Uncovering Atoms: History and Properties

1. Lesson 5 Atomic Theory Anything in black letters = write it in your notes (‘knowts’)

2. Section 1 – Defining the Atom Section 2 – The Spectrum of Light Section 3 – Development of Modern Atomic Theory Section 4 – The Nucleus Section 5 – Electrons in Atoms

3. How do we know that atoms exist?

4. 5.1 – Defining the Atom Democritus (460 B.C. – 370 B.C.) one of the first to propose the idea of the atom; based on pure speculation comes from the Greek word atomos which means uncuttable or indivisible

5. John Dalton (~1800) proposed 1st atomic theory

6. Daltons Atomic Theory (~1800) • All elements are composed of tiny indivisible particles called atoms. • Atoms of the same element are identical. The atoms of any one element are different from those of any other element. • Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. • Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Compound made by atoms of elements A and B Atoms of element A Atoms of element B Mixture of atoms of elements A and B

7. What we know now of Dalton’s Atomic Theory • All elements are composed of tiny indivisible particles called atoms. Atoms are not indivisible – they are made of subatomic particles • Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Every atom has at least one isotope; one atom’s isotope is NOT identical to another isotope of the same atom. • Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. This is known as the Law of Definite Proportions – very important. • Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Atoms of one element can change into an atom of another element as a result of a nuclear reaction.

8. Law of Definite Proportions - Atoms combine in fixed whole number ratios. CO2 H2O Law of Multiple Proportions - Sometimes, atoms combine in more than one ratio. CO, CO2 H2O, H2O2

9. Atom - smallest particle of an element that still has the properties of the element

10. 5.2 – The Spectrum of Light Light waves can be described by their wavelength or frequency.

11. Wavelength– length of the wave! measured in meters Frequency– number of waves per second. How frequent the waves are measured in waves per second or Hertz (Hz)

13. Low energy ( = 700 nm) High energy ( = 380 nm) Frequency  (s-1) 3 x 106 3 x 1012 3 x 1022 10-8 10-14 102 The Electromagnetic Spectrum

15. Visible light ranges from 380 – 700 nm 1 nm = 10-9 m

16. Light behaves as a wave but consists of particles called photons. Photon - particle of light

17. Incandescence – emission of light by hot objects. Incandescent light bulbs give off more heat energy than light energy and so are not very efficient.

18. Fluorescence – absorption of light of high freq. and re-emit at lower freq.

19. Fluorescent Lights When Hg is energized, it produces UV light. The white powder in a fluorescent light bulb will fluoresce and visible white light is emitted. VIDEO – half-coated fluorescent tube

20. Blacklight – emits UV light and small amount of visible light.

21. A fluorescent material will absorb the high energy UV light and re-emit lower energy visible light – the material then appears to “glow” in the dark.

22. Phosphorescence – absorption of light followed by a delay in the re-emission of light. Phosphorescent materials will keep emitting light for a while in the dark.

23. Phosphorescent pigments In visible light ZnS SrAl2O4

24. Phosphorescent pigments In the dark ZnS SrAl2O4

25. Phosphorescent pigments In the dark after 4 minutes ZnS SrAl2O4

26. Chemiluminescence – emission of light by a chemical reaction.