Atoms Unit

1. Atoms Unit

2. History of the Atomic Theory • Democritus His theory: Matter could not be divided into smaller and smaller pieces forever. There was a piece that was indivisible.

3. Dalton’s Theory He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements.

4. Thomson’s Plum Pudding Model • In 1897, the English scientist J.J. Thomson proved an atom is not solid and has even smaller particles. • Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding

5. Thomson Experiment Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles.

6. Rutherford’s Gold Foil Experiment In 1908, the English physicist Ernest Rutherford was working on an experiment that involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick). He assumed that his + particles would go right through the gold foil, since there was not any actual positive particles in Thomson’s model of the atom.

7. Atoms weren’t a pudding filled with a positively charged material. • Rutherford concluded that an atom must have a small, dense, positively charged center that repelled his positively charged “bullets.” • Rutherford named this center the nucleus

8. Bohr Model • According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certaindistances from the nucleus. Bohr

9. The show Homer is watching, When Dinosaurs Get Drunk, is replaced with another called The Boring World of Niels Bohr.

10. The Wave Model • According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun. • They move in certain wavelike pathways described by complicated equations. Erwin Schroedinger

11. Wave Model

13. End Atomic History

14. Atomic Structure

15. Electron Cloud Model

16. What defines an element? • Atomic number – the number of protons in the nucleus of an atom • This is the same for every • atom of a certain element.

17. What is the mass number of an element? Mass number – the sum of the number of protons and the number of neutrons in the nucleus of an atom. (Electrons are too small to contribute to the mass.) It is the mass of that atom. There can be different Mass numbers for different atoms of an element.

18. Isotopes • Atoms of the same element with a different number of neutrons are called isotopes. • Many elements have one or more isotopes.

19. Average atomic mass So why is the number 6.941 written as the mass for Lithium? This is the weighted average of the mass of the isotopes of Lithium. The isotopes are not equally represented. 92.5% of all lithium atoms are the Li-7 isotope; 7.5% are the Li-6 isotope. The other isotope is very rare.

20. How is it calculated? • 92.5% Li-7 7.5% Li-6 • Percent abundance of each isotope. • [0.925 (7)] + [0.075(6)] = • 6.475 + 0.45 = 6.925

22. End Atomic Structure General

23. Electron Properties

24. Atoms and Light • About 100 years ago some scientists noticed that certain elements emitted different colors of visible light when heated in a flame. • More research helped them connect • this light to the arrangement of each • element’s electrons.

25. LIGHT • To understand the structure of the atom we need to learn about light and how it can act like a wave and a particle at the same time . • Simply stated, light is nature's way of transferring energy through space , light is energy.

26. Light can behave like a wave wave pattern

27. Light as Waves –The Electromagnetic Spectrum Low frequency Long wavelength Low energy High frequency Short wavelength High energy 400nm 700nm Visible light ROYGBIV

28. Light behaves like a particle photoelectric effect Photoelectric effect colorado

29. Photons Light is absorbed by electrons in “chunks” or particles. A particle of light is a photon. • Absorption at the quantum level happens when an individual photon has the exact energy that corresponds to an energy gap between two possible energy states. • The type of energy gap corresponds directly to the frequency of the photon Photon absorption and emission

30. Putting it all together Atomic emission spectra – each element has a unique spectrum of colors that it produces as the electrons absorb energy. A spectrum is like the element’s “fingerprint”. Spectra

31. Not all spectra are visible; some are too high frequency; some are too low frequency Visible Hydrogen spectrum

32. End Atoms Unit