php/7700/mod_resource/content/0/Quantum/Notes_Handouts/BohrModel.swf

Lab Time Flame test lab You will be provided a handout with complete instructions. You are to make the observations indicated and then answer the questions afterwards. Oh, this is lots of fun. And we do this in a darkened room.

In review… https://youtu.be/thnDxFdkzZs

Review Who is the father of atomic theory? Dalton What was the first model of the atom? Dalton’s Tiny Ball Model What are Dalton’s 3 Laws? Law of Conservation of Mass, Law of Constant Composition, Law of Multiple Porportion

Review How were Thomson’s and Dalton’s model different? Dalton’s model was 1 sphere that cannot be divided, Thomson had the plum pudding where electrons are randomly spread throughout a positively charged sphere. What did Thomson find out? Atoms have electrons, they have a - charge

Review What were Rutherford’s conclusions from the Gold Foil Experiment? Atom has a positively charged nucleus electrons are outside, atoms are mostly empty Nucleus contains most of the mass.

Review What conclusions were made about the atom due to the work of Bohr? His model does describe an atom with one electron accurately (hydrogen) but not atoms with more than 1 electron Electrons revolve about the positively charged nucleus in fixed orbits (energy levels) When an electron moves from one energy level to another, the energy lost or gained is done so ONLY in very specific amounts of energy

There is more to the Atomic theory, but… Mrs. Pieper,