* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 6 Electronic Structure of Atoms
Survey
Document related concepts
Double-slit experiment wikipedia , lookup
Quantum electrodynamics wikipedia , lookup
Molecular orbital wikipedia , lookup
Ferromagnetism wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Matter wave wikipedia , lookup
Franck–Condon principle wikipedia , lookup
X-ray fluorescence wikipedia , lookup
Wave–particle duality wikipedia , lookup
Electron scattering wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Hydrogen atom wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
Tight binding wikipedia , lookup
Chemical bond wikipedia , lookup
Atomic orbital wikipedia , lookup
Transcript
Chapter 6 Electronic Structure of Atoms Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Where are electrons (electrons distribution) ? -First question we will ask is: how are electrons distributed if we know that they interact through electromagnetic forces with nuclei and other electrons? -Second question we ask is how well could the such interaction mechanisms be described? (physical/mechanical model) Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. EM radiation Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. What do you know about light : simple experiments (experiment 3) Prentice - Intelliome A. S. LLC 2005 Electronic Structure of4Atoms What do you know about light : simple experiments (experiment 3) Prentice - Intelliome A. S. LLC 2005 Electronic Structure of5Atoms What makes the light- the light sources • Atoms can release EM light or absorb it • Atoms don’t emit EM of all colors, only very specific wavelengths – in fact, the spectrum of wavelengths can be used to identify the element Prentice - Intelliome A. S. LLC 2005 Electronic Structure of6Atoms Emission Spectrum Prentice - Intelliome A. S. LLC 2005 Electronic Structure of7Atoms Spectra Prentice - Intelliome A. S. LLC 2005 Electronic Structure of8Atoms What we know about the EM radiation: Electromagnetic Waves: check your radio, telephone or TV frequency or wavelength? Prentice - Intelliome A. S. LLC 2005 Electronic Structure of9Atoms • The Electromagnetic Spectrum light passed through a prism is separated into all its colors - this is called a continuous spectrum • the color of the light is determined by its wavelength Prentice - Intelliome A. S. LLC 2005 Electronic Structure of 10Atoms Spectrum of white light Radiation composed of only one wavelength = monochromatic (one color). Radiation that spans a whole array of different wavelengths = continuous. White light = continuous spectrum of colors. Dark= no light. When light interacts with material it can be scattered, absorbed or emitted. Depending on the material, the interaction with different radiation will be different. Prentice - Intelliome A. S. LLC 2005 Electronic Structure of11Atoms Types of Electromagnetic Radiation • Classified by the Wavelength – Radiowaves = l > 0.01 m • low frequency and energy – Microwaves = 10-4m < l < 10-2m – Infrared (IR) = 8 x 10-7 < l < 10-5m – Visible = 4 x 10-7 < l < 8 x 10-7m • ROYGBIV – Ultraviolet (UV) = 10-8 < l < 4 x 107m – X-rays = 10-10 < l < 10-8m – Gamma rays = l < 10-10 • high frequency energy Prentice -and Intelliome A. S. LLC 2005 Electronic Structure of 12Atoms Light as well as electrons can be described through wave-like observables • Waves are described by wavelength, l, amplitude, A and frequency, n. • The speed of a wave, c, is given by its frequency multiplied by its wavelength: Velocity ( c ) = nl • C = 2.9979 * 108 m/s (in vacuum) Electronic Prentice - Intelliome A. S. LLC 2005 Structure of 13Atoms Low Frequency Wave l l High Frequency Wave l Prentice - Intelliome A. S. LLC 2005 Electronic Structure of 14Atoms Waves • To understand the electronic structure of atoms, one must understand the nature of electromagnetic radiation. • The distance between corresponding points on adjacent waves is the wavelength (l). Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Waves • The number of waves passing a given point per unit of time is the frequency (n). • For waves traveling at the same velocity, the longer the wavelength, the smaller the frequency. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Electromagnetic Radiation • All electromagnetic radiation travels at the same velocity: the speed of light (c), 3.00 108 m/s. • Therefore, c=n l Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. The Sources of Light • For atoms and molecules one does not observe a continuous spectrum, as one gets from a white light source. • Only a line spectrum of discrete wavelengths is observed. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Absorption Spectrum Emission Spectrum Absorption Spectrum 656.3 486.1 Emission Spectrum Prentice - Intelliome A. S. LLC 2005 434.1 410.2 Electronic Structure of 19Atoms # Note that electromagnetic structures can be very complex, this is just a simple wavelike approximation Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. EM radiation interacting with electrons Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. What is inside atom-experimental evidence Electronic Structure of Atoms A Simple Book-keeping Model of Electrons in EM radiation field drivers in Atom Electronic Structure of Atoms Experiments show that structures have well defined wavelike patterns Electronic Structure of Atoms What are electron distributions? electrons behave like wave like physical structures Electronic Structure of Atoms Interferences - Wave-like observables Electronic Structure of Atoms The Nature of Electron Distribution in EM fields • Consequences : causal relationships between observables can explain phenomenological relations such as the one between mass and wavelength at certain conditions h l = mv Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Consequence: causal relations (“uncertainty principle”) (x) (mv) h 4 For some systems h can be used , h= Planck’s constant, 6.626 10−34 J-s. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Causal relationships can help: For example approximation phenomenological relation: Electron energy+ Bonding energy = h n E = hn where h is Planck’s constant, 6.626 10−34 J*s. Electronic Structure of Atoms How to measure electron distributions in atoms? Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. How to see electrons inside atoms? Can EM radiation help? Electronic In 1800s Hertz started kicking electrons with EM radiation Structure Prentice - Intelliome A. S. LLC 2005 of 31Atoms How to see electrons inside atoms? Can EM radiation help? In 1800s Rydberg’s analysis of atomic line spectra (Na,H): *observables using h constant Prentice - Intelliome A. S. LLC 2005 Electronic Structure of 32Atoms The Electronic Structure of Atoms • Experimental evidence: Electrons in an atom are seen as going from one well defined area to the other. Each of these areas (orbitals) corresponds to certain energy, so the colors we see correspond to the energies of photons that are equal to the energy difference between these orbital energies. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. The Electronic Structure of Atoms • Electrons stay in well defined areas like standing waves stay in a resonance box and they do not gain or lose energy for certain time: we say they are in a certain state. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. The Electronic Structure of Atoms Energy can be absorbed or emitted: if absorbed electron will slow down to an outermost orbital, if emitted an electron will go down closer to the nucleus . The energy difference is Ei –Ef =Ephoton = hn = h c/l Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Brings us back to the question how can we see electrons inside atoms with EM radiation. Experiment Model ? Experiment Prentice - Intelliome A. S. LLC 2005 Electronic Structure of 36Atoms The Electronic Structure of Atoms The energy absorbed or emitted from the process of electron promotion or demotion can be calculated by the equation: E = −RH ( 1 1 - 2 nf2 ni ) where RH is the Rydberg constant, 2.18 10−18 J, and ni and nf are the initial and final energy levels of the electron. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. What about a mechanical model? • How to describe these quantum waves? • Quantum or wave mechanics and • The future Electronic Structure of Atoms How to describe the mechanics of electron motion in many-electron atoms and 3 dimensions? • Answer: Use any mechanics that describes what you see in experiments. If you use wavemodels make sure that you accurately represent observed properties of electrons! Example: (Schroedinger wave equation) If electrons have wavelike patterns use a wave mechanics to describe it !! Also take into account 3 dimensions (x, y, z). Electronic Structure of Atoms Mechanics of electrons (probability as density) in atoms Electronic Structure of Atoms Orbits vs. Orbitals Pathways vs. Probability-Pathways Kicking “waves” is not as precise as kicking localized objects! Electronic Structure of Atoms Wave-like baseball: • Here, you cannot have “straight” hits, the path of the ball has wavelike distribution and interferes with other objects, players etc.! • The person catching the ball should expect “wavelike” distribution of incoming balls! The forces and interactions here are wavelike. • See again our slit experiment and assume that the slit is a bat and detector is a “catcher”. This analogy requires more detailed knowledge of physics! Electronic Structure of Atoms How we describe the path of the ball ?: position (x, y, z coordinates) And how we describe the path of the electron ?: Wave-like distribution (quantum numbers n, l, m) Electronic Structure of Atoms Quantum or Wave Mechanics Schrödinger’s equation and 3 quantum numbers:Principal Quantum Number, n. This is the same as Bohr’s n. As n becomes larger, the atom becomes larger and the electron is further from the nucleus. Angular Quantum Number, l. This quantum number depends on the value of n. The values of l begin at 0 and increase to (n - 1). We usually use letters for l (s, p, d and f for l = 0, 1, 2, and 3). Usually we refer to the s, p, d and forbitals. Magnetic Quantum Number, ml. This quantum number depends on l. The magnetic quantum number has integral values between -l and +l. Magnetic quantum numbers give Electronic the 3D orientation of each orbital. Structure of Atoms Quantum Mechanics • Erwin Schrödinger developed a mathematical treatment into which both the wave and particle nature of matter could be incorporated. • It is known as quantum or wave mechanics. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Quantum Mechanics • The wave equation is designated with a lower case Greek psi (Y). • The square of the wave equation, Y2, gives a probability density map of where an electron has a certain statistical likelihood of being at any given instant in time. • Note: y is the solution of wave mechanical (Schrödinger) equation, so it represents a wavelak distribution- the symbol l could also have been chosen Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Quantum Numbers • Solving the wave equation gives a set of wave functions, or orbitals, and their corresponding energies. • Each orbital describes a spatial distribution of electron density. • An orbital is described by a set of three quantum numbers. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Principal Quantum Number (n) • The principal quantum number, n, describes the energy level on which the orbital resides. • The values of n are integers ≥ 1. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Angular Momentum Quantum Number (l) • This quantum number defines the shape of the orbital. • Allowed values of l are integers ranging from 0 to n − 1. • We use letter designations to communicate the different values of l and, therefore, the shapes and types of Electronic orbitals. Structure of Atoms © 2009, Prentice-Hall, Inc. Angular Momentum Quantum Number (l) Value of l 0 1 2 3 Type of orbital s p d f Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Magnetic Quantum Number (ml) • The magnetic quantum number describes the three-dimensional orientation of the orbital. • Allowed values of ml are integers ranging from -l to l: −l ≤ ml ≤ l. • Therefore, on any given energy level, there can be up to 1 s orbital, 3 p orbitals, 5 d orbitals, 7 f orbitals, etc. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Magnetic Quantum Number (ml) • Orbitals with the same value of n form a shell. • Different orbital types within a shell are subshells. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. s Orbitals • The value of l for s orbitals is 0. • They are spherical in shape. • The radius of the sphere increases with the value of n. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. s Orbitals Observing a graph of probabilities of finding an electron versus distance from the nucleus, we see that s orbitals possess n−1 nodes, or regions where there is 0 probability of finding an electron. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. p Orbitals • The value of l for p orbitals is 1. • They have two lobes with a node between them. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. d Orbitals • The value of l for a d orbital is 2. • Four of the five d orbitals have 4 lobes; the other resembles a p orbital with a doughnut around the center. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Energies of Orbitals • For a one-electron hydrogen atom, orbitals on the same energy level have the same energy. • That is, they are degenerate. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Energies of Orbitals • As the number of electrons increases, though, so does the repulsion between them. • Therefore, in manyelectron atoms, orbitals on the same energy level are no longer degenerate.Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Spin Quantum Number, ms • In the 1920s, it was discovered that two electrons in the same orbital do not have exactly the same energy. • The “spin” of an electron describes its magnetic field, which affects its energy. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Spin Quantum Number, ms • This led to a fourth quantum number, the spin quantum number, ms. • The spin quantum number has only 2 allowed values: +1/2 and −1/2. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Pauli Exclusion Principle • No two electrons in the same atom can have exactly the same energy. • Therefore, no two electrons in the same atom can have identical sets of quantum numbers. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Electron Configurations • This shows the distribution of all electrons in an atom. • Each component consists of – A number denoting the energy level, Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Electron Configurations • This shows the distribution of all electrons in an atom • Each component consists of – A number denoting the energy level, – A letter denoting the type of orbital, Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Electron Configurations • This shows the distribution of all electrons in an atom. • Each component consists of – A number denoting the energy level, – A letter denoting the type of orbital, – A superscript denoting the number of electrons in those orbitals. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Orbital Diagrams • Each box in the diagram represents one orbital. • Half-arrows represent the electrons. • The direction of the arrow represents the relative spin of the electron. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Hund’s Rule “For degenerate orbitals, the lowest energy is attained when the number of electrons with the same spin is maximized.” Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Periodic Table • We fill orbitals in increasing order of energy. • Different blocks on the periodic table (shaded in different colors in this chart) correspond to different types of orbitals. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Some Anomalies Some irregularities occur when there are enough electrons to halffill s and d orbitals on a given row. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Some Anomalies For instance, the electron configuration for Cu and Cr. For Cr it is [Ar] 4s1 3d5 rather than the expected [Ar] 4s2 3d4. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Some Anomalies • This occurs because the 4s and 3d orbitals are very close in energy. • These anomalies occur in f-block atoms, as well. Electronic Structure of Atoms © 2009, Prentice-Hall, Inc. Electron Configurations Electronic Structure of Atoms 7s 6s Energy 5s 4s 6p 5p 6 d 5d 5f 4f 4d 4p 3d 3p 3s 2p 2s 1s Electronic Structure of Atoms Order of Subshell Filling in Ground State Electron Configurations start by drawing a diagram putting each energy shell on a row and listing the subshells, (s, p, d, f), for that shell in order of energy, (left-to-right) next, draw arrows through the diagonals, looping back to the next diagonal each time 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s Electronic Structure of Atoms Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. 2. Draw 9 boxes to represent the first 3 energy levels s and p orbitals 1s 2s 2p 3s 3p Electronic Structure of Atoms Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. 3. Add one electron to each box in a set, then pair the electrons before going to the next set until you use all the electrons • When pair, put in opposite arrows 1s 2s 2p 3s 3p Electronic Structure of Atoms Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. 4. Use the diagram to write the electron configuration – Write the number of electrons in each set as a superscript next to the name of the orbital set 1s22s22p63s2 = [Ne]3s2 1s 2s 2p 3s 3p Electronic Structure of Atoms Valence Electrons • the electrons in all the subshells with the highest principal energy shell are called the valence electrons • electrons in lower energy shells are called core electrons • chemists have observed that one of the most important factors in the way an atom behaves, both chemically and physically, is the number of valence Electronic electrons Structure of Atoms Valence Electrons Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 • the highest principal energy shell of Rb that contains electrons is the 5th, therefore Rb has 1 valence electron and 36 core electrons Kr = 36 electrons = 1s22s22p63s23p64s23d104p6 • the highest principal energy shell of Kr that contains electrons is the 4th, therefore Kr has 8 valence electrons and 28 core electrons Electronic Structure of Atoms Electrons Configurations and the Periodic Table Electronic Structure of Atoms Electron Configurations from the Periodic Table • elements in the same period (row) have valence electrons in the same principal energy shell • the number of valence electrons increases by one as you progress across the period • elements in the same group (column) have the same number of valence electrons and they are in the same kindElectronic Structure of Atoms of subshell Electron Configuration & the Periodic Table • elements in the same column have similar chemical and physical properties because their valence shell electron configuration is the same • the number of valence electrons for the main group elements is the same as the group number Electronic Structure of Atoms s1 1 2 3 4 5 6 7 s2 p 1 p 2 p 3 p 4 p 5 s2 p6 d1 d2 d3 d4 d5 d6 d7 d8 d9 d10 f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 Electronic Structure of Atoms Electron Configuration from the Periodic Table • the inner electron configuration is the same as the noble gas of the preceding period • to get the outer electron configuration, from the preceding noble gas, loop through the next period, marking the subshells as you go, until you reach the element – the valence energy shell = the period number – the d block is always one energy shell below the period number and the f is two energy shells below Electronic Structure of Atoms Electron Configuration from the Periodic Table 8A 1A 1 2 3 4 5 6 7 3A 4A 5A 6A 7A 2A Ne P 3s2 3p3 P = [Ne]3s23p3 P has 5 valence electrons Electronic Structure of Atoms Electron Configuration from the Periodic Table 8A 1A 1 2 3 4 5 6 7 3A 4A 5A 6A 7A 2A 3d10 4s2 Ar As 4p3 As = [Ar]4s23d104p3 As has 5 valence electrons Electronic Structure of Atoms The Noble Gas Electron Configuration • the noble gases have 8 valence electrons – except for He, which has only 2 electrons • we know the noble gases are especially nonreactive – He and Ne are practically inert • the reason the noble gases are so nonreactive is that the electron configuration of the noble gases is especially stable Electronic Structure of Atoms Everyone Wants to Be Like a Noble Gas! The Alkali Metals • the alkali metals have one more electron than the previous noble gas • in their reactions, the alkali metals tend to lose their extra electron, resulting in the same electron configuration as a noble gas – forming a cation with a 1+ charge Electronic Structure of Atoms Everyone Wants to Be Like a Noble Gas! The Halogens • the electron configurations of the halogens all have one fewer electron than the next noble gas • in their reactions with metals, the halogens tend to gain an electron and attain the electron configuration of the next noble gas – forming an anion with charge 1- • in their reactions with nonmetals they tend to share electrons with the other nonmetal so that each attains the electron configuration of a noble gas Electronic Structure of Atoms Everyone Wants to Be Like a Noble Gas! • as a group, the alkali metals are the most reactive metals – they react with many things and do so rapidly • the halogens are the most reactive group of nonmetals • one reason for their high reactivity is the fact that they are only one electron away from having a very stable electron configuration – the same as a noble gas Electronic Structure of Atoms Stable Electron Configuration And Ion Charge • Metals form cations by losing enough Atom electrons to get the same electron configuration as the Na previous noble gas Mg • Nonmetals form Al anions by gaining O enough electrons to F get the same electron configuration as the next noble gas Atom’s Electron Config [Ne]3s1 Na+ Ion’s Electron Config [Ne] [Ne]3s2 Mg2+ [Ne] [Ne]3s23p1 Al3+ [Ne] [He]2s2p4 O2- [Ne] [He]2s22p5 F- [Ne] Ion Electronic Structure of Atoms Electronic Structure of Atoms © 2009, Prentice-Hall, Inc.