Models of the Atom

1. Models of the Atom

2. Review of Ancient Models of Matter • Leucippus and Democritus - the idea of the “atom” – the indivisible particle • Empedocles and Aristotle - the 4-element model • Aristotle, Aquinas and The Church – the 4-element model persists for over 2000 years. • The Alchemists – develop new chemical lab equipment and techniques but do not apply the Scientific Method

3. Enter the Scientists • The great breakthroughs made by Galileo, Copernicus and Isaac Newton made way for a revolution in pure science in Europe. • Great scientists such as Boyle, Lavoisier, Priestly and Scheele began to make strides in the field of Chemistry. • Scientists such as Francis Bacon and Rene Descartes developed the process we know as the Scientific Method. • The first scientist to apply scientific methodology to the study of matter and atoms was John Dalton

4. John Dalton • Dalton was a schoolteacher and later a professor of Mathematics and Natural philosophy (Science) • His strongest interests were in the field of Meteorology (Dalton’s Law of Partial Pressures) but he also made contributions in Chemistry • He began to make measurements of the masses of elements and created a simple table of elements. • He then went on to conduct experiments with chemical reactions and determined the masses and ratios of elements in the newly formed compounds.

5. Dalton’s Chemical Formulae

6. Dalton’s Atomic Theory • In 1803, Dalton suggested an atom is a solid featureless sphere. • Billiard Ball Model of the Atom • Dalton made two assertions about atoms: • atoms of each element are all identical to one another but different from the atoms of all other elements • atoms of different elements can combine to form more complex substances.

7. Five main points of Dalton's atomic theory • Elements are made of extremely small particles called atoms. • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. • Atoms cannot be subdivided, created, or destroyed. • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged.

8. Problems with Dalton’s Model • Atoms exhibit electrical properties – charged atoms, static electricity, etc… • The true nature of atoms could not be determined since atoms were extremely small and no known methods of testing individual atoms was known

9. J.J. Thomson • In 1897, J.J. Thomson conducted experiments using a cathode ray tube. • Thomson’s was able to create a stream of particles that flowed through the tube and fluoresced when they hit a detectoe screen • Using magnets Thomson was able to bend the flow of particles.

10. JJ Thomson’s Model of the Atom • The particles were called “electrons” because of their electrical properties • Electrons were determined to have very small masses and a negative charge (they were repelled by the negative force of a magnet)

11. J.J. Thomson’s Model of the Atom • Atoms contain particles called electrons • Electrons have a small mass and a negative charge • The rest of the atom is a sphere of positive charge • Electrons are embedded in this sphere, so that the resulting atoms are neutral or uncharged • Thomson compared the (negative) electrons to plums embedded in a positively charged pudding

12. J.J. Thomson’s Model of the Atom • Hence the Plum Pudding Model or Raisin Bun Model

13. Ernest Rutherford • In 1895, the German scientist Roentgen discovered X-rays and started a rush of work to study the electromagnetic radiation emitted by radioactive elements. • In the early part of the 20th century, many scientists such as the Curies, Becquerel, Geiger, and Rutherford studied these invisible rays. • Scientists began using these minute waves to study the properties of atoms.

14. Ernest Rutherford • Rutherford decided to test the model of his mentor, Thomson. • In 1911, Ernest Rutherford conducted his famous gold foil experiment. • He aimed alpha particles (positively charged particles) at a thin sheet of gold foil • PREDICTION: Particles would pass straight through the gold foil

15. The Gold Foil Experiment PREDICTED OBSERVATION: A small number of alpha particles bounced almost straight back from the gold foil OBSERVED

17. Ernest Rutherford’s Model of the Atom • An atom has a tiny, dense, positive core called the nucleus (which deflected the alpha particles and contains protons) • The nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons (through which the alpha particles passed unhindered) Ernest Rutherford

18. Ernest Rutherford’s Model of the Atom • Rutherford’s atom had a tiny, dense, positive nucleus orbited by electrons. Most of the atom’s volume is empty space

19. James Chadwick • In 1932, Chadwick conducted experiments that suggested that at least half of the mass of the nucleus was unaccounted for. • Chadwick’s research suggested the existence of a neutral particle in the nucleus with similar mass to the proton. James Chadwick – the real Jimmy Neutron

20. Updated Model of the Atom

21. Modern Development of the Model of the Atom

22. A Problem with the Planetary Model • Classical Newtonian Physics suggest that any object travelling in a curved path (even if it is moving at a constant speed) is said to be accelerating. • An electron accelerating around the nucleus would continuously emit electromagnetic radiation and lose energy • Therefore, it would eventually fall into the nucleus and the atom would collapse • However, not consistent with real-world observations – atoms are stable

23. Learning Checkpoint Assigned Questions p. 15 UC # 1, 2, 3, 4, 5, 6