Atomic History Timeline

1. Atomic History Timeline Major Contributors Main Discoveries

2. Objectives • 4.1.1) List the main points of Dalton’s Atomic Theory and describe which parts are still believed to be true • 4.1.2) Explain the atomic models of the following scientists: Dalton, Thomson, Rutherford, Bohr, Schrödinger • 4.1.3) Describe the experiments performed by Thomson and Rutherford and what the observations suggested about the structure of the atom • Explain the contributions of the following scientists to the development of the Atomic Theory: Democritus, Crookes, Becquerel, Curie, Millikan, Chadwick

3. Democritus’s Theory(Foundation of Atomic Theory) • Thought that all things were made of tiny, invisible, indestructible particles, called atoms • Thought atoms varied in size, shape and weight

4. Aristotle's Theory • Opposed the Atomic Theory • Thought matter was made of air, water, fire, and Earth

5. Dalton’s Atomic Theory • Chemical elements are made of atoms • Atoms are indestructible (can’t be divided into smaller parts) • Atoms have no electric charge • Each atom of the same element is identical • When elements react, their atoms combine in simple, whole-number ratios

6. Crooke’s Tube

7. Henri Becquerel • When Becquerel placed uranium salt in a desk drawer with a photographic plate, radiation from the uranium formed an image on the plate. • This proved that radiation could occur without outside energy such as the sun • When he placed metal between the uranium and photographic plates, the metal blocked the radiation • Discovered natural radiation • Discovered that a magnetic field could block radiation

8. Marie Curie • Confirmed the work of Becquerel • Invented the term “radioactivity” • Discovered two new elements: polonium and radium in 1898. • Her research indicated that some atoms are unstable and will disintegrate over time on their own. • If they fall apart, the must not be the smallest piece of matter out there

9. JJ Thomson • Discovered electrons using the Cathode Ray Tube (CRT) • Beam will bend towards a positive charge, therefore, it must be made of negatively charged particles

10. Thomson’s Atomic Model“Plum Pudding” Model • He realized that neutral atoms must have equal amounts of positive and negative charge. • His model shows negative charges as plums and positive charges as the pudding.

11. Rutherford's Gold Foil Experiment • Consisted of shooting alpha particles (positive charge) at a very thin piece of gold. • Most went straight through, some deflected at an angle, and a small percentage bounced straight back

12. Rutherford’s Contribution to Atomic Theory • Atoms are mostly empty space. • Atoms have a tiny dense center called ‘the nucleus’. • Nucleus contains most of the mass of an atom. • Electrons are found outside of the nucleus. • Nucleus is positively charged.

13. Bohr’s Atomic Model • Electrons exist in orbits around the nucleus • Electrons can sometimes jump from one orbit to another by gaining or releasing energy

14. Robert Millikan • Discovered mass and electric charge of the electron by using the “Falling Drop” Method

15. Erwin Schrödinger • Stated that one could only predict the probability of where an electron could be (based on a mathematical formula), not its exact location • These probabilities form a region of space around the nucleus that he called orbitals (the most probable location of the electrons).

16. James Chadwick • Discovered the neutron in 1932 • Neutrons have approximately the same mass as protons • Neutrons have no electric charge