Transcript Slide 1
CHEMISTRY 161 Chapter 7 1. Structure of an Atom subatomic particles electrons protons neutrons mass number 17e 17p 35 17 18n Cl 18 neutrons atomic number Why are atoms stable? e- classical physics predicts that electron falls into nucleus 2. Waves . wavelength [m] l . amplitude Intensity direction of propagation distance, x EXP1 v = x/t t = x/v T . direction of Intensity . period [s] propagation time, t Frequency [s-1] 1 T [1 Hz = 1 s-1] Hertz Frequency and Wavelength l = c wavelength meter (m) frequency Hz or s-1 speed of radiation m s-1 LONG WAVELENGTH, LOW FREQUENCY light can be described as a wave POSTULATE visible light consist of electromagnetic waves speed of propagation (speed of light) James Maxwell c = 3 108 ms-1 light has two components 1.electric field 2.magnetic field ELECTROMAGNETIC SPECTRUM l INCREASING FREQUENCY & ENERGY EXP2 VISIBLE SPECTRUM Wavelength l (nanometers) l = c Which color has the higher frequency? 1 = orange 2 = blue The wavelength of the yellow light from a lamp is 589 nm. What is the frequency of the radiation? c l l 589nm 5.89 10 m 7 1 2.998 10 ms 14 1 5.09 10 s 7 l 5.89 10 m c 8 3. Postulates Li Na K different atoms emit distinct light EXP3 E h h is Planck’s constant h = 6.626 x 10-34 J s Max Planck energy can be emitted or absorbed only in discrete quantities (little packages) EXP4/5 Emission Nebula CLASSICAL any amount of energy can be emitted or absorbed E h NON-CLASSICAL energy can be emitted or absorbed only in discrete quantities (little packages) E n h energy is not continuous QUANTUM smallest amount of energy which can be absorbed/ emitted 3. Properties of Photons POSTULATE Electromagnetic radiation can be viewed as a stream of particle-like units called photons Albert Einstein E h ABSORPTION OF A PHOTON Efinal atoms and molecules E h absorb discrete photons (light quanta) Einitial EXP6 Duality of Wave and Corpuscle light has properties of a wave and of a particle p de Broglie h l mc SUMMARY 1. light can be described as a wave of a wavelength and frequency E h l = c 2. light can be emitted or absorbed only in discrete quantities (quantum - photon) E n h 3. duality of wave and corpuscle p h l mc 4. Properties of Electrons de Broglie wavelength p h l mc h l mc h l mu each particle can be described as a wave with a wavelength λ WAVE-PARTICLE DUALITY matter and light (photons) show particle and wave-like properties h h l mu p MASS INCREASES WAVELENGTH GETS SHORTER MASS DECREASES WAVELENGTH GETS LONGER WAVE-PARTICLE DUALITY large pieces of matter are mainly particle-like small pieces of matter are mainly wave-like MASS Baseball Proton Particle-like Electron Photon Wave-like 1. light behaves like wave and particle 2. electron behaves like wave and particle 3. electrons are constituents of atoms 4. light is emitted/absorbed from atoms in discrete quantities (quanta) E h 5. Electrons, Photons, Atoms EMISSION OF A PHOTON atoms and molecules Einitial emit discrete photons E h Efinal electrons in atoms and molecules have discrete energies EMISSION SPECTRA analyze the wavelengths of the light emitted only certain wavelengths observed only certain energies are allowed in the hydrogen atom Balmer found that these lines have frequencies related 1 1 15 1 v 2 3.29 10 s 4 n n = 1, 2, 3, 4, 5… THE BOHR ATOM electrons move around the nucleus in only certain allowed circular orbits QUANTUM NUMBERS n=4 n=3 n=2 n=1 e- each orbit has a quantum number associated with it n is a QUANTUM NUMBER n= 1,2,3,4……... THE BOHR ATOM QUANTUM NUMBERS and the ENERGY n=4 n=3 n=2 n=1 En AZ n 2 2 Z = atomic number of atom A = 2.178 x 10-18 J = Ry THIS ONLY APPLIES TO ONE ELECTRON ATOMS OR IONS BOHR ATOM ENERGY LEVEL DIAGRAM En Z=1 AZ n 2 2 HYDROGEN ATOM! A En 2 n BOHR ATOM ENERGY LEVEL DIAGRAM ENERGY En -A A En 2 n A E1 2 A 1 n=1 BOHR ATOM ENERGY LEVEL DIAGRAM En ENERGY -A/4 -A A En 2 n A A E2 2 2 4 n=2 n=1 BOHR ATOM ENERGY LEVEL DIAGRAM En -A/4 n=2 ENERGY -A/9 n=4 n=3 -A e- n=1 A En 2 n Ephoton = h ELECTRON EXCITATION ELECTRON DE-EXCITATION En n=4 n=3 0 -A/9 e- Energy -A/4 -A n=2 emission of energy as a photon e- n=1 ABSORPTION OF A PHOTON nf only a photon of the correct energy will do ni E h E photon ABSORPTION OF A PHOTON E E f Ei h nf AZ Ei 2 ni 2 AZ Ef 2 nf ni 2 ABSORPTION OF A PHOTON E E f Ei h nf ni AZ 2 AZ 2 E 2 2 n n f i 1 1 2 E AZ n2 n2 f i ABSORPTION OF A PHOTON nf 1 1 2 E AZ n2 n2 f i n f ni 1,2,3...(absorption) E0 ni energy is absorbed EMISSION OF A PHOTON ni 1 2 1 E AZ n2 n2 f i ni n f 1,2,3...(emission) E0 nf This means energy is emitted! IONIZATION OF AN ATOM nf ni 1 1 2 E AZ n2 n2 f i nf E0 This means energy is absorbed! IONIZATION ENERGY E = 2.178 x 10-18 J for one atom the ionization energy for one mole is = 2.178x 10-18 J atom-1 x 6.022x1023 atoms mol-1 =13.12 x 105 J mol-1 = 1312 kJ mol-1 THE BOHR ATOM 1 1 QUANTUM NUMBERS E AZ n2 n2 f i 2 n=4 n=3 n=2 n=1 e- E0 E0 absorption emission ni 1; n f ionization energy 6. HEISENBERG’S UNCERTAINTY PRINCIPLE in the microscopic world you cannot determine the momentum (velocity) and location of a particle simultaneously x is the uncertainty in the particle’s position p is the uncertainty in the particle’s momentum h xp 4 p mv EXPVII THE HEISENBERG UNCERTAINTY PRINCIPLE h x mv 4 h 34 2 1 34 0.527 10 Js 0.527 10 kgm s 4 h 1 x v 4 m if particle is big then uncertainty small This means we have no idea of the velocity of an electron if we try to tie it down! Alternatively if we pin down velocity we have no idea where the electron is! So for electrons we cannot know precisely where they are! we cannot know precisely where electrons are! we cannot describe the electron as following a known path such as a circular orbit Bohr’s model is therefore fundamentally incorrect in its description of how the electron behaves. Schroedinger Born (1926) (1927) electron has wave properties 2 The probability of finding an electron at a given location is proportional to the square of 2 – The Bus - Probabilities orbit of an electron at radius r (Bohr) probability of finding an electron at a radius r (Schroedinger, Born) 1. Schroedinger defines energy states an electron can occupy 2. square of wave function defines distribution of electrons around the nucleus high electron density - high probability of finding an electron at this location low electron density - low probability of finding an electron at this location atomic orbital wave function of an electron in an atom each wave function corresponds to defined energy of electron an orbital can be filled up with two electrons (box) EXPVIII most atoms have more than two electrons each electron in an atom is different electrons have different ‘labels’ called quantum numbers QUANTUM NUMBERS 1.principle quantum number 2. angular momentum quantum number 3. magnetic quantum number 4. spin quantum number 1. principle quantum number n n = 1, 2, 3, 4, 5… hydrogen atom: n determines the energy of an atomic orbital measure of the average distance of an electron from nucleus n increases → energy increases n increases → average distance increases n=1 2 3 4 5 6 ‘shell’ K L M N O P maximum numbers of electrons in each shell n=4 n=3 n=2 n=1 e- 2 2 n EXP8 2. angular momentum quantum number l = 0, 1, … (n-1) l=0 1 2 3 4 5 s p d f g h define the ‘shape’ of the orbital EXPIX 1s, 2s, 3s 1s 2s 3s 3. magnetic quantum number ml = -l, (-l + 1), … 0…… (+l-1) +l defines orientation of an orbital in space 2px, 3px, 4px 2px 3px 4px d orbitals 4. spin quantum number ms = -1/2; + 1/2 ORBITALS AND QUANTUM NUMBERS 1.principle quantum number n = 1, 2, 3, 4, 5… 2. angular momentum quantum number l = 0, 1, … (n-1) 3. magnetic quantum number ml = -l, (-l + 1), … 0…… (+l-1) +l 4. spin quantum number ms = -1/2; + 1/2 ATOMIC ORBITALS (n, l, ml, ms) n 1 l 0 ml 0 orbitals 1 designation 1s 2 0 0 1 2s 1 -1,0,+1 3 2px,2py,2pz 0 0 1 3s 1 -1,0,+1 3 3px,3py,3pz 2 -2,-1,0,+1,+2 5 … … … 3dxy,3dyz,3dxz, 3dx2-y2,3dz2 … 3 4