Chapter 5. Electrons in Atoms

Transcription

1 Chapter 5 Electrons in Atoms

2 Warm - Up What kind of light causes you to sunburn? Why does only this type of light burn your skin and not other light? What does sunscreen do on a chemical level?

3 Today s Agenda Question for Today: How are the wavelength and frequency of a wave related? Understand and utilize mathematical electromagnetic wave relationships. Continuous vs. electromagnetic spectra. Homework: Read Section 5.1 problems evens Due tomorrow! Lab books due Thurs

4 Development of Atomic Structure electron nucleus protons and neutrons Certain elements emit light when heated. Why? Li, Na, and K all react violently with water. Why? Chemical behavior related to the arrangement of the electrons.

5 Light as a WAVE Electromagnetic radiation A form of energy exhibiting wavelike behavior as it travels through space. Examples: Microwaves, X rays, radio waves

6 Parts of a Wave Wavelength (lambda) shortest distance between to peaks (or troughs) on a wave. Expressed in nm (1x10 9 nm = 1m) Frequency (nu) number of waves that pass a given point/second (1/s = Hz) Amplitude height from origin. peak trough

7 Parts of a Wave Amplitude Which wave has a shorter Which wave has a higher frequency?

8 What s nu? If wavelength is short, is frequency high or low? INVERSELY PROPORTIONAL - as increases decreases, Both related to the speed of light c = 3x10 8 m/s c = Pretend, c = 10, = 5, whats Now c = 10, = 2, whats c = c =

9 Practice 1. Rearrange c = to solve for (include units!) 2. Calculate the frequency of each of the following wavelengths of electromagnetic radiation: nm (632.8x10-9 m), and 503 nm. 3. Rearrange c = to solve for (include units!) 4. Calculate the wavelength of each of the following frequencies of electromagnetic radiation: x 10 6 Hz, and 1035 khz

10 More c = Practice! What is the wavelength of a microwave with a frequency of 1.56 x 10 9 Hz? The red-colored light in a fireworks display might be produced when Strontium salts are heated. What is the frequency of such red light with a wavelength of 6.5x10-7 m? After careful analysis, an electromagnetic wave is found to have a frequency of 7.8x10 14 Hz. What is the wavelength of the wave?

11 Electromagnetic Spectrum VIB G YOR

12 Continuous Spectrum White light is separated into its visible components by the prism. Continuous because each color corresponds to a unique wavelength and frequency.

13 Vocabulary to Know Emit Electromagnetic radiation Wavelength Frequency Amplitude Electromagnetic wave relationship Electromagnetic spectrum

14 Vocabulary to Know Emit to give off (energy, light, heat, etc.) Electromagnetic radiation energy that travels through space as a WAVE Wavelength distance from peak to peak of a wave Frequency # of waves that pass a point per second Amplitude height of wave Electromagnetic wave relationship c = Electromagnetic spectrum all wavelengths of energy from radio to gamma (visible nm)

15 Energy of Waves We want to be able to understand the energy given off by these waves Are they harmful to us? (too much energy) Can we use them for medicinal imaging? Technology? Transmission of information? What is the relationship between and ENERGY?!

16 What else is nu? Energy, E = h h = Plank s constant = x Js Named for Max Plank German physicist. Whats J? Joule = (kg x m 2 )/s 2 unit of energy

17 Practice Rearrange E = h to solve for How much energy is possessed by a single photon of UV-A electromagnetic radiation with a frequency x Hz? What is the frequency of a wave with an energy of 1.64 x J?

18 Energy and We know c = and E= h Rearrange both equations to solve for = c/ = E/h c = E/h E = hc/ We can relate wavelength, energy, and frequency with those relationships!

19 Practice Problem A Zn salt emits orange light at a wavelength of 450 nm. What is the energy of this light? 1. Identify the appropriate equation. E = hc/ 2. Rearrange to solve for unknown. Check (E is unknown) 3. Solve and show unit cancellations. E = 6.626x10-34 Js x 3x10 8 m 450 x 10-9 m s = 4.42 x J

20 Warm Up What is the energy? (show equations) Light with a wavelength of 640 nm. Wave with a frequency of 3.24 x Hz.

21 Today s Agenda Question of the Day: If light transfers energy like a wave, how does light also act like a particle? Warm Up Discuss light as a particle quantum, photoelectric effect, Use mathematical relationships to demonstrate the photoelectric effect. Compare and contrast a continuous spectrum to an atomic emission spectrum Test and Lab Return correction guidelines

22 Warm Up Find the energy and frequency of a ray with a wavelength of 24 nm. What is the energy and wavelength for a photon with a frequency of 5.21 x Hz?

23 Warm - Up is the symbol for is the symbol for The value of c is always The value of h is always The unit for wavelength is The unit for frequency is or c =, E = h / and E = h Find the energy and frequency of a ray with a wavelength of 24 nm. What is the energy and wavelength for a wave with a frequency of 3.24 x Hz?

24 Today s Agenda Does like always act like a wave? Photoelectric Effect Dual nature of light Photoelectric effect calculations Lab notebooks due today! Worksheets due tomorrow!

25 Light as a Wave Light transfers energy through a wave with a specific frequency and wavelength. c =, E = h E=hc HOWEVER, because science laughs at us for trying to figure out the universe, light doesn t ALWAYS act like a wave Science

26 Quantum Theory Light as a Particle Quantum concept: Matter can gain or lose energy in small specific amounts called quanta. Quanta: minimum amount of energy that can be gained or lost by an atom

27 Photoelectric Effect - Einstein Electrons (photoelectrons) are emitted from a metal s surface when a certain energy light shines on the surface. Photon is a massless particle that carries a quantum of energy. If light was a wave, photoelectric effect would NOT be observed.

28 Photoelectric Effect in Practice What is the value of Energy C? Energy C Energy = 2.4 x J Energy B Energy A

29 Building on PE Effect New Vocabulary and Notation: Incident Beam radiation that is used to eject an electron from a surface. (E i, i, i) Threshold energy, frequency, wavelength the required radiation to eject an electron dictated by a surface. (E t, t, t) Kinetic energy (KE) energy something possesses due to its motion.

30 Photoelectric Effect Problems Find the energies associated with each or. Use c= E=h or E =hc/ Is the incident energy more than the threshold energy? If so, electron is ejected! If the electron is ejected, find it s kinetic energy! Use E KE = E i E t

31 Photoelectric Effect Problems A photon in the IR region of the electromagnetic spectrum has a wavelength of 4.52 x 10-5 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold frequency of 5.51 x Hz? Make a table!

32 Warm Up! Remember that E = h & E = hc/ Aluminum has a threshold wavelength of 295 nm. If a sample of aluminum foil is exposed to four types of radiation, which radiation source would eject a photon? Microwave? = 3.82 x /s Optical wave? = 5.18 x /s UV wave? = 1.02 x /s X ray wave? = 4.31 x /s 295 nm E =? J

33 Using the Photoelectric Effect (from yesterday) A photon in the IR region of the electromagnetic spectrum has a wavelength of 7.23 x 10-9 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold threshold frequency of sodium is 5.51 x Hz? E = hc/ E = (6.626x10-34 Js x 3x10 8 m/s) = 2.74 x J E = h Yes! 7.23x10-9 m E = 6.626x10-34 Js x 5.51x /s = 3.65 x J

34 Practice You have a beam of light with a wavelength of 424 nm. What is the energy of this light? A metal surface has a threshold frequency of 4.84 x Hz. What is the energy of this light? Can we eject electrons?

35 Photoelectric Effect Problems A metal surface requires a photon with a minimum 450 nm to emit electrons from sodium metal. If an electron is ejected with a kinetic energy of 4.21 x J, what was the incident energy and wavelength? Light with a wavelength of 424 nm is shone upon a metal surface. The metal has a threshold frequency of 4.84 x Hz. What is the KE of the ejected electrons?