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Chapter 5 Scientific Models Models are things used to represent real phenomena. simplify and explain complex realities. can take many forms scale models, e.g. a globe mathematical models, e.g. P/V = k computer models, etc., e.g. weather predictions It explained much about the structure Nucleus: positive, very dense, most of atom’s mass Electrons: outside the nucleus Empty space: most of the volume of the atom It could not explain chemical behavior of elements, such as…. Why did elements give off light when heated? Why did one element react with another to form a new compound? Rutherford’s Model Rutherford’s model could not explain why matter gave off light when heated The Bohr Model Neils Bohr Danish Physicist 1913: Proposed new model of the atom Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Each possible electron orbit in Bohr’s model has a fixed energy. The fixed energies an electron can have are called energy levels. Higher energy levels are farther away from the nucleus A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level. Energy levels are like rungs on a ladder Higher energy levels are closer together Takes less energy to change between higher levels The Bohr Model When electrons absorb exactly the right quanta of energy…. They jump to higher energy level When it jumps back down… It gives off (emits) the same energy as light. Bohr’s planetary model only worked for hydrogen But it could not explain motion of electrons Schrödinger and others developed a new mathematic model of the atom…. Called the quantum mechanical model ERWIN SCHRÖDINGER Like Bohr’s model, electrons are restricted to certain energy levels Unlike Bohr’s model, the exact pathway of the electron is uncertain Locations of electrons are described terms of probability…. i.e. the likelihood of finding the electron at a given point in time Electrons are found within an “electron cloud” outside the nucleus An analogy: a spinning fan blade Forms a ‘fuzzy’ image You know the fan blade is within the fuzzy region, but at any point in time you don’t know exactly where it is Electrons are located in regions of probability called “orbitals” The electron cloud is more dense where the probability of finding the electron is high. Quantum Number Defines Describes Values 1st Principal Energy Level ----- n = 1 to 7 2nd Angular Momentum Energy Sublevel Shape s, p, d, f 3rd Magnetic Orbital 3-D orientation x, y, z, etc. 4th Spin ----- Magnetic spin +1/2 or -1/2 Orbitals are represented by “electron density maps” Probability is represented by the density of color The more probable location of the electron is in the darker blue region AN “s” ORBITAL Regions of space in which there is a high probability of finding an electron Various types of orbitals exist, depending upon the sublevel S sublevels have one orbital P sublevels have 3 orbitals each d sublevels have 5 orbitals each f sublevels have 7 orbitals each Energy Level # Sublevels # Orbitals Electron capacity n n n2 2n2 1 1 1 2 2 2 4 8 3 3 9 18 4 4 16 32 5 5 25 50 6 6 36 72 7 7 49 98 Each orbital can contain up to 2 electrons! Sublevel # Orbitals per sublevel Electron capacity per sublevel s 1 2 p 3 6 d 5 10 f 7 14 In most natural phenomena, change trends toward lower energy Systems are more stable when they have less energy. Electrons also tend to arrange themselves in their lowest energy states. The arrangement of electrons within an atom is called an electron configuration. Three rules are used to determine electron configurations Aufbau Principle Pauli Exclusion Principle Hund’s Rule Electrons occupy the lowest energy level first This diagram is known as an electron orbital diagram 4th quantum number is the “spin” number Electrons “spin”, either clockwise & counterclockwise Spin is symbolized ↑ or ↓ PEP says…. Two electrons in the same orbital must have opposite spins. No two electrons in an atom can have the same identical set of 4 quantum numbers. Electrons fill orbitals within a sublevel such that they have maximum number of unpaired spins This is because they have the lowest energy this way Determine the number of electrons in the diagram. How? Begin filling orbitals at the lowest energy level (Aufbau principle) Continue filling, applying Hund’s rule All “up” spins Follow by “down” spins Stop when you have assigned all the electrons to orbitals A shorthand way for writing orbital diagrams Write the energy level, sublevel, and number of electrons in the sublevel Li 1s2 2s1 C 1s2 2s2 2p2 N 1s2 2s2 2p3 O 1s2 2s2 2p4 F 1s2 2s2 2p5 Ne 1s2 2s2 2p6 Na 1s2 2s2 2p6 3s1 Periods (rows) in the PT correspond to energy level Certain groups (columns) correspond to the sublevels (s, p, d, f) (see page 166) Transition elements (groups 3-12) tend to prefer half-filled or completely filled d-orbitals at the expense of the s-orbital. For chromium, you would expect but in fact one of the 4s electrons is promoted to 3d, resulting in ….4s2 3d4, ….4s1 3d5 Try copper…. Also called “noble gas notation” An element’s electron configuration contains the e-config of a noble gas (group VIIIA, 18) Begin with the preceding noble gas Then complete the e-config Much of what is known about the atom is due to the study of light Light has properties of waves Waves have amplitude, wavelength, and frequency Inversely proportional c c = speed of light = 3.00 x 108 m/s (constant) lambda = wavelength (meters) nu = frequency (Hertz, Hz, s-1) Visible light is a small portion of the electromagnetic spectrum All EM radiation travels at the same speed c = 3.00 x 108 m/s EM radiation varies in wavelength and frequency Longer wavelength → Lower frequency Shorter wavelength → Higher frequency Light separates into different colors (wavelengths) when it passes through a prism It is a continuous spectrum Electrons of an element can absorb energy and emit the energy as EM radiation These emission spectra are not continuous Each element has a unique emission spectra Like a bar code for an element Electron at ground state absorbs a quantum of energy Excited electron returns to ground state, emitting the quantum as light Frequency of the light is directly proportional to the energy change of the electron Lyman Series is in the UV range Balmer series is visible Paschen series is in IR range Einstein determined that light behaved like a particle “Particle” of light is the photon Photon is a quantum of light So light can behave as a wave and a particle, which is it? Both If light (a wave) can behave as a particle, can a particle behave as a wave? Yes So electrons can be thought of as waves. Uncertainty principle It is not possible to know the location and momentum (speed) of an electron at the same time Schrodinger equation Mathematically described sublevels and orbitals