Lecture 2.2 Atomic Models

1. Lecture 2.2 Atomic Models

2. We now have much evidence that an atom is made of a nucleus and electrons orbiting around the nucleus • What did people think before?

3. Even back in the days of Plato and Aristotle, people were debating whether or not matter could be cut in half indefinitely • A Greek philosopher named Democritus stated that all things are composed of tiny indestructible particles (around 400BC) • It was not until early 1800’s when scientists had a pretty accurate understanding of an atom

4. Even though scientists can not see atoms, experiments can be performed to create a model • A scientific model is a representation of reality that tries to explain how things work (theory)

5. Dalton’s Atomic Theory (1804) • All matter is composed of extremely small particles called atoms. • Atoms cannot be subdivided, created, or destroyed. (Is this true?) • In chemical reactions, atoms are combined, separated, or rearranged in specific ratios

6. J.J Thomson discovered the electron around 1897 • He proposed the “plum pudding model” to describe an atom • Within a field of positive charge, tiny negative electrons were spaced out

7. Ernest Rutherford performed an experiment around 1911 that provided much evidence for our current understanding of an atom • He stated that atoms are mostly empty space, with a very tiny, yet dense, positively charged center. • Negatively charged electrons orbit around the nucleus in the “empty space”

8. Gold Foil Experiment • He focused a beam of radiation through a very thin gold foil • The gold foil is only about 6000 atoms thick • The area around the gold foil was enclosed with a special film that can detect radiation

9. According to the plum pudding model, most of the radiation was suppose to pass straight through the gold atoms, maybe with slight angle deflection • However Rutherford found that some radiation was being deflected backwards!

12. All atoms have very small masses, the heaviest atom is 4 x 10 -22 grams Using the unit of grams it too cumbersome to express so the unit of atomic mass unit (amu) is used Proton= 1.0073 amu ≈ 1 amu Neutron= 1.0087 amu ≈ 1 amu Electron= 0.0005486 amu ≈ 0 amu

13. What is the mass number of a carbon if it has 6 protons, 6 neutrons, and 6 electrons? • Since the mass number is 12, we can refer to this specific carbon as “Carbon-12” Element - Mass number • Another way to write carbon-12 is this: Mass number Atomic number C 12 6

14. How many neutrons are there in carbon-13? • How many neutrons are there in carbon-14? • What is the number of protons, neutrons, and mass number in K? • What are two ways you will write an element with 17 protons and 18 neutrons? • What are two ways you will write an element with 78 protons and 100 neutrons? 40 19

15. If carbon has a mass of 12 amu, how come it says “12.01” on the Periodic Table?

16. Isotopes • Carbon-12 and carbon-13 are the same elements but have different number of neutrons • These are called isotopes (same number of protons, different number of neutrons) • Most elements in the world are a mixture of isotopes • For instance, 99% of carbon is carbon-12, 1% is carbon-13

17. Carbon is listed to have an atomic mass of 12.01 amu on the periodic table because the average mass of every carbon in this world is 12.01 amu • Atomic mass: average atomic mass of the isotopes of elements (different from mass number)

18. Using your textbook or periodic table, or internet, find the atomic mass for the following elements: • Fluorine • Thorium • Hydrogen • Oxygen

19. In-class Research and write at least 10 sentences for each of the following scientists. Write in your own words. Focus on their accomplishments towards chemistry (try to focus on atoms): • John Dalton • William Crookes, • WilheimRoentgen, • Henri Becquerel, • JJ Thompson, • Robert Millikan, • Ernest Rutherford, and • James Chadwick.