Atomic Theory

Catalyst (5 min) What does the atomic number of each atom represent? What does the mass number of each atom represent? How many protons and neutrons are in the nuclei of the following elements? phosphorus (P) cadmium (Cd) lead (Pb)

Let’s Take a Trip Through Time!

``Nothing exists except atoms and empty space; everything else is opinion.''  Democritus

Democritus 460 – 370 B.C. the first to propose that matter was not infinitely divisible Believed that matter was made up of tiny indivisible particles called atomos Everything is composed of small atoms moving in a void Ideas rejected by leading philosophers because they did not believe in empty space

First Concept of an Atom

John Dalton 1766-1844 English School Teacher Introduced his ideas in 1803 Used experiments that studied the ratio that elements combine in chemical reactions to transform Democritus’s ideas on atoms into a scientific theory Became the head of his quaker school at 12, was also reading newton’s principia at that time. French chemist pelletier traveled to meet the famous scientist and was shocked to see him teaching arithmetic to elementary boys.

Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atoms Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can physically mix together or chemically combine to form compounds In a chemical reaction, atoms are separated, combined or rerranged. Atoms of one element are never changed into atoms of another element. Atoms cannot be created, divided into smaller particles or destroyed.

Dalton’s Model

But if the scientists couldn’t SEE the atom, how did they even know it existed?!

``To the electron: may it never be of any use!'' J.J. Thomson

J.J. Thomson 1856-1940 Hypothesized that the structure of an atom was like ‘plum pudding’ Discovered the electron in the Cathode Ray Experiment Discovered that this subatomic particle has a negative charge Discovered isotopes 1913

Cathode Ray Tube Experiment Identified the first subatomic particle, the electron Determined the charge to mass ratio of the electron Mass of the electron is MUCH less then the mass of a hydrogen atom Meant there are particles smaller than atoms!

Video!

Cathode Ray Experiment Altering the gas in the tube and the material used for the cathode have no effect Proves: the ray’s negative particles (electrons) are found in all forms of matter Cathode Ray is deflected toward the positively charged plate by an electric field Proves: the particles in the ray must have a negative charge

Plum Pudding Model

“Fullness of knowledge always means some understanding of the depths of our ignorance; and that is always conducive to humility and reverence. “ Robert Millikan

Robert A. Millikan 1868-1953 Conduced oil drop experiment to determine the mass and quantity of an electrons charge

“You should never bet against anything in science at odds of more than about 1012 to 1.” Ernest Rutherford

Ernest Rutherford 1871-1937 Student of J.J. Thomson Conducted the gold foil experiment Discovered that an atom consists of mostly empty space through which the electrons move Concluded there is a tiny dense region called the nucleus which is located in the center of the atoms and contain all its mass Electrons are held in the atom by attraction with positively charged protons. Wasn’t particularly great at mathematics, in lectures would often get lost in his own equations and ask his students to finish. According to colleague James chadwick, also wasn’t particularly clever at experimentation. He was simply tenacious and open minded. He was prepared to work longer and harder then most. His greatest discovery came when he spent tedious hours counting alpha particles – work that would normally be farmed out.

Gold Foil Experiment Shot alpha particles at a thin piece of gold foil Most of particles passed through foil which indicated that atoms are composed of mostly empty space Alpha particle sometimes bounced back which indicated they were deflected from the nucleus

Video!

“If an atom were expanded to the size of a cathedral, the nucleus would be about the size of a fly - but a fly many thousands of times heavier than the cathedral.”

“This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you” Ernest Rutherford

Rutherford Model

But what keeps the electrons from spiraling into the nucleus?

``An expert is a person who has made all the mistakes that can be made in a very narrow field.'' Enter niels bohr Niels Bohr

Niels Bohr 1885-1962 Developed theory that electrons orbited the nucleus in neat, planet-like orbits Planetary Model 1913 Nucleus surrounded by orbiting electrons at different energy levels Electrons have definite orbits Utilized Planck’s Quantum Energy theory Worked on the Manhattan Project (US atomic bomb) Bohr was a student of Rutherford’s, who postponed his honeymoon when he first came up with the idea that electrons had specific orbits. One the nobel prize for his work the year after einstein

Bohr Model for Nitrogen

How can the protons bundle inside the nucleus without blowing the atom apart?

James Chadwick 1932 showed that the nucleus contained a neutral particle called the neutron. Enter James chadwick, a colleague of rutherfords at cambridge who spent 11 years of his life to finding the neutron

Quantum Model (learn next class) Developed from wave equation by Erwin Schrodinger (1926)

Fill in the chart using your notes (5 min) Name Democritus Dalton Thomson Rutherford Bohr Time Frame Key Points Model

Who Did It? (5 min) Six scientists were sitting around a table discussing their contributions to Atomic Theory. Use the clues to determine where they were sitting and what contribution they made.

Subatomic Particles Table Summary In your notes fill in the table comparing the relative charge, mass and location of each of the subatomic particles (3 min)

In your notes, make a timeline graph showing the development of modern atomic theory (5 min)

You Do: Complete the following book problems on a separate sheet of paper: p.104 #1, 4 p.105 #13, 15 p.122 #39, 62

Closing How did the concept of the atom change over time and what key experiments and scientists led to our current understanding of the atom?

Homework Describe the location of the electrons in Thompson’s ‘plum pudding’ model of the atom In the Rutherford atomic model, which subatomic particles are located in the nucleus? Describe Thomson’s and Millikan’s contributions to atomic theory.