Lesson 15 History of Atomic Theory Objectives: * The student will list and explain the five points in Dalton’s atomic theory. * The student will describe and reenact Rutherford’s Gold Foil Experiment, and explain his theory of an atom. * The student will describe Neils Bohr’s atomic theory.

History of Atomic Theory Virtual Lab 1 video

I. Dalton’s Atomic Theory a.John Dalton looked over knowledge that had been accumulated before his time, and write five basic principles dealing with matter. i. All matter is made of indivisible and indestructible atoms i. All matter is made of indivisible and indestructible atoms ii. Atoms of a given element are identical in their physical and chemical properties ii. Atoms of a given element are identical in their physical and chemical properties iii. Atoms of different elements have different physical and chemical properties iii. Atoms of different elements have different physical and chemical properties

iv. Atoms of different elements combine in simple, whole-number ratios to form compounds – also known as the Law of Multiple Proportions v. Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions – also known as the Law of Conservation of Mass

b. Dalton’s theory of the atom makes it out to be something similar to a marble. to be something similar to a marble.

II. Finding the Structure of the Atom a. Electrons are negatively charged particles that have a small mass i. Cathode ray tubes (CRT) are used in televisions and other devices. i. Cathode ray tubes (CRT) are used in televisions and other devices. ii. Small particles travel from the cathode to the anode. ii. Small particles travel from the cathode to the anode. 1. Cathode – a negative electrode through which current flows 2. Anode – a positive electrode through which current flows iii. J.J. Thompson discovered that electrically charged plates could deflect these beams of particles. He determined they were negatively charged. iv. Draw a Cathode Ray Tube in this space: iv. Draw a Cathode Ray Tube in this space:

Cathode Ray Tube Drawing:

Thompson's Experiment Virtual Lab 1 video

V. His model of the atom was called the Plum Pudding model. V. His model of the atom was called the Plum Pudding model. 1. He is given credit for the discovery of these negatively charged electrons embedded in a ball of positive charge. 2. Resembles “plum pudding”, a bread with fruit pieces embedded in it. 2. Resembles “plum pudding”, a bread with fruit pieces embedded in it. 3. Draw the “Plum Pudding Model” in this space:

b. Each atom has a positively charged inner core i. Ernest Rutherford did an experiment known as the gold foil experiment. 1. He fired tiny positively charged particles (alpha particles) at a very thin piece of gold foil, and determined where the electrons ended up going. 2. He found that most passed straight through, but some were deflected anywhere from a little to almost straight back 3. He determined from this that the atoms were mostly empty space and had a positively charged “core”, and it was named the nucleus.

4. Draw the setup of the Gold Foil Experiment in this space:

The majority of the particles passed through with an unaltered path. But a few alpha particles had their pathway drastically changed

Rutherford’s Experiment Virtual Lab 1 video

i. His model of the atom is similar to that of the solar system, with electrons traveling around the nucleus in well-defined paths. ii. Draw a Rutherford “solar system model” in this space:

Conclusions from Rutherford’s Experiment Most of the atom is empty spaceMost of the atom is empty space Atoms have a solid core called the nucleusAtoms have a solid core called the nucleus Nucleus is positively chargedNucleus is positively charged They measured the approximate size of the nucleusThey measured the approximate size of the nucleus

c. Electrons occupy energy levels within an atom i. Neils Bohr came up with the idea that electrons would be found only in specific energy levels, similar to the rungs on a ladder ii. The energy levels closer to the nucleus have lower energy than those farther away. iii. The difference in energy between any one level and the next is called a quantum. iv. Electrons can only be found in those energy levels, never in between. They are “quantized” v. Bohr’s model of the atom can be compared to a stepladder

d. Neutrons add mass to the nucleus i. It was found that the entire mass of the nucleus couldn’t be accounted for with just protons ii. Neutrons were discovered by James Chadwick in 1932, and some of his research was based on discoveries by Rutherford and Irène and Frédéric Joliot-Curie. iii. This particle was very difficult to discover, because of the fact that it does not possess a charge. iv. Originally, the neutron was thought to be a combination of a proton and an electron, but later it was determined that it was a unique particle.