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

Dalton’s atomic theory Democritius (460-370 BC) was the first to propose the idea of “atoms” . This theory was rejected by Aristotle (384-322) and lay dormant for 2000 years Aristotle (c. 384-322 BCE) disagreed with Democritus because he did not believe empty space could exist. Aristotle’s views went unchallenged for 2,000 years until science developed methods to test the validity of his ideas. John Dalton looked over knowledge that had been accumulated before his time, and wrote five basic principles dealing with matter.

I. Dalton’s Atomic Theory Dalton’s atomic theory stated:.       i.      All matter is made of indivisible and indestructible atoms              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           

Law of Conservation of Mass 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.

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.     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. He was able to determine the mass of what is now called the electron, identifying the first subatomic particle, disproving Dalton’s Theory v. Millikan was then able to determine the exact charge of this electron.    

Cathode Ray Tube Drawing:

Thompson's Experiment Virtual Lab 1 video

                      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. 3. Draw the “Plum Pudding Model” in this space:

Robert A. Millikan 1868-1953 1909 – Using Thomson’s work, he was able to determine the exact charge and mass of an electron.

b. Each atom has a positively charged inner core i 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 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                         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 space Atoms have a solid core called the nucleus Nucleus is positively charged They measured the approximate size of the nucleus

c. Electrons occupy energy levels within an atom i 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

Rutherford’s Model vs. Bohr’s Model Unstable Atom Stable Atom electrons continuously emit energy and eventually fall into the nucleus electrons continuously absorb and emit energy by changing orbitals

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.

Neutron Experiment 1930 James Chadwick used this experiment to mathematically prove the existence of the neutron. In 1930 it was discovered that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation.  However, further investigations into the properties of the radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and since they were not deflected upon passing through a magnetic field, neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect). Irène Joliot-Curie and Frédéric Joliot (husband) discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin, protons were knocked loose which could be easily detected by a Geiger counter.   In 1932, Chadwick proposed that this particle was Rutherford's neutron. In 1935, he was awarded the Nobel Prize for his discovery. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton. another scientist Irène Joliot-Curie and Frédéric Joliot (husband)

Atomic Theory Summary All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron. Atoms are spherically shaped. Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus.

Atomic Theory Summary

Relative Sizes If a hydrogen atom was enlarged to the size of a golf ball, then an actual golf ball would have to be enlarged to the size of the earth to keep the scale correct.

Relative Sizes If the nucleus was enlarged to the size of a 0.75 cm pea, then the atom would have to be as large as a baseball field.

Relative Sizes If you imagine a proton to be the same mass as a honeydew melon (1200 grams), then an electron would be as massive as a pit (0.6g).