1. LIGHT AND THE ELECTRON Quantized Energy

2. The Wave-Particle Duality  Light sometimes behaves like a wave. At other times, it acts as a particle.  Scientists have found strong evidence that light is both a particle and wave.  Therefore, light is considered to be both.

3. Light as a Wave  Light, and other types of waves, can be described in terms of wavelength, frequency, and amplitude.  Wavelength ( ) - length of one complete wave  Frequency ( ) - number of waves that pass a point during a certain time period (measured in hertz where 1 Hz = 1 cycle/s.)  Amplitude (A) - distance from the origin to the trough or crest Low frequency High frequency Amplitude long wavelength  short wavelength 

4. Visible light is just one portion of the electromagnetic spectrum. The Electromagnetic Spectrum AM radio Short wave radio Television channels FM radio Radar Microwave Radio Waves Gamma Rays X- Rays infrared Increasing photon energy Increasing frequency Decreasing wavelength Red Orange Yellow Green Blue Indigo Violet UV Rays VisibleLightVisibleLight R O Y G B I V HIGHENERGYHIGHENERGY LOWENERGYLOWENERGY

5. Light as a Particle  The observations of Planck and Einstein contributed to the particle theory of light.  Light is a stream of small particles or packets of energy called photons.  A photon is a massless particle of electromagnetic radiation that contains one quantum of energy.

6. EXCITED STATE VS. GROUND STATE

7. Electron States  Niels Bohr (1913): electrons can possess only certain amounts of energy, and can therefore be only certain distances from nucleus.  When all electrons are in the lowest possible energy state, an atom is in the _________________________.  If the “right” amount of energy is absorbed by an electron it can “jump” to a higher energy level. This is an unstable, momentary condition called the __________________________.

8. Energy Level Concept e-e- e-e- Ground state Excited state Electrons can only be at specific energy levels, NOT between levels.

9. Color = Energy of Photons Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329

10. Energy Level Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329 A B CD Ground state Energy Four excited states

11. An Excited Lithium Atom Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 326 Photon of red light emitted Li atom in lower energy state Excited Li atom Energy

12. Excitation of Hydrogen Atoms Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 328

13. Return to Ground State

14. Release of Energy  When an electron falls back to a lower-energy, more stable orbital (it might be the orbital it started out in, but it might not), the atom releases the “right” amount of energy as light.  Any-old-value of energy to be absorbed or released is NOT OK. This explains the lines of color in an emission spectrum.

15. Atomic Emission Spectra  The atomic emission spectrum of an element is a set of specific wavelengths of light (certain colors) that are emitted by atoms of that element.  Each element’s emission spectra is unique—it something like a fingerprint for an element. An emission spectra can be used to identify an element.  The emission spectrum of an element can be seen be using a prism to separate the light from a glowing sample of the element (such as neon in a neon light).

16. Emission Spectrum of Hydrogen 1 nm = 1 x 10 -9 m = “a billionth of a meter” 410 nm434 nm486 nm656 nm

17. Continuous and Line Spectra

18. Why are emission spectra important?  Historically, the emission spectra of the elements were a puzzle for many scientists. Based on classical (older-style) physics, many scientists thought the emissions spectra should be continuous (like a rainbow).  The fact that the elements had line spectra defied the common theory of the time.  This discrepancy eventually led to discovery of energy levels and the development of quantum mechanics.

19. Lab Questions 1. How can the existence of line spectra help to prove that energy levels in atoms exist? 2. How can the single electron in a hydrogen atom produce all of the lines found in its emission spectrum? 3. How can spectra be used to identify the presence of specific elements in a substance?