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Chapter 4: Glow in the Dark Introductory Activity List as many things as you can think of that “glow” What do you have to do to make these “glowing” things “glow”? Glow in the dark This chapter will introduce the chemistry needed to understand how glowing things work Section 4.1: History of Atomic Theory Section 4.2: Atomic Structure Section 4.3: Electron Structure Section 4.4: Periodic table Section 4.5: Periodicity Section 4.6: Light Section 4.7: Light and matter Glow in the dark Is based on Electron structure Gives off Changes in produce Is a part of Light Which can be determined using Periodic Table Atomic structure Arranged to show Is based on Atomic theory Periodicity Section 4.1—Development of Atomic Theory Dalton’s Atomic Theory John Dalton’s theory based on experiments in early 1800’s All matter is made of tiny particles “atoms” Atoms cannot be created, divided, destroyed or changed into other types of atoms Atoms of the same element have identical properties Atoms of different elements have different properties Atoms of different elements combine in wholenumber ratios to form compounds Chemical changes join, separate or rearrange atoms in compounds Cathode Ray Tubes A cathode ray is a ray of light traveling in a vacuum (no other particles inside) The ray travels from one metal plate to another as the plates are connected to electricity Cathode ray Metal plate (cathode) releases stream Metal plate (anode) to which stream travels Cathode Ray Tubes & Charge In the late 1800’s, JJ Thomson put charged plates outside the tube Negatively charged plate - + Positively charged plate Ray is deflected away from negative plate and towards positive plate It made no difference what type of metal he used in the tube—all material produced this stream that curved towards the positive charge Thomson’s conclusions The evidence from Thomson’s work showed that there was something negatively charged in atoms Since all types of metal produced the same result, the negative charge is in all types of atoms Since atoms were overall neutral, if there was a negative charge there had to also be a positive charge In 1897, Thomson announced that the rays were electrons and they had a negative charge Theories change Thomson’s evidence showed Dalton’s idea of solid, uniform atoms was incorrect. Eugene Goldstein conducted experiments to label the positive part “protons” and determined it has the same charge as the electron (with opposite sign) but is 1837 times heavier! Thomson developed the “plum pudding” model. Since most of us aren’t familiar with plum pudding, you can think of it as a chocolate cookie dough theory Thomson’s Theory The “chips” are the negative electrons. The “dough” is the positive portion The “chips” are stationary and don’t move within the “dough” Remember, officially this theory is called “plum pudding” but it’s the same idea! Gold Foil Experiment Hans Geiger performed experiments in the early 1900’s where he bombarded very thin gold foil with radioactive particles (alpha particles “”) They expected these relatively heavy particles to go through the atoms with a small deflection What happened in the experiment? Gold foil What did he see? Most of the alpha particles passed straight through with no deflection These particles did not run into anything Some did deflect slightly These particles ran into something much smaller than themselves A few were reflected back the direction they came from These particles ran into something very dense What did that mean? Atoms are mostly empty space Electrons (the smaller particles) were the cause of the small deflections There must be a small area of the atom with most of its mass (the protons) that caused the reflections. He called this small, dense area the nucleus A third particle The protons and electrons could explain the charges of the various parts of the atom They could not explain the total mass of the atoms Neutrons were proposed in 1920’s but not confirmed until 1932 by James Chadwick Neutrons had mass similar to protons and no charge. They were located in the nucleus More changes to the theory Niels Bohr performed experiments with hydrogen atoms & light He determined that electrons are in levels according to how much energy they have and that only certain energy amounts were allowed. The Bohr Model It consists of the nucleus with protons & neutrons and electrons in concentric orbits (circles) outside the nucleus The circle closest to the nucleus contains the lowest energy electrons The first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18. Pictures of the Bohr Models Electron Proton Neutron Hydrogen-1 Helium-4 Lithium-6 Use of the Bohr Model now We no longer believe electrons are in concentric circles, but this is still a convenient way to show energy levels on 2-dimensional paper Modern Atomic Theory In the 1920’s, Bohr’s research lead the way for the study of quantum mechanics (the study of tiny particles) Modern atomic theory uses calculus equations to show how the subatomic particles act as both particles and waves These equations show the most probable location of electrons in the atom (known as atomic orbitals) Section 4.2—Atomic Structure What are atoms? Atom - smallest piece of matter that has the chemical properties of the element. What’s in an atom? An atom is made of three sub-atomic particles Particle Location Mass Charge Proton Nucleus 1 amu = 1.6710-27 kg +1 Neutron Nucleus 1 amu = 1.6710-27 kg 0 Electron Outside the nucleus 0.00055 amu 9.1010-31 kg -1 1 amu (“atomic mass unit”) = 1.66 10-27 kg What gives an atom its identity? What makes an atom “carbon” as opposed to “oxygen”? Every atom has a different number of protons. The number of protons determines the identity of the atom The atomic number shows the number of protons. Atomic number = protons The Nucleus & Mass Since the nucleus has protons & neutrons, and the mass of each one is 1 amu… The mass of the nucleus (in amu’s) is the number of protons + neutrons Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom Mass # = protons + neutrons Charges Protons have a positive charge Electrons have a negative charge Neutrons have no charge Overall charge = protons + (-1)×electrons Charge = protons - electrons How do we show information about an element? Element symbols Element Symbol 1 or 2 letters, found on the periodic table Mass number # protons + # neutrons Charge A C X Z # # protons - # electrons (assumed to be “0” if blank) Atomic number # of protons Number How many atoms do you have? Example: Element symbols Element Symbol O = Oxygen Charge -2 Mass number 16 16 -2 O 8 Atomic number 8 Number Assumed to be “1” if blank Let’s Practice Example: Fill in the missing values Symbol Name Atomic # Magnesium-25 Mass # Charge Proton Neutron Electron +2 82 126 82 Isotopes What are isotopes? Isotopes - n. Atoms of the same element with a different number of neutrons Some isotopes are radioactive—but not all…many are quite stable! Isotopes Example Mass # = 2 amu Mass # = 1 amu Hydrogen-1 Hydrogen-2 If they have different number of neutrons, and neutrons have a mass of 1 amu… Then isotopes of the same element will have different masses! But because their protons are the same, they are the same element! Identifying Isotopes Isotopes can be differentiated by their different mass numbers in the element symbol 12 C Carbon-12 13 C Carbon-13 Or by the mass number following their name. Mass Number versus Atomic Mass Mass Number Average Atomic Mass # of protons + # of neutrons Average of actual masses Always a whole number Not a whole number For one specific isotope only Weighted average of all isotopes Is not found on the periodic table Is found on the periodic table Calculating Average Atomic Mass Average atomic mass is a weighted average (it takes into account how often each isotope occurs). Actual mass (not mass number) “Sum of” Average = atomic mass ( Abundance of isotope Mass of ) isotope What fraction of the time is that isotope present? Example of Finding Avg Atomic Mass Example: Find the atomic mass of chlorine if Chlorine-35 has a mass of 34.969 amu and Chlorine-37 has a mass of 36.966 amu and is present 24.22% of the time. This chart summarizes the information in the problem: Remember that percents add up to 100. So they said the second isotope is present 24.22% of the time. This means that the first isotope is present 100-24.22 = 75.78% of the time Isotope Mass Percent Decimal 1 34.969 amu 75.78 0.7578 2 36.966 amu 24.22 0.2422 Avg Mass 0.7578 34.969amu 0.2422 36.966 = 35.45 amu (this is what’s on the periodic table for Cl!) Section 4.3—Electron Structure The Electron Hotel The story of the Electron Hotel Shopping Center Parking Garage Restaurant A man built an hotel for electrons with a restaurant next door. But he was making so much money that he decided to add on with some more rooms and a parking garage. He still had high demand and decided to add on some more rooms and a shopping center. He used the last space he could to put some rooms above the shopping center. How the Electron Hotel Fills Shopping Center Parking Garage Restaurant This man had some very strange ideas about how to run his hotel. He insisted four things: • The lowest possible must be used first (actually it was the fire inspector that insisted on this one) • There can only be one person in a room until all rooms at that level have someone • No more than 2 people to a room • When two people are in a room, they must be of opposite sex If 8 people come to the hotel, where would he put them? Another Example Shopping Center Parking Garage Restaurant This man had some very strange ideas about how to run his hotel. He insisted four things: • The lowest possible must be used first (actually it was the fire inspector that insisted on this one) • There can only be one person in a room until all rooms at that level have someone • No more than 2 people to a room • When two people are in a room, they must be of opposite sex If 21 people come to the hotel, where would he put them? You Try Shopping Center Parking Garage Restaurant This man had some very strange ideas about how to run his hotel. He insisted four things: • The lowest possible must be used first (actually it was the fire inspector that insisted on this one) • There can only be one person in a room until all rooms at that level have someone • No more than 2 people to a room • When two people are in a room, they must be of opposite sex If 42 people come to the hotel, where would he put them? Where do electrons really live? Electron Clouds They don’t live in a hotel…They are in the area outside of the nucleus where the electrons reside. Electron Clouds Electron Hotel Electron cloud Which section of the hotel Principal energy levels The electron cloud is made of energy levels Which floor Subshells Energy levels are composed of subshells Which room Orbitals Subshells have orbitals. Subshell versus Orbital Subshell – A set of orbitals with equal energy Orbital – Area of high probability of the electron being located. Each orbital can hold 2 electrons Energy increases Types of Subshells Subshell Begins in energy level Number of equal energy orbitals Total number of electrons possible s 1 1 2 p 2 3 6 d 3 5 10 f 4 7 14 Electron Configuration What are electron configurations? They show the grouping and position of electrons in an atom. The number and configuration of electrons determines how something glows…so it’s important to know “where the electrons live” for an atom! Electron configurations use boxes for orbitals and arrows for electrons. Energy and Subshells 6p 6s 5p 5d 4f 4d 5s 4p 3d 4s 3p 3s 2p Energy 2s Subshells are filled from the lowest energy level to increasing energy levels. Does this look familiar? Electron Hotel! 1s Aufbau Principle The first of 3 rules that govern electron configurations 1 Aufbau Principle: Electrons fill subshells (and orbitals) so that the total energy of atom is the minimum What does this mean? Electrons must fill the lowest available subshells and orbitals before moving on to the next higher energy subshell/orbital. Where did we see this “rule” in the Electron Hotel? Hund’s Rule 2 Hund’s Rule: Place electrons in unoccupied orbitals of the same energy level before doubling up. How does this work? If you need to add 3 electrons to a p subshell, add 1 to each before beginning to double up. Where did we see this “rule” in the Electron Hotel? Pauli Exclusion Principle 3 Pauli Exclusion Principle: Two electrons that occupy the same orbital must have different spins. “Spin” describes the angular momentum of the electron How does this work? “Spin” is designated with an up or down arrow. If you need to add 4 electrons to a p subshell, you’ll need to double up. When you double up, make them opposite spins. Where did we see this “rule” in the Electron Hotel? Determining the Number of Electrons Charge = # of protons – # of electrons Atomic number = # of protons Example: How many electrons does Br-1 have? Writing Electron Configurations 1 Aufbau Principle: Electrons fill subshells (and orbitals) so that the total energy of atom is the minimum 2 Hund’s Rule: Place electrons in unoccupied orbitals of the same energy level before doubling up. 3 Pauli Exclusion Principle: Two electrons that occupy the same orbital must have different spins. Example: Write the boxes & arrows configuration for Cl Spectroscopic Notation Spectroscopic Notation Shorthand way of showing electron configurations The number of electrons in a subshell are shown as a superscript after the subshell designation 1s 2s 2p 3s 1s2 2s2 2p6 3s2 3p5 3p Writing Spectroscopic Notation 1 Determine the number of electrons to place 2 Follow Aufbau Principle for filling order 3 Fill in subshells until they reach their max (s = 2, p = 6, d = 10, f = 14). 4 The total of all the superscripts is equal to the number of electrons. Example: Write spectroscopi c notation for S No charge written Charge is 0 Atomic number for S = 16 = # of protons 0 = 16 - electrons 1s 2 2s 2 2p 6 3s 2 3p 4 Electrons = 16 2 + 2 + 6 + 2 + 4 = 16 Noble Gas Configuration Noble Gases & Noble Gas Notation Noble Gas – Group 8 of the Periodic Table. They contain full valence shells. Noble Gas Notation – Noble gas is used to represent the core (inner) electrons and only the valence shell is shown. Br Spectroscopic Noble gas 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5 [Ar] 4s 2 3d 10 4p 5 The “[Ar]” represents the core electrons and only the valence electrons are shown Which Noble Gas Do You Choose? How do you know which noble gas to use to symbolize the core electrons? Think: Price is Right. How do you win on the Price is Right? By getting as close as possible without going over. Choose the noble gas that’s closest without going over! Noble Gas # of electrons He 2 Ne 10 Ar 18 Kr 36 Xe 54 Noble Gas Notation Example 1 Determine the number of electrons to place 2 Determine which noble gas to use 3 Start where the noble gas left off and write spectroscopic notation for the valence electrons Example: Write noble gas notation for As Section 4.4—The Periodic Table History of the Periodic Table Different scientists organized the elements differently—this lead to confusion In 1869, Dimitri Mendeleev designed a periodic table based on atomic mass. This way showed patterns in properties that repeated across rows and similarities down columns He couldn’t find elements to fit all the property trends, so he left holes History of the Periodic Table The holes he left were later filled in as more elements were discovered The modern periodic table is arranged by atomic number rather than atomic mass This caused a few “switches” in placement, but overall is very similar to Mendeleev’s Organization of the Periodic Table Groups and Periods Periods Rows are called “periods” Groups Columns are called “groups” or “families” Information for Each Element Most periodic tables give the following information, but it can be in a different location Atomic Number Element Symbol If there’s a second letter, it’s lower-case Atomic Mass Number with decimals Gives the mass for 1 mole of atoms, in grams 6 C Carbon 12.01 Whole number— elements are ordered by this on the periodic table. Element Name Parts of the Periodic Table The rows at the bottom Most periodic tables are written with 2 rows at the bottom. This is done to allow the font to be bigger on a piece of paper. The rows at the bottom Most periodic tables are written with 2 rows at the bottom. This is done to allow the font to be bigger on a piece of paper. But they really belong here! Follow the atomic numbers on your periodic table to see it! Electron Configurations and the Periodic Table. Configurations Within a Group Look at the electron configurations for the Halogens F 1s2 2s2 2p5 Cl 1s2 2s2 2p6 3s2 3p5 Br 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 I 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 All of the elements in Group 7 end with 5 electrons in a p subshell. In fact, every Group ends with the same number of electrons in the highest energy subshell Configurations and the Periodic Table s-block p-block d-block s1 s2 p1 p2 p3 p4 p5 p6 d1 d2 d3 d4 d5 d6 d7 d8 d9 d10 f-block f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 How to remember the filling order? 1s 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6p 6s 4f 5d 7s 5f 6d s subshells begin in level 1, so begin the s-block with “1s” p subshells begin in level 2, so begin the p-block with “2p” d subshells begin in level 3, so begin the d-block with “3d” f subshells begin in level 4, so begin the f-block with “4f” How to remember the filling order? 1s 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6p 6s 4f 5d 7s 5f 6d To see the filling order of subshells, read from left to right, top to bottom! Note that this tool shows that the 3d energy level is filled after the 4s energy level! Where Does the Noble Gas Leave Off? How do you know where to start off after using a noble gas? Use the periodic table! 1s He 2s 2p Ne 3s 3p Ar 4s 3d 4p Kr 5s 4d 5p Xe 6p Rn 6s 4f 5d 7s 5f 6d The noble gas fills the subshell that it’s at the end of. Begin filling with the “s” subshell in the next row to show valence electrons. Section 4.5—Periodicity What is periodicity on the periodic table? The predictable pattern by which properties of elements change across or down the periodic table. There are always exceptions to these periodicity trends…each of the trends is a “general” trend as you move across a period or down a group. Trend 1: Atomic Radii What is atomic radius? Half of the distance between the nuclei of two bonded atoms. H H Distance between nuclei Atomic radius of hydrogen atom Atomic Radii Trends Decreases Increases Atomic Radii Trends Reasoning, Part 1 Why do atomic radii decrease across a period? Moving left to right, the number of protons, neutrons and electrons all increase. e e e n p n p n p e Move across the periodic table Radius decreases Lithium atom e e pn p p p n nn Beryllium atom As the # of protons electrons increase, the attraction between the positive nucleus and negative electron cloud increases. This attraction “pulls” in on the electrons. e Atomic Radii Trends Reasoning, Part 2 Why do atomic radii increase down a group? Protons, neutrons and electrons are also added as you move down a group. e e e + Move down the periodic table e e Radius increases e e e e e e e Lithium atom e + e Sodium atom However, the electrons are added in new energy levels. The inner electrons “shield” the new outer electrons from the pull of the nucleus, therefore it doesn’t pull in like the last slide. Trend 2: Ionization Energy What is Ionization Energy? The energy needed to remove the outermost electron. Ionization Energy Trends Increases Decreases Ionization Energy Trends Reasoning, Part 1 Why does Ionization Energy increase across a period? Moving left to right, the radius of the atom decreases as more protons pull on more electrons. Move across the periodic table e e n p n p n p e e Radius decreases IE increases Lithium atom e e pn p p p n nn e Beryllium atom When an atom is smaller, the electrons are closer to the nucleus, and therefore feel the pull more strongly. It is harder to pull electrons away from these smaller atoms. Ionization Energy Trends Reasoning, Part 2 Why does ionization energy decrease down a group? As you move down a group, the radius increases as more electrons shells are added. e e e + Move down the periodic table Radius increases IE decreases e e e e e e e + e e e e Lithium atom Sodium atom As the outer electrons (those involved in bonding) are farther from the nucleus, they will feel the “pull” of the nucleus less. It is easier to remove an electron from a larger atom. Trend #3: Electron Affinity What is Electron Affinity? energy released when an electron is added to an atom Electron Affinity Trends Increases Decreases Electron Affinity Trends Reasoning, Part 1 Why does Electron Affinity increase across a period? Moving left to right, the radius of the atom decreases as more protons pull on more electrons. Move across the periodic table e e n p n p n p e e Radius decreases EA increases Lithium atom e e pn p p p n nn e Beryllium atom When an atom is smaller, the electrons are closer to the nucleus, and therefore feel the pull more strongly. A smaller atom can handle an extra electron more easily as it can be more “controlled” by the closer nucleus Electron Affinity Trends Reasoning, Part 2 Why does electron affinity decrease down a group? As you move down a group, the radius increases as more electrons shells are added. e e e + Move down the periodic table Radius increases e e e e e e EA decreases e e e Lithium atom e + e Sodium atom As the outer electrons (those involved in bonding) are farther from the nucleus, they will feel the “pull” of the nucleus less. The larger atom is less able to “control” a new electron added. Ionic Charge & Radii Review Some Definitions Ion – atom with a charge. Cation – positively charged ion. Results from loss of electrons. Anion – negatively charged ion. Results from gain of electrons. Ionic Radii—Cations How does the radius of a cation compare to the parent Atoms lose electrons to create positive ions atom? e e e + Lithium atom Creating a cation, losing electrons Radius decreases e e + Li+ ion When electrons are lost, there are now more protons than electrons Therefore, the protons have a greater “pull” on each of the electrons. Ionic Radii—Anions How does the radius of an anion compare to the parent atom? Atoms gain electrons to create negative ions e e e e Creating an anion, gaining electrons e + e e e e e e e e + Radius increases e e e Oxygen atom e e O2- ion When electrons are gained, there are now more electrons than protons Therefore, the protons have a weaker “pull” on each of the electrons. Let’s Practice Example: List Li, Cs and K in order of increasing Atomic radii Example: List Li, N and C in order of increasing Atomic radii Ionization Energy Electron Affinity Ionization Energy Electron Affinity Section 4.6—Light Light is Electromagnetic Radiation Electromagnetic energy is energy that has electric and magnetic fields There are many types of Electromagnetic Radiation…visible is just one type! Electromagnetic energy travels in waves at the speed of light “c” (3.0 × 108 m/s) Types of Electromagnetic Radiation 400 nm 700 nm Visible light Cosmic Gamma X- Ultraviolet Rays Rays Rays Rays 10-12 10-10 10-8 Infrared Microwaves Rays 10-6 10-4 10-2 Radio waves 1 102 Electric Power 104 106 Wavelengths (cm) Wave Properties—Frequency Frequency () is the number of times a wave completes a cycle in one second (cycles per second is “Hertz” or “Hz”) Lower frequency Higher frequency Wave Properties—Wavelength Wavelength () is the distance from trough to trough of a wave (measured in meters “m”) wavelength Relationship between wave properties The shorter the wavelength, the higher the frequency The higher the frequency, the higher the energy The speed of light is equal to wavelength (in meters) times frequency (in sec-1 or s-1 or Hz) c = Visible Range Wavelength increases Frequency decreases Energy decreases 400 nm 700 nm Visible light White light is made of all the colors…a prism can separate white light into a rainbow! Light is Quantized Light is quantized, which means it comes in packets—you can only have certain amounts of it The “packets” are called “photons” The Energy of a photon (in Joules, “J”) is equal to Planck’s constant “h” times frequency) E = h where h = 6.63 × 10-34 Js Section 4.7—Light & Matter Electrons Absorbing Energy Photon coming into atom collides with electron. Photons are energy. + Electrons Absorbing Energy Photon coming into atom collides with electron. Photons are energy. + The electron is “excited” to a higher energy level with is newly increased energy from absorbing the photon. Excitation The process of an electron absorbing a photon of light (energy) and being promoted to a higher energy level from its “ground state” And later… The electron cannot remain in that excited state indefinitely + And later… The electron cannot remain in that excited state indefinitely + Energy is released during relaxation Relaxation The process of an electron releasing a photon of light (energy) and falling back down to a lower energy level. Energy of photon and levels jumped The higher the energy of the photon, the greater the electron jump! A photon of UV light has more energy than a photon of Infrared light The UV photon would cause a higher energy jump (jump up more levels) than the IR photon. Total energy in = Total energy out However much energy was absorbed must be released again, but it can be released in smaller packets A high energy photon might be absorbed, but two lower energy photons might be released as the electron falls in a “stepwise” manner. Photons must match energy changes The energy of the photon must exactly match the energy change of the electron. If the photon is not an exact match, the photon will pass through unabsorbed. + Measuring light absorption Not all colors come out All colors of light go in Sample of hydrogen Hydrogen Spectrum The black bars are the colors that a hydrogen atom absorbs. The other colors pass through the atom un-absorbed. Absorption of Molecules Because the structure of a molecule is much more complicated than a single atom, they absorb regions of light rather than single wavelengths. Absorption spectrum of water Picture from http://www.lsbu.ac.uk/water/vibrat.html Ways of producing light Fluorescence: visible light is absorbed and visible light is emitted at the same time— the relaxation happens very quickly after excitation Phosphorescence: Visible light is absorbed and then a while later is emitted—relaxation occurs after a period of time Ways of producing light Incandescence: Energy is put in from heat and given off as visible light Chemiluminescence: Energy released during a chemical reaction is absorbed to cause excitation. Relaxation produces visible light Biolouminescence: Chemiluminescence that occurs in a biological organism. Triboluminescence: Physical pressure or torque provides energy for excitation. Relaxation produces visible light. What did you learn about glowing things? Glow in the dark Is based on Electron structure Gives off Changes in produce Is a part of Light Which can be determined using Periodic Table Atomic structure Arranged to show Is based on Atomic theory Periodicity