Chem 17 Reviewer (1st Exam) Essays

Chem 17 Reviewer (1st Exam) Essays Chem 17 Reviewer (1st Exam) Essay Chem 17 Reviewer (1st Exam) Essay CHEMISTRY 17 1ST LONG EXAM REVIEWER I. ELECTRONIC STRUCTURE OF ATOMS -arrangement of atoms’ electrons -the number of electrons in an atom -distribution of electrons around the nucleus and their energies ATOMS *Billiard Ball Model (JOHN DALTON) *Plum Pudding Model (JOSEPH J. THOMPSON) *RUTHERFORD’s Model of Atom -atoms are tiny, hard, indivisible spheres -electrons (negative charge) occupy the atom Atom is mostly empty space with a dense nucleus PROBLEMS with Rutherford’s Model According to laws of physics, Rutherford’s atom should collapse *The model CANNOT explain differences in properties of elements *The model CANNOT explain the colors emitted by elements when heated A. THE QUANTUM MODELS (WAVE) -formed through studies about light DESCRIBING ELECTROMAGNETIC WAVE *Wavelength (? †lambda†) -distance between two adjacent peaks or troughs (m) *Frequency (? †nu†) number of cycles(complete wavelengths) that pass a given point each sec ond (Hz or /s) **Wavelength is inversely proportional to frequency. Where: c= 3. 00 x 10? m/s B. QUANTIZED ENERGY AND PHOTONS Blackbody Radiation oPerfect absorber and emitter of light oAt high temperature, solids emit radiation oRadiation depends on the temperature not in the element the solid is made from. oPREDICTION: â€Å"As temperature increases, the frequency of light emitted also increases. † oOBSERVATION: There was a peak. Photoelectric Effect and Photons oWhen a beam of light shines on a certain surface (e. g. some metals), electrons are ejected. oThe effect is instantaneous. PREDICTION: †It will take time for effect to be observed†; â€Å"and light will produce the effect, given enough time† oOBSERVATION: Effect is instantaneous. There is a minimum frequency of light needed to produce the effect. o**†If frequency of light is below a certain value called the threshold frequency, the number of electrons will be ejected. † o**†If i ntensity of light is increased, flow of current also increases. † QUANTUM THEORY (Max Planck *1858-1947*) oQuantum (fixed amount) – smallest quality oEach particle corresponds to certain energy. Planck proposed that the minimum amount of radiant energy that can gain or lose is related to the frequency of radiation. o†Radiation consists of packets/quantum of energy (PHOTONS *name given by G. N. Lewis*). † oQuantum Theory: â€Å"Energy is quantized. It can only have certain allowed values. † oEnergy of radiation is proportional to the frequency of radiation. Higher frequency ? More energy; Lower wavelength ? More energy oMonochromatic light consists of photons with the same frequency or same energy. oIntensity of the light depends on the number of photons emitted per second. Einstein’s Explanation on the Photoelectric Effect Light consisting of energy packets hit the metal and their energy is absorbed by the electrons. oOne packet of energy (phot on) hits the metal surface, one electron is emitted. oIf energy of a bundle is not enough, it will not be able to dislodge electron from the attraction of the atomic nuclei. No photoelectric effect. oIf energy of a bundle is enough (equal to the threshold energy). It will dislodge an electron from the attraction of the atomic nuclei. oEnergy in excess of energy to dislodge electron (in excess of the threshold energy) becomes kinetic energy of the electron. Higher energy/ frequency of light ? more excess energy ? Higher kinetic energy of electron C. LINE SPECTRA AND THE BOHR MODEL Line Spectra oMonochromatic radiation is composed of single wavelength. oSpectrum is produced when radiation from light is separated into different wavelength components. oContinuous spectrum ?ROYGBIV, containing light of all wavelength oLine spectrum ?Spectrum containing radiation of only specific wavelengths Bohr’s Model (Spectra of Hydrogen Atom) oAssumed that electron move in circular orbits arou nd the nucleus oPOSTULATES: Only orbits of certain radii, corresponding to certain definite energies, are permitted for the electron in a hydrogen atom. ?An electron in a permitted orbit has a specific energy and is in an allowed energy state. An electron in an allowed energy state will not spiral into the nucleus. ?Energy is transmitted or absorbed by the electron only as the electron changes from one allowed energy state to another. This energy is emitted or absorbed as a photon, E= h?. For electron to move farther from the nucleus, it needs more potential energy. oDistance from the nucleus of an allowed orbit, n. Energy of the electron in the allowed orbit, n. oApplications ?Fireworks ?Use of emission spectroscopy to analyse for the presence of elements. Success of Bohr Model oIt successfully predicted the frequencies of the lines in the hydrogen spectrum oIt introduced the concept of discrete energy levels of electron in atoms and the first quantum number, n. oThe model failed to predict energy levels for atoms with more than 1 electron. (if 2/more electrons are present, the electrons repel) D. THE WAVE BEHAVIOR OF MATTER Louis de Broglie (1892-1987) â€Å"Depending on the experimental circumstances, radiation appears to have either a wavelike or a particle-like (photon) character. † o**As the electron moves about the nucleus, it is associated with a particular wavelength. The characteristic wavelength of the electron, or of any particle, depends on its mass, m, and moving at speed, v. oThe quantity mv for any object is its momentum. oMatter waves, wave characteristic of material particles. oThis hypothesis is found to be applicable to all matter; any object of mass and velocity would give rise to a characteristic matter wave. Uncertainty Principle -There is an inherent uncertainty in the precision with which we can simultaneously specify the position and momentum of a particle. This uncertainty is significant only for particles of extremely small mass, such as electrons. oWerner Heisenberg (1901-1976) ?â€Å"The dual nature of matter places a fundamental limitation on how precisely we can know both the location and the momentum of any object. The limitation becomes important only when we deal with matter at the subatomic level (that is with masses as small as that of an electron). † E. QUANTUM MECHANICS AND ATOMIC ORBITALS Erwin Schr? dinger (1887-1961) oQuantum physics opened a new treatment of atoms, electrons, ions and molecules: by describing it mathematically oUsed Broglie’s hypothesis that electrons can be described as a wave. oSolutions to the equation are called wave functions (energy state of an atom). ?WAVE FUNCTIONS: ?Only certain wave functions are allowed. An electron can only have certain energy states: the energy of the electron is quantized. ?They are called orbitals. ?Its square (? ) is called either probability density or the electron density. Mathematical description of a region in space where an electron has some probability of being found. ?Each wave function (or each energy state/orbital) is characterized by a set of 3 quantum numbers: n, l, ml oSchr? dinger’s theory chose to define the energy of the electron precisely. Therefore, according to the uncertainty principle, this would result in a large uncertainty finding the locatio n of the electron. oWhen the Schr? dinger’s equation for the H-atom is solved, the resulting wave functions contain 3 integral numbers called quantum numbers. Orbitals and Quantum Numbers oWhat for? ?Solutions to wave equations given ?Energy of electron ?Probable location of electron ?Identify an orbital oTo identify an orbital, we use THREE quantum numbers. oTo describe an electron, we use FOUR quantum numbers. Principal Quantum Number (n) a. n defines a main energy level or a principal shell b. the size of orbital and energy of orbital depends mainly on n c. As n increases, there will be higher energy of electrons and the farther is the distance of the electron from the nucleus. Azimuthal Quantum Number (l) a. Each value of l defines a type of sublevel or subshell, or an orbital type. b. Thus, we speak of op sublevel/ p orbitals od sublevel/ d orbitals c. Allowed values: positive integer values (from O ? (n-1)) d. Orbitals and sublevels are designated by letters. e. Determines the shape of the orbital. Value of l012345 Letter designatedspdfgh Magnetic Quantum Number (ml) a. Describe the 3D orientation of the orbital b. Gives the number of orbitals in a given substance c. Orbitals in some sublevel have same energy but differ in orientation in space called degenerate orbitals. . Value depends on l. e. Allowed values from –l to +l. Spin Quantum Number (ms) a. Pauli Exclusion Principle oNo 2 electrons in the same atom can have exactly same energy. b. Energies of orbital for H-atom. oFor 1 electron H-atom, orbitals on the same energy level have same energy. They are degenerate. oEnergy of the orbitals of H-atom depends only on the principal quantum number, n. oEnergies of Orbitals As num ber of electron increase, so do the repulsion interactions among them. oSubshells in a Principal Shell have different energies: s