Atoms: The building blocks of Matter

1. Atoms: The building blocks of Matter The Structure of the Atom

2. Dalton’s Atomic Theory Democritus – atomos Lavoisier Law of Conservation of Matter Dalton Atomic Theory Proust Law of Definite Proportions

3. Dalton’s Atomic Theory Questions: • What accounts for differences in properties between atoms of different elements? • What forces hold atoms together in compounds? Dalton’s Model: Hard, round object

4. Discovery of the Electron Experiments by J.J. Thomson with cathode ray tubes showed that atoms contain electrons. (1897)

5. J.J. Thomson: The Plum Pudding Model Atoms are mostly empty space.

6. Millikan’s Oil Drop Experiment-1909 • Confirmed that electrons have a negative charge • The mass of an electron was determined (a small fraction of the mass of an atom).

7. Two important inferences: • Atoms are neutral, so they must contain positive charge to balance the negatively charged electrons. • The small mass of electrons compared to atoms indicated that there must be other particles which account for an atom’s mass.

8. Discovery of the Atomic Nucleus • Rutherford’s Experiment (1911)

9. Alpha Particle Scattering Experiment Expected results Actual Results

10. Rutherford’s Conclusion • Atom’s contained a densely packed bundle of matter with a positive charge. • This bundle of matter was called the nucleus • Suggested that electrons move around the nucleus like planets around the sun

11. Other Important Discoveries • (Mosely) Atoms of each element have a unique positive charge • Protons are positively charged particles in the nucleus • Neutrons are neutral particles in the nucleus • The charge of a proton is equal but opposite of the charge of an electron • Atoms contain an equal number of protons and electrons. • The # of protons in the nucleus gives each element its identity. • Strong nuclear forces hold protons and neutrons together in the nucleus