1. ATOMIC THEORY Honors Chemistry Mr. McKenzie

2. Atomic Structure Essential Questions: –How was the modern theory of atomic structure developed? –What are the significant experiments that led to the advancement of our understanding of atomic structure? –What are the components of the atom?

3. The Atom Dalton – first to develop an atomic theory Used experimental data on chemical reactions to develop his theory Three parts to his theory: –All matter consists of tiny particles called atoms –All atoms of the same element are the same –Atoms are indivisible (cannot be broken down into smaller parts

4. Problems with Dalton’s Theory Marie Curie Studied radioactivity – she isolated radium and discovered polonium Only way to explain radioactivity is to hypothesize that radioactive atoms are disintegrating or decomposing into smaller particles If atoms can disintegrate, they must be made up of smaller particles

5. Smaller particles? The search was on for smaller particles If atoms could be broken down, what were the parts of the atom? Atoms are part of everything – that makes it important to understand them!

6. The Electron Cathode Ray Tubes Cathode Rays –flowed from the negative electrode (cathode) to the positive electrode (anode) –caused gases to glow –caused metals to glow red-hot –can be deflected by a magnetic field –are attracted to positive charges

7. Cathode Ray Tubes

9. The Electron J.J. Thompson Used his knowledge of the properties of electricity and magnetism to study the mass to charge ratio of cathode rays (electrons) Found that the cathode rays (electrons) were produced from a range of materials, suggesting that the particles were universal

10. Charge of an Electron - Millikan

11. Charge of an Electron – Millikan

12. The Electron Millikan – Oil Drop Experiment Used knowledge of electricity and magnetism to calculate the charge of ionized oil drops The measured charges were whole-number multiples of a constant, 1.60 x 10 -19 C Once the charge of an electron was discovered, its mass could be calculated using the mass-to-charge ratio determined by Thompson

13. The Proton The discovery of negatively charged electrons lead scientists to look for the accompanying positively charged particles Canal Rays Experiments showed that the particles must be significantly more massive than electrons

14. Canal Rays

16. The Proton Rutherford Bombarded different elements with alpha radiation Noticed that some particles were deflected rather than passing straight through the element Deflection depended on atomic mass Results showed that the positive charge had to be concentrated in the center of the atom When N was bombarded, protons were emitted and observed

17. The Neutron The number of protons in an atom didn’t account for the observed mass of an atom Hypothesized that there was another, neutral particle with a significant mass present in the atom to account for the discrepancy between the number of protons and the observed atomic mass

18. The Neutron Chadwick Bombarded a beryllium target with polonium, creating a “penetrating radiation” which he directed at a wax target Wax target emitted protons “penetrating radiation” was really neutrons, non-charged particles with about the same mass as a proton

19. The Nucleus Rutherford Gold Foil Experiment Created a thin sheet of gold, only a few atoms thick Hit the foil with alpha particles – products of radioactive decay that have mass and positive charge

20. Gold Foil Experiment

21. The Nucleus The discovery of the nucleus was a surprise Discovery completed much of the work regarding atomic structure Electron configurations will be covered later in the course