Introduction to Atomic Structure

Transcription

1 hemistry Introduction to tomic Structure hemistry is the study of The type of matter that is changing and what types of changes it undergoes determines the field of chemistry that is being studied. ranches of hemistry Physical hemistry: the study of matter and the physics behind its changes Matter You will recall that we define matter as anything that has mass and takes up space. iochemistry: the study of the makeup and changes undergone by living species nalytical hemistry: The study of the composition (or analysis) of substances Organic hemistry: The study of compounds which primarily contain arbon Inorganic hemistry: The study of compounds which do NOT contain arbon toms You will also recall that all matter is made of very small particles called atoms toms are so small that it is only with recent technological and imaging break throughs that we are able to see even the most general shape of an atom. Light and Sound In 1905 Einstein derived an equation relating mass and energy. You should be familiar with this equation: E = mc 2 This equation has been changed a bit since, but a relationship has now, for the first time in history, been established between matter and energy, and between physics and chemistry. 1

2 The Nature of Light: Wave or Particle? The nature of light has been debated for thousands of years. In the 1600's, Newton argued that light was a stream of particles. Huygens countered that it was a wave. oth had good arguments, but neither could prove their case. Sound Waves: Interference If two waves are perfectly in sync with one another or, if the extra distance one sound has to travel is exactly one wavelength longer, the interference will be constructive, causing the waves to feed off one another. particle! wave! λ λ Listener Sound Waves: Interference Sound waves from 2 sources ut they will destructively interfere if one sound travels half a wavelength longer than the other. λ So for sounds waves, we expect to get a pattern of maxima (light bands) and minima (dark bands) like this. d loud quiet loud quiet λ/2 Listener loud ut because waves are waves, this would be the case for all waves, not just sound waves. Ripple Tank Waves Interference Patterns: Maxima source 1 source 2 We can see what is called an "interference" pattern when we look at how 2 waves interact with one another. This pattern is clearly seen in the dark and light bands noticed as the end of the tank. Light and (Maxima) Light and (Maxima) s you can see in the picture, 2 sources interacting with one another creates a pattern of black bands and light bands Light and (Maxima) Light and (Maxima) Light and (Maxima) 2

3 Interference Patterns: Minima iffraction ark and (Minima) ark and (Minima) s we can see this is true if we have 2 waves forming from 2 different sources, but it is the same when 1 wave from 1 source enters a double slit. ut why? ark and (Minima) ark and (Minima) When a wave, including light, meets an obstacle it bends around it to some extent. When it meets a small opening, the opening generates a new wave on the other side. ` 1 What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? 2 What principle is responsible for light spreading as it passes through a narrow slit? refraction polarization dispersion interference refraction polarization diffraction interference Young's ouble Slit Experiment This photo is of light (of one color) striking a distant screen after passing through 2 slits. This only makes sense if light is a wave. slit screen measurement screen iffraction and Interference The double slit experiment relies on two properties of waves: diffraction and interference Each slit generates a new wave due to diffraction. Those waves then either constructively or destructively interfere on a far away screen. d x S 1 light source S 2 L viewing screen 3

4 ouble Slit Maxima and Minima Interference occurs because each point on the screen is not the same distance from both slits. epending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot). Electromagnetic Waves The electric and magnetic waves are perpendicular to each other, and to the direction of propagation. Light is an Electromagnetic Wave Young showed that light is a wave. Maxwell showed that electromagnetic waves exist and travel at the speed of light. Light was shown to be an electromagnetic wave. The frequency of an electromagnetic wave is related to its wavelength. For electromagnetic waves (including light), in a vacuum: ll electromagnetic radiation travels at the same velocity: the speed of light (c) c = For all waves: Therefore for light: Electromagnetic Radiation velocity = wavelength x frequency 3 ll electromagnetic waves travel through a vacuum at 4 In a vacuum, the velocity of all electromagnetic waves the same speed. speeds that are proportional to their frequency. speeds that are inversely proportional to their frequency. none of the given answers is zero. is m/s. depends on the frequency. depends on their amplitude. 4

5 5 For a wave, the frequency times the wavelength is the wave's speed. amplitude. intensity. power. 6 Electromagnetic radiation travels through vacuum at a speed of 186,000 m/s 125 m/s 3.00 x 10 8 m/s It depends on wavelength 7 The wavelength of light that has a frequency of 1.20 x Hz is 25 m 2.5 x 10 5 m m 2.5 m 8 What is the frequency of light whose wavelength is 600 nm? 5.0 x Hz 1.0 x Hz 1.5 x Hz 2.0 x Hz c = λf c = 3.00 x 10 8 m/s c = λf c = 3.00 x 10 8 m/s ll objects emit electromagnetic radiation which depends on their temperature: thermal radiation. blackbody absorbs all electromagnetic radiation (light) that falls on it. lackbody Radiation Planck s Quantum Hypothesis The wave nature of light could not explain the way an object glows depending on its temperature: its spectrum. In 1900, Max Planck explained it by assuming that atoms only emit radiation in quantum amounts. ecause no light is reflected or transmitted, the object appears black when it is cold. However, black bodies emit a temperaturedependent spectrum termed blackbody radiation. For example, the temperature of the above Pāhoehoe lava flow can be estimated by observing its color. These days, this assumption is regarded as the birth of quantum physics and the greatest intellectual accomplishment of Planck's career. 5

6 Planck s Postulate The Photoelectric Effect When light strikes a metal, electrons sometimes fly off causing an electric current. where h is Planck s constant (6.63 x J s) and f is the frequency of the light lassical physics couldn't explain some specific features about how the effect works. So Einstein used Planck's idea to solve it. The Photon If atoms can only emit light in packets of specific sizes; maybe light itself travels as packets of energy given by Planck's formula. Wave Particle uality evacuated chamber E = hf Radiant energy metal surface Earlier we proved that light is a wave. where h is Planck s constant (6.63 x J s) e Now we've proven that light is a particle. voltage source urrent indicator He called these tiny packets of energy or light photons. So which is it? 9 The ratio of energy to frequency for a given photon gives its amplitude. its velocity. Planck's constant. its work function. 10 What is a photon? an electron in an excited state a small packet of electromagnetic energy that has particle like properties one form of a nucleon, one of the particles that makes up the nucleus an electron that has been made electrically neutral E = hf c = λf h = 6.63 x J s c = 3.00 x 10 8 m/s 6

7 11 The energy of a photon depends on its amplitude. its velocity. its frequency. none of the given answers 12 The photoelectric effect can be explained assuming that light has a wave nature. that light has a particle nature. that light has a wave nature and a particle nature. none of the above 13 The energy of a photon that has a frequency 110 GHz is J J J J 14 The frequency of a photon that has an energy of 3.7 x J is Hz Hz J J E J E = hf c = λf h = 6.63 x J s c = 3.00 x 10 8 m/s E = hf c = λf h = 6.63 x J s c = 3.00 x 10 8 m/s 15 The energy of a photon that has a wavelength of 12.3 nm is J J J J E J 16 If the wavelength of a photon is halved, by what factor does its energy change? 4 2 1/4 1/2 E = hf c = λf h = 6.63 x J s c = 3.00 x 10 8 m/s E = hf c = λf h = 6.63 x J s c = 3.00 x 10 8 m/s 7

8 alton's Postulates alton's Four Postulates uilding on emocritus' idea, in the early 1800s, English chemist John alton was the first scientist to observe the physical world and matter and via these observations, this draw some conclusions about atoms. (1) Matter is made of (2) ll atoms of one element are Various atoms and molecules as depicted in John alton's 1808 book: New System of hemical Philosophy alton s Four Postulates (3) toms of an element are into atoms of a different element by ; atoms are in chemical reactions. (4) are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. Law of onservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. click here for an explanation of conservation of mass Hydrogen Oxygen Water [ ] 17 Which one of the following is not one of the postulates of alton's atomic theory? iscovery of The Electron toms are made of protons, neutrons, and electrons. ll atoms of a given element are identical toms are neither created nor destroyed in chemical reactions. ompounds are formed when atoms of more than one element combine Streams of negatively charged particles were found to emanate from cathode tubes. J. J. Thomson is credited with their discovery (1897). E Each element is composed of extremely small particles called atoms. High voltage Electric field plate Magnetic coils 8

9 Plum pudding model The prevailing theory was that of the plum pudding model, put forward by J. J. Thomson around It featured a positive sphere of matter with negative electrons imbedded in it. Robert Millikan So we know that charge is a continuous variable. This was thought in much the same way that many of the properties of light can be explained by treating it as a continuous wave rather than as a stream of photons. The e is one of the fundamental physical constants and its accurate value is of great importance. In 1923, Millikan won the Nobel Prize in physics in part because of his experiment involving the electron. iscovery and Properties of the Electron The currently accepted value of e is: Radioactivity Radioactivity is the spontaneous emission of radiation by an atom. It was first observed by Henri ecquerel. Marie and Pierre urie also studied it. Knowing e allows the electron mass to be calculated: electrons x integers = charge The 3 shared the 1903 Nobel prize in physics for their work. Marie urie also went on to win the 1911 Nobel Prize for hemistry. Radioactivity Three types of radiation were discovered by Ernest Rutherford: alpha particles beta particles gamma rays [ ] 18 Of the three types of radioactivity characterized by Rutherford, which are particles? E β rays α rays, β rays, and γ rays γ rays α rays and γ rays α rays and β rays 9

10 Ernest Rutherford shot alpha particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. Geiger Marsden Experiment (Gold Foil Experiment) iscovery of the Nucleus ccording to the Plum Pudding Model of the atom, if we fire a particle at a wall made of solid particles we would expect most of the particles to bounce right back. few of the particles would fly through the "cracks" between the atoms but these gaps would be so small compared to the atoms that most of the particles would bounce off. iscovery of the Nucleus [ ] 19 The gold foil experiment performed in Rutherford's lab. What actually happened was very surprising. Most of the particles flew right through the foil! E confirmed the plum pudding model of the atom led to the discovery of the atomic nucleus was the basis for Thomson's model of the atom utilized the deflection of beta particles by gold foil proved the law of multiple proportions 20 In the Rutherford nuclear atom model: the heavy subatomic particles reside in the nucleus the principal subatomic particles all have essentially the same mass the light subatomic particles reside in the nucleus mass is spread essentially uniformly throughout the atom Subatomic Particles Protons were discovered by Rutherford in Neutrons were discovered by James hadwick in Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it. Particle harge Mass (amu) Proton Neutron Electron 10

11 tomic Spectra n atomic spectrum is a line spectrum only certain frequencies appear. If white light passes through such a gas, it absorbs at those same frequencies. tomic Spectra Why don't atoms radiate, or absorb, all frequencies of light? Why do they radiate light at only very specific frequencies, and not at others? ohr's Model n represents the energy level where is the lowest naturally occupied level or "ground state" The ohr tom These possible energy states for atomic electrons were quantized only certain values were possible. The spectrum could be explained as transitions from one level to another. Electrons would only radiate when they moved between orbits, not when they stayed in one orbit. + upper e upper e lower lower bsorption Emission bsorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. The electromagnetic energy is transformed to other forms of energy for example, to heat. Emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a photon, resulting in the production of light. When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is re emitted in the form of a photon. 11

12 Emission Spectrum Flame Test The wavelength of the photon is determined by the difference in energy between the two states. These emitted photons form the element's emission spectrum. Emission spectrum of Hydrogen Emission spectrum of Iron Light consists of electromagnetic radiation of different wavelengths. Therefore, when the elements or their compounds are heated either on a flame they emit energy in form of light. flame test is a procedure used in chemistry to detect the presence of certain metal ions, based on each element's characteristic emission spectrum. The ohr tom The ohr tom Using the oulomb force, he calculated the energy of each orbit. For hydrogen he arrived at this result: 13.6 ev E = n 2 n = 1, 2, 3, 4,... Notice that the energy levels are all negative, otherwise the electron would be free of the atom. The levels get closer together, and closer to zero, as n increases. n The lowest energy level is called the ground state; the others are excited states. ohr's Model ccording to ohr's model, first an electron is excited from its ground state. In this case we will consider an electron being excited from it's ground state (, since would be its first excited state) () to its second excited state + ohr's Model In order to make that jump, we would need to calculate the amount of energy required for this electron to increase from ground state () to the second excited state (). + 12

13 ohr's Model To do that we take the Final Energy (E f) the initial Energy (E 0). ohr's Model Here we see 2 separate emissions coming from the same electron. The electron can either go from right to or it can go from to to oth are acceptable and both will occur. Now you can solve for the frequency of light, but be sure to use the correct value for h! lso, since frequency and wavelength cannot be negative, you must use the absolute value of energy. Since energy is still energy no matter how you look at it, the negative or positive sign simply tells us if its coming (absorption) or going (emission). That's why we can do this and still be doing legitimate calculations. ohr's Model ohr's Model We can calcuate wavelength one of two ways, we can use our Lymen, almer, or Paschen equations we learned about earlier, or we can use the frequency we just found. + + de roglie s Hypothesis pplied to toms These are circular standing waves for n = 2, 3, and 5. Quantum Physics While a big step forward, ohr's model only worked for atoms that had one electron, like hydrogen or certain ionized atoms. It failed for all atoms other than hydrogen. The idea that the electron was a particle in orbit around the nucleus, but with wavelike properties that only allowed certain orbits, worked only for hydrogen. Semi classical explanations failed except for hydrogen. It turned out that only a lucky chance let it work even in that case. 13

14 Quantum Mechanics Our goal was to explain why electrons in an atom don't fall into the nucleus. n electron, as a charged particle, would fall in because of Newton's Second Law. ΣF = ma ut electrons, in atoms, aren't particles, they're waves. Waves don't follow Newton's Second Law. Schrodinger had to invent a new equation for wave mechanics. to be continued in chapter 2. slide to reveal Hψ new = Eψ equation 14