Models of the Atom

1. Models of the Atom

2. Ancient Greece • 300 BC in Greece • School of thought that matter is made up of tiny indivisible, invisible, indestructible, fundamental units of matter called atoms (means “that which cannot be divided”) • Democritus of Abdera most well known atomist. • Did no experiments • No proof • Philosopher

3. 1770s • Antoine Lavosier made observations and did experiments resulting in the Law of Conservation of Mass • Law of Conservation of Mass • Matter cannot be created or destroyed • Mass of materials before the chemical reaction is the same mass after the reaction

4. 1799- Joseph Proust • Law of Definite Proportions: that the proportion by mass of the elements in a compound is always the same (CO vs. CO2) • Part of our definition of a chemical - any substance with a definite composition

5. 1808- John Dalton • English School Teacher • Theory based on the Greek idea of the atom • Atoms were tiny indestructible particles

6. Dalton’s Atomic Theory • All matter is made of invisible and indestructible particles (atoms) • Atoms of same element are identical • Atoms of different elements differ in physical and chemical properties • Atoms of different elements combine in simple whole number ratios to form compounds • Chemical Reactions occur when atoms separate, join or rearrange. Atoms of one element NEVER change into atoms of another element.

7. Problems with Dalton’s Model • Atoms are destructible (protons, electrons, neutrons) • Atoms of the same element are not completely identical, isotopes exist • Atoms of the same element combine with each other (O2) • Atoms can “turn into” other elements through nuclear decay

8. 1897 Sir J.J.Thomson • British physicist • Theory: cathode ray is a stream of negatively charged particles • Experiment: • cathode ray experiment

9. Experiment • Cathode ray: a tube filled with gas that would glow when an electric current was passed through tube (like a neon sign) • Thomson held the “+” pole of magnet next to tube, and the light bent towards magnet • Therefore he concluded that there must be some negative charge in atom!! (discovered electron) • Also hypothesized that there must be some positive charge in atom (as atoms are neutral)

10. Plum Pudding Model • Electrons are embedded in atom • Positive charge floats around rest of atom

11. Thomson Misconceptions? • Placement of electron • Location of positive charge

12. 1909 -Ernst Rutherford • Physicist and Chemist from New Zealand • Proposed: • that the theory of Thomson was actually correct • Any charge that occurs in atom must occur in whole number ratios • Ex: +1 or -1, not + 1.5

13. Rutherford's Gold Foil Experiment • Fired small radioactive particles (basically a helium nucleus) at a piece of gold foil • Expected that most of these particles would pass right through the gold foil, deflecting only a little bit • * what actually happened was that while that majority did pass right through, those that deflected deflected at many different angles • Think of throwing a grain of salt at a chain link fence!

14. Rutherford’s Experiment

15. Rutherford’s Conclusion • Key Idea -NUCLEAR ATOM • All of the mass of the positive particles (protons) is at the center region of the atom • Center region called the NUCLEUS • Electrons surround the nucleus in a “cloud” • Atom is mostly empty space

16. Rutherford’s Misconceptions • Still could not place electrons correctly • Was not aware of the neutron

17. Niels Bohr • Danish Physicist • Proposed his model in 1915 (this is the one most of you know) • He coined the term, “Planetary Model” of atom • Electrons orbit the nucleus of atom like planets around the sun • Different orbits exist, each having a specified level of energy • Suggested that outermost energy levels can hold more electrons than inner energy levels

18. Bohr’s Experiment • Studied line emission spectrum of hydrogen • Noticed that when electrons were excited (basically electrified) they would “jump”. • Further, he noticed that these jumps were similar in energy • This helped to solidify his theory of electrons orbiting the nucleus in specific energy levels

19. Bohr’s Atom • Conclusions: • There are certain circular ORBITS in which an electron can travel around the nucleus • The farther away from the nucleus, the higher the energy level

20. Problems with Bohr • Electrons do not exist in orbits!

21. Quantum Model/Schrodinger - 1925 • Theory: • states that electron can act as a particle and a wave • electrons can exist in any of an unlimited number of energy levels • Experiment: Schrodinger wave equation

22. Quantum Model/Schrodinger • Conclusions: • Electrons do not orbit nucleus, they exist in orbitals • Orbitals are probable locations of the electron • There is an unlimited number of energy levels in which electrons can exist • Energy increases as you get farther from nucleus • Misconceptions?

23. Quantum Model/Schrodinger

24. What do we know now?? • All models are not exactly correct, but they lead to further understanding and discovery • All have important key ideas • Atoms are made up of subparticles • Atoms are divisible, but not by ordinary chemical means • Atoms of elements can vary (isotopes) • Electrons reside in ORBITALS, not orbits

25. Parts of Atom

26. What happens when you change… • Protons: Since the number of protons is the same as the atomic number, protons are what define an element. • If you change the protons you change the element • Neutrons: The neutrons add to the mass of the element. • If you change the neutrons you change the mass of the element (make an isotope) • Electrons: The electrons balance the charge of the protons. • If you change the electrons you change the charge of the element (make an ion)

27. Quotes about atoms • From A Short History of Nearly Everything by Bill Bryson • “protons give an atom its identity, electrons its personality” p. 140 • “if an atom were expanded to the size of a cathedral, the nucleus would be only about the size of a fly- but a fly many thousands of times heavier than the cathedral” p. 141