Dr. Williamson s Notes for Atoms Continued

Transcription

1 Structure of the Periodic Table Atoms Continued: Bohr student version Dr. V.M. Williamson Arranged by or : vertical columns of elements with similar chemical and physical properties : horizontal rows of elements whose properties change progressively Family Names Group IA: Group IIA: Group I-XB : Group VA : choking-gas formers (pnicogens) Group VIA : chalk formers (chalcogens) Group VIIA: Group VIIIA: Physical Properties of Metals and Nonmetals Metals Nonmetals Good conductor Poor conductor electrical and heat electrical and heat Almost all are Solids, liquids, gases Metallic luster Malleable Ductile No metallic luster Nonductile 1

2 Metals and Nonmetals Metals: are on the of the chart. Some elements come as multiples, rather than single atoms Nonmetals: are on the of the chart. Metaloids: have a mixture of characteristics. Note: elements that touch stair steps from group IIIA, except and Diatomic: (not At) Other multiples: and Gases = H 2, O 2, N 2, F 2, Cl 2, noble gases Liquids = Hg, Br 2 Li Na Mg Al K Ca Periodic Trends in Reactivity with Water (Metallic Character) Periodic Property: Atomic Radii Used to describe atomic size Measured indirectly from bonding distances in molecules Metallic Character Copyright 1995 by Saunders College Publishing 2

3 Atomic Radii of Some Elements Periodic Trend: Atomic Radii Atomic Radii Atomic Radii of Some Transition Elements Periodic Property: Ionization Energy First ionization energy (IE 1 ) or potential: Amount of energy required to remove the most loosely bound electron from an isolated gaseous atom e.g., Always positive (energy absorbed) Ion formed (atom with unequal e- and p+). Second e _ harder to remove: First Ionization Energies of Selected Elements Periodic Trend: Ionization Energy Ionization Energy 3

4 The Trend For I.E. Is General for Each Period Ionic Radii Atoms with unequal protons and electrons = Ionic Radii of Some Elements Periodic Trend: Ionic Radii Ionic Radii Absorption and Emission Electromagnetic Radiation Elements give light of specific colors when given electrical energy Light is a form of called Electromagnetic Radiation or radiation Copyright 1999 by HOLT Publishing 4

5 Electromagnetic Radiation Described in terms of waves: White light is a mixture of all colors. For a wave traveling at some speed: υ = c λ ν = the frequency in cycles per second (1/s or s -1 ; 1 Hz = 1 cycle/s) λ = the wavelength c = the speed of light = m/s Copyright 1999 by HOLT Publishing Electromagnetic (EM) Radiation Visible light is only 1 form of EM radiation. Note the wavelengths on in. Planck Equation Describes light in terms of particles called with a particular quantum of energy: E=hυ= hc λ where E = energy of the photon h = Planck s constant = 6.63 x J s ν = c = λ = E=hυ=hc λ Note that Energy and wavelength are inverse. So wavelengths = lower energy and wavelengths = higher energy. light is longer wavelengths; is shorter. What does that tell you about E for red and blue? Units?? What is the smallest amount of energy (1photon) that an object can absorb from light of frequency 7.35 x s -1 (violet)? E = hν = hc / λ = = 5

6 Blackbody Radiation See Flame Test demo- different ions are heated in a flame So elements are absorbing energy as heat or electricity and giving off energy as light (emission). Copyright 1995 by Saunders College Publishing The hotter an object, the the λ But if electrons are at all energy levels, won t the light given off always be -all wavelengths??? Explaining the Spectrum If Rutherford s model was correct, the electrons should be continuously emitting energy of, resulting in a continuous spectrum of electromagnetic radiation- The Bohr Atom Niels Bohr ( ) Nobel Prize 1922 The light evidence, plus the jumps in the ionization energy lead to a new theory of the atom, proposed by Niels Bohr. He proposed that the electrons in a hydrogen atom move around the nucleus in certain allowed orbits The Bohr Atom Electrons are only at certain specific distances from nucleus 6

7 Absorption and Emission The Bohr Atom Copyright 1999 by HOLT Publishing Copyright 1995 by Saunders College Publishing According to Bohr s atomic model, electrons move in definite around the nucleus, much like planets circle the sun. These orbits, or, are located at certain distances from the nucleus. Electrons move in a circular orbit about the nucleus Electrons in an atom are in definite and discrete An electron can move from one to another with corresponding absorption or emission of radiation The Bohr Atom n = 1 e- n = 2 n = 3 e- e- n = 4 e- 0 Potential Energy n = n = 4 n = 3 n = 2 n = 1 First Ionization Energies of Selected Elements Bohr Orbits n=1 ; electrons n = 2 ; add l e- n = 3 ; add l e- Bohr config.: He 2) both e- in n=1 He to Li? Copyright 1999 by HOLT Publishing C 2)4) e- in n=1; e- in n=2 7

8 Bohr Model Explains Ionization Energy jumps between He and Li, Ne and Na, etc. Has stability with shells Explains atomic radii (size) larger size down a family because more orbits (shells). Li 2) 1) vs. Na 2)8)1) Size across a row is harder. Why is C 2)4) SMALLER than Li 2)1)??? Explains metal reactivity with water Na 2)8)1) while K 2)8)8)1) so it is easier to K because it s electron is further from the nucleus. Effective Nuclear Charge Nuclear charge actually felt by an electron in an outer energy level Denoted by Less than actual nuclear charge,, since electrons in inner filled shells shield or screen outer electrons from full nuclear charge Size across a row : Carbon IS SMALLER than lithium C 2)4) Li 2)1) So actually the n=2 level of carbon has a pull from the nucleus of +4, while lithium has a pull on n=2 of +1. The n=2 level of Carbon is pulled tighter, so it is smaller. Explain Ionic Radii Cations are always smaller than parent neutral atoms. Why?? Li 2)1) Li +1 2) Both have in nucleus Anions are always larger than parent neutral atoms. Why?? F 2)7) F -1 2)8) Both have in nucleus Consider: Li +1 and Be +2 Both only have 2 electrons. They are isoelectronic ions (ions will the same number of electrons). Consider the pull from the nucleus, which will have the larger radius? Li 2)1) Be 2)2) Li +1 2) Be +2 2) 8

9 Calculation Electron Transitions Rydberg Equation Note there are 2 forms of this and that if the negative is removed the initial and final will switch: 1 = λ R = x 10 7 m -1 ΔΕ = ΔΕ = R H = x J for energy of a certain level n Calculate the wavelength when an electron moves from n=4 to n=1 1 λ = R $ 1 2 n 1 ' & 2 ) % f n i ( 1 = e7 m -1 [ 1-1 ] λ / λ = e7 m -1 [ ] 1/ λ = e7 m -1 1 = λ = m x 10 7 m -1 Sample problem continued λ = 9.73 x 10-8 m The only means light (given off). Absorption and Emission Spectra Another unit you will see is nanometer. Add this to your list. 1 nanometer = 1 x 10-9 m 1,000,000,000 nanometers = 1 m So the answer could be λ = nm The Bohr Atom The Bohr model was able to explain the line spectrum He was even able to calculate the energy levels available to the electron in the hydrogen atom Hydrogen Atom With Bohr Model Transition λ predicted actual 2 -> UV 3 -> UV 4 -> UV 3 -> red 4 -> bl/gr 5 -> bl 6 -> vio. 4 -> IF 9

10 Hydrogen and beyond Unfortunately, this did not work for any of the other elements with more complex spectra Bohr Model will be replaced because of a number of reasons. For example: It couldn t predict spectrum results beyond hydrogen It didn t explain the transition elements (why periods vary in length) It didn t explain beyond element 20. BUT, easy to write and can use for a number of things... Models of the Atom So Far 10