The Evolution of Our Understanding of the Atom (Chapters 4 and 5)

1. The Evolution of Our Understanding of the Atom(Chapters 4 and 5)

2. Aristotle (460 B.C. – 370 B.C.) • emphasized that nature consisted of four elements: air, earth, fire, and water. • he did not believe in discontinuous or separate atoms, but felt that matter was continuous

3. Democritus (460 B.C. – 370 B.C.) • greek philosopher • first to suggest the existence of “atoms” (Greek word “atomos” = indivisible) • atoms indivisible and indestructible • no experimental support http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg

4. John Dalton (1766-1844) • english chemist and schoolteacher • used scientific method to test Democritus’s ideas • Dalton’s atomic theory • elements composed of atoms • atoms of the same element are alike atoms of different elements are different • different atoms can combine in ratios to form compounds • chemical reactions can occur when atoms are separated, joined, or rearranged (but can’t change one element into another) • revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy

5. Problems with Dalton’s Atomic Theory? 1. matter is composed, indivisible particles Atoms Can Be Divided, but only in a nuclear reaction 2. all atoms of a particular element are identical Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)! 3. different elements have different atoms YES! 4. atoms combine in certain whole-number ratios YES! Called the Law of Definite Proportions 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Yes, except for nuclear reactions that can change atoms of one element to a different element

6. J.J. Thompson (1856-1940) • English physicist • discovered the electron • thought atom was negative charges stuck in a positive charged lump • referred to as the “plumb pudding model”

8. We still use Thompson’s “cathode ray tube” in TV and other applications

9. Robert A. Millikan (1868-1953) • American physicist that continued Thompson’s work on electrons • Found the quantity of charge carried by an electron (one unit of negative charge) • Calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

10. Ernest Rutherford (1871-1937) • New Zealand physicist and former student of Thompson • proposed that the atom is mostly empty space • positive charges and almost of the mass are in a small, centralized region called the nucleus

11. Evidence for Rutherford’s Conclusions http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Generalchemistry/Atomic/BasicStructure/ErnestRutherford/ruther.gif Rutherford Flash Animation

12. Results of foil experiment if Plum Pudding model had been correct.

13. What Actually Happened

14. Niels Bohr (1855-1962) • former student of Rutherford • electrons found only in specific circular paths (orbits) around the nucleus • based on information about how the energy of an atom changes when it absorbs and emits light • called these fixed energies “energy levels”

16. Erwin Schrodinger (1926) • Austrian physicist • quantum mechanical model • probability of electron locations around the nucleus • not an exact orbit • led to the electron cloud model

17. Werner Heisenberg (1927) • German physicist • Heisenberg Uncertainty Principle – impossible to know the exact position and velocity of a particle (electron) at the same time • the observer affects the observed http://www.deutsches-museum-bonn.de/ausstellungen/heisenberg/bilder/heisenberg_2.jpg