Lect_W5 - Ge ral C m ne he istry C HEM 151 Week 5 UA...

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Unformatted text preview: Ge ral C m ne he istry C HEM 151 Week 5 UA GenChem Week 5 Reading Assignment UA GenChem Unit 2: Zooming In t are they made of? Why are they joined in t What y could be atoms, molecules, ions holds them together? Why are there forces between them? (intermol UA GenChem Electrical Nature Electrical How do we explain this? We assume atoms and molecules have an electrical nature (they are made of + and - charges) arge distribution” in atoms and molecules is of central importa UA GenChem old Atoms new Made of electrons (-) , protons (+), and neutrons (no charge). ass Number, A Atoms are characterized by the number of protons in the nuc protons+ In a neutral atom, the number of protons is equal to the numb eutrons) A Z X O UA GenChem 16 8 Atomic Number, Z, and Atomic and Mass Number, A e same number of protons in the nucleus, Z, but they same number be • Mass Num r A= # protons + # ne utrons boron • A boron atomcan have 5p + 5n A= A Z 10 5 B UA GenChem Isotopes Atom of thesam e m nt (sam Z) but diffe nt m s e le e e but re ass Atom num r (A). be num Boron-10 (10B) has 5 p and 5 n B) Boron-11 (11B) has 5 p and 6 n B) 11 How many p and n in this atom B 12 6 C ?? UA GenChem 10 B What is this? 208 82 Atomic Mass he scale of atomic masses is defined relative to a standard: 12 6 12 amu (atomic mass units) amu is exactly 1/12 the mass of an atom of C or 1.66 x 10-24g 12 6 C 1 1 H B 1.00728 amu 11.0093 amu 10 5 79 35 B 10.0129 amu 11 5 Br 78.918 amu Because of the existence of isotopes, the mass of Because isotopes the an atom in the periodic table is expressed as an average value of the mass of the different natural isotopes. isotopes. UA GenChem Average Atomic Mass 11 .9% 10B (10.013 amu) and 80.1% 11B (11.009 amu) .9% 10 (10.013 (11.009 10 B What is the average atomic mass? What 11 B Average Mass = fraction(1) x M1 + fraction(2) x M2 +…. = 0.199(10.013 amu) + 0.801(11.009 amu) = 10.81 amu Periodic Table 5 B 10.811 UA GenChem Average Atomic Mass Average By EXPERIMENT for Every element Symbol number 17 e Atomic mass 35.45 Chlorine Cl UA GenChem Electrical Interactions + r repel attract s have exactly the same charge (magnitude) but opposite signs - q = ±1.602 x 10 −19 Coulomb(C ) Coulomb’s Law q1 q 2 F= 2 r Electrons have both kinetic and potential energy. st stable when the total energy (kinetic + potential) is UA GenChem Other Interactions ctrons have an additional property called “spin”. Same Spin dditional Repulsion Opposite Spin Some Attraction Spin generates an additional force between e (magnetic) Magnetic force much less than Electric force UA GenChem A Look into Look Electronic Structure Electronic s of elements and compounds depend on the electron How are electrons distributed in the sys (space-wise, energy-wise) UA GenChem Chemical Analysis e inside atoms, we need to use indirect methods to an are based on the use of light or electromagnetic radiation itted or absorbed by different atoms to make models of their st UA GenChem Light Interactions Because atoms have an electrical nature, they can inter Waves can be characterized Waves by their wavelength istance between the top of nearby crests. (meters-m) istance UA GenChem Electromagnetic Spectrum UA GenChem Light from Glowing objects Most heated solid objects (light bulbs, electric stove) give all the colors of the spectrum (white light) but in different amounts – we see the most intense. “red hot” (mostly red) and “white hot” (more blues) UA GenChem Atomic Emission Spectra prism, only a few colored lines are seen. This is called a l H atoms UA GenChem Line spectra Each element in the gas phase has a distinctive set of colored lines – this was the way new elements were often found at very low concentrations UA GenChem Line Spectra of Excited Atoms • Excited atoms emit light of only certain wavelengths • The wavelengths of emitted light depend on the type of atom. Long λ Long Low ν Low 700. nm Short λ Short High ν High How do we explain this behavior? UA GenChem How do we explain How Atomic Line Spectra? Atomic ectrons in atoms can only exist in certain discrete orbits, w quantized energy states. Niels Bohr Electron orbit + UA GenChem Atomic Spectra and Bohr’ Model If electrons are in quantized energy states, then If ∆ E of states can have only certain values. This of explains sharp line spectra. explains can use the wavelength of the light emitted to calculate the ene n: principal quantum number UA GenChem Interaction Light-Matter Basic Relationships λ • ν= c where c = speed of light = 2.998 x 108 m/s 2.998 racts with matter, energy is transmitted in packets of energy ca Energy per photon E=h•ν h = Planck’s constant = 6.6261 x 10-34 J•s Planck’s UA GenChem Excited H atoms release light of different colors (wavelengths) Consider one of the lines that is red light UA GenChem 700 nm Electromagnetic Radiation One H-emission has λ = 700. nm (red light) Calculate the frequency in Hertz. Calculate UA GenChem Energy of Radiation M: Calculate a) the energy of a photon of red light (λ = M: Calculate a) the UA GenChem Model Predictions Perfect Match !! UA GenChem Electromagnetic Radiation .. The optic nerve needs a minimum of 2.0 x 10-17 J of energy to tr How many photons of blue light (475. nm) are needed? UA GenChem roperties can help us understand electroni electron First Ionization Energy first electron from a gaseous atom to create a positive M(g) M+ (g) + e(g) UA GenChem 1st Ionization Energy Data M → M+ + 1eE= I.E. Trend? UA GenChem What trends do you see? Implications? What H He Li H 1st IE decreases down a group 1st IE increases across a period UA GenChem Shell Model of the Atom tion energy suggest that electrons are arranged in energy shel n=2 n=1 Observations Shell n=1 # of e 2 e8 e8 e18 eUA GenChem n=2 n=3 n=4 ...
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This note was uploaded on 04/07/2009 for the course CHEM 151 taught by Professor Staff during the Spring '08 term at University of Arizona- Tucson.

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