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2007, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA.

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Presentation on theme: "2007, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA."— Presentation transcript:

1 2007, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

2 The Behavior of the Very Small electrons are incredibly small a single speck of dust has more electrons than the number of people who have ever lived on earth electron behavior determines much of the behavior of atoms directly observing electrons in the atom is impossible, the electron is so small that observing it changes its behavior Tro, Chemistry: A Molecular Approach2

3 A Theory that Explains Electron Behavior the quantum-mechanical model explains the manner electrons exist and behave in atoms helps us understand and predict the properties of atoms that are directly related to the behavior of the electrons why some elements are metals while others are nonmetals why some elements gain 1 electron when forming an anion, while others gain 2 why some elements are very reactive while others are practically inert and other Periodic patterns we see in the properties of the elements Tro, Chemistry: A Molecular Approach3

4 The Nature of Light its Wave Nature light is a form of electromagnetic radiation composed of perpendicular oscillating waves, one for the electric field and one for the magnetic field an electric field is a region where an electrically charged particle experiences a force a magnetic field is a region where an magnetized particle experiences a force all electromagnetic waves move through space at the same, constant speed 3.00 x 10 8 m/s in a vacuum = the speed of light, c Tro, Chemistry: A Molecular Approach4

5 Electromagnetic Radiation Tro, Chemistry: A Molecular Approach5

6 Characterizing Waves the amplitude is the height of the wave the distance from node to crest or node to trough the amplitude is a measure of how intense the light is – the larger the amplitude, the brighter the light the wavelength, ( ) is a measure of the distance covered by the wave the distance from one crest to the next or the distance from one trough to the next, or the distance between alternate nodes Tro, Chemistry: A Molecular Approach6

7 Wave Characteristics Tro, Chemistry: A Molecular Approach7

8 Characterizing Waves the frequency, ( ) is the number of waves that pass a point in a given period of time the number of waves = number of cycles units are hertz, (Hz) or cycles/s = s -1 1 Hz = 1 s -1 the total energy is proportional to the amplitude and frequency of the waves the larger the wave amplitude, the more force it has the more frequently the waves strike, the more total force there is Tro, Chemistry: A Molecular Approach8

9 The Relationship Between Wavelength and Frequency for waves traveling at the same speed, the shorter the wavelength, the more frequently they pass this means that the wavelength and frequency of electromagnetic waves are inversely proportional since the speed of light is constant, if we know wavelength we can find the frequency, and visa versa Tro, Chemistry: A Molecular Approach9

10 Examples Calculate the wavelength of red light with a frequency of 4.62 x 10 14 s -1 Calculate the wavelength of a radio signal with a frequency of 100.7 MHz

11 Color the color of light is determined by its wavelength or frequency white light is a mixture of all the colors of visible light a spectrum RedOrangeYellowGreenBlueViolet when an object absorbs some of the wavelengths of white light while reflecting others, it appears colored the observed color is predominantly the colors reflected Tro, Chemistry: A Molecular Approach11

12 Amplitude & Wavelength 12

13 Electromagnetic Spectrum Tro, Chemistry: A Molecular Approach13

14 The Electromagnetic Spectrum visible light comprises only a small fraction of all the wavelengths of light – called the electromagnetic spectrum short wavelength (high frequency) light has high energy radiowave light has the lowest energy gamma ray light has the highest energy high energy electromagnetic radiation can potentially damage biological molecules ionizing radiation Tro, Chemistry: A Molecular Approach14

15 Interference the interaction between waves is called interference when waves interact so that they add to make a larger wave it is called constructive interference waves are in-phase when waves interact so they cancel each other it is called destructive interference waves are out-of-phase Tro, Chemistry: A Molecular Approach15

16 Interference Tro, Chemistry: A Molecular Approach16

17 Diffraction when traveling waves encounter an obstacle or opening in a barrier that is about the same size as the wavelength, they bend around it – this is called diffraction traveling particles do not diffract the diffraction of light through two slits separated by a distance comparable to the wavelength results in an interference pattern of the diffracted waves an interference pattern is a characteristic of all light waves Tro, Chemistry: A Molecular Approach17

18 Diffraction Tro, Chemistry: A Molecular Approach18

19 2-Slit Interference Tro, Chemistry: A Molecular Approach19

20 Tro, Chemistry: A Molecular Approach20 The Photoelectric Effect it was observed that many metals emit electrons when a light shines on their surface this is called the Photoelectric Effect classic wave theory attributed this effect to the light energy being transferred to the electron according to this theory, if the wavelength of light is made shorter, or the light waves intensity made brighter, more electrons should be ejected

21 Tro, Chemistry: A Molecular Approach21 The Photoelectric Effect The Problem in experiments with the photoelectric effect, it was observed that there was a maximum wavelength for electrons to be emitted called the threshold frequency regardless of the intensity it was also observed that high frequency light with a dim source caused electron emission without any lag time

22 Tro, Chemistry: A Molecular Approach22 Einstein’s Explanation Einstein proposed that the light energy was delivered to the atoms in packets, called quanta or photons the energy of a photon of light was directly proportional to its frequency inversely proportional to it wavelength the proportionality constant is called Planck’s Constant, (h) and has the value 6.626 x 10 -34 J∙s

23 Examples Calculate the number of photons in a laser pulse with wavelength 337 nm and total energy 3.83 mJ What is the frequency of radiation required to supply 1.0 x 10 2 J of energy from 8.5 x 10 27 photons?

24 Tro, Chemistry: A Molecular Approach24 Ejected Electrons 1 photon at the threshold frequency has just enough energy for an electron to escape the atom binding energy,  for higher frequencies, the electron absorbs more energy than is necessary to escape this excess energy becomes kinetic energy of the ejected electron Kinetic Energy = E photon – E binding KE = h - 

25 Tro, Chemistry: A Molecular Approach25 Spectra when atoms or molecules absorb energy, that energy is often released as light energy fireworks, neon lights, etc. when that light is passed through a prism, a pattern is seen that is unique to that type of atom or molecule – the pattern is called an emission spectrum non-continuous can be used to identify the material Rydberg analyzed the spectrum of hydrogen and found that it could be described with an equation that involved an inverse square of integers

26 Tro, Chemistry: A Molecular Approach26 Identifying Elements with Flame Tests NaKLiBa

27 Tro, Chemistry: A Molecular Approach27 Emission vs. Absorption Spectra Spectra of Mercury

28 Tro, Chemistry: A Molecular Approach28 Bohr’s Model Neils Bohr proposed that the electrons could only have very specific amounts of energy fixed amounts = quantized the electrons traveled in orbits that were a fixed distance from the nucleus stationary states therefore the energy of the electron was proportional the distance the orbital was from the nucleus electrons emitted radiation when they “jumped” from an orbit with higher energy down to an orbit with lower energy the distance between the orbits determined the energy of the photon of light produced

29 Tro, Chemistry: A Molecular Approach29 Bohr Model of H Atoms

30 Tro, Chemistry: A Molecular Approach30 Wave Behavior of Electrons de Broglie proposed that particles could have wave-like character because it is so small, the wave character of electrons is significant electron beams shot at slits show an interference pattern the electron interferes with its own wave de Broglie predicted that the wavelength of a particle was inversely proportional to its momentum

31 Tro, Chemistry: A Molecular Approach31 however, electrons actually present an interference pattern, demonstrating the behave like waves Electron Diffraction if electrons behave like particles, there should only be two bright spots on the target

32 examples Calculate the wavelength of an electron traveling at 2.65 x 10 6 m/s Determine the wavelength of a neutron traveling at 1.00 x 10 2 m/s (Mass neutron = 1.675 x 10 -24 g)

33 Tro, Chemistry: A Molecular Approach33 Uncertainty Principle Heisenberg stated that the product of the uncertainties in both the position and speed of a particle was inversely proportional to its mass x = position,  x = uncertainty in position v = velocity,  v = uncertainty in velocity m = mass the means that the more accurately you know the position of a small particle, like an electron, the less you know about its speed and visa-versa

34 Tro, Chemistry: A Molecular Approach34 Uncertainty Principle Demonstration any experiment designed to observe the electron results in detection of a single electron particle and no interference pattern

35 Tro, Chemistry: A Molecular Approach35 Determinacy vs. Indeterminacy according to classical physics, particles move in a path determined by the particle’s velocity, position, and forces acting on it determinacy = definite, predictable future because we cannot know both the position and velocity of an electron, we cannot predict the path it will follow indeterminacy = indefinite future, can only predict probability the best we can do is to describe the probability an electron will be found in a particular region using statistical functions

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