BELLWORK 9/25/18

1. BELLWORK 9/25/18 • What is the atom? • Write and/or draw any information that you know about the atom

2. VIDEO – Models of the Atom TimelineEdPuzzle

3. Democritus (400 B.C.) • A Greek philosopher • Was the first person to think about an atom’s existence. • Believed that matter was composed of tiny indivisible particles called atomos. • Had no evidence Hmmm… matter…

4. Aristotle (380 B.C.) • A Greek philosopher • Did not believe in atoms. • Believed all matter was made from air, earth ,fire, and water. • He believed that stars and planets were made of a substance called aether. “atomos” Hmmm… matter…

5. John Dalton (1803) • A meteorologist • Unlike Democritus, he had experimental evidence to support his theory. • Dalton had fourmajor points (postulates) to his theory.

6. Dalton’s Atomic Theory 1.) All elements are composed of indivisible particles called atoms. 2.) Atoms of the same element are identical. The atoms of any one element are different from those of another.

7. Dalton’s Theory (cont) 3.) Atoms of different elements mix or combine in whole number ratios. Example: Oxygen combines with hydrogen to form water in a 2:1 ratio. 4.) Chemical reactions occur when atoms separate, join, or rearrange. Law of Conservation of Mass - the sum of the reactants = sum of products

8. JJ. Thomson’s Plum Pudding Model - 1897 • Discovered the electron. • From his experimental evidence, he believed that the atom was a solid positive sphere with electrons shoved into the sides of it.

9. Thomsons’ Cathode Ray Tube Experiment • In the tube was an inert gas, and two plates, a positiveand a negative. • The particles in the gas were attracted to the positiveplate. • Therefore, the particles MUST have anegativecharge. (Opposites attract.)

10. Ernest Rutherford- 1911 • Discovered the nucleus • Shot high energy beam of alphaparticlesinto gold foil. • Planetary Model

11. WATCH THE VIDEO ON ERNEST RUTHERFORD’S GOLD FOIL EXPERIMENT

12. Ernest Rutherford Conclusions • The atom is mostly empty space. • The alpha particle came close to something small and positive (nucleus). • The alpha particles hit a small, very dense, and positively charged center (nucleus).

13. The problem with Rutherford’s planetary model • It didn’t explain why the atom didn’t collapse into itself. • If the electrons circle around the nucleus, why are they not drawn to the positive charges inside of the nucleus.

14. Niels Bohr - 1913 • In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths. • Won a Nobel Prize

15. Bohr Model of the Atom

16. Erwin Schrodinger - 1926 • In 1926, he further explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found

17. Quantum Mechanical Model

18. JAMES CHADWICK (1932) • DISCOVERED THE NEUTRON

19. Side note… Not all of Dalton’s postulates were correct. • We now know that atoms are indeed divisible – atoms can be broken down into their subatomic particles,protons, neutrons, and electrons (and these too can be broken down even further!). • We also know that not all atoms of the same element areidentical. Isotopes exist for different elements. (We’ll talk about this later.)

20. Properties of Subatomic Particles • p+ • +1 • Proton • 1.007276 • 0 • Neutron- most massive • n • 1.008665 • Electron – least massive • e- • -1 • 0.0005486

21. Location of Particles • Protons– In the nucleus • Neutrons – In the nucleus • Electrons – In the electron cloud outside the nucleus in energy levels.

22. Neutral Atom An atom is considered neutralwhen it has the same number of electrons and protons. (p+ = e-)