James T. Shipman Jerry D. Wilson Charles A. Higgins, Jr. Omar Torres Chapter 9 Atomic Physics
Atomic Physics What evidence suggests that matter is composed of atoms?
Evidence for Atoms: Law of Conservation of Mass in a chemical reaction Mass is not gained or lost
Evidence for Atoms: Law of Definite Proportions A compound always has the same relative amounts of the elements that compose it. For example, when water is broken down by electrolysis into oxygen and hydrogen, the mass ratio is always 8 to 1.
Atomic Theory Dalton s Atomic Theory (1803 1807): 1. Each element is composed of small particles called atoms. 2. All atoms of a given element are chemically identical 3. Atoms in chemical reactions combine in simple, fixed, whole-number ratios to form compounds.
Atomic Theory The Discovery of Electrons By J. J. Thomson In 1903 J.J. Thomson discovered the negatively charged electron. Since atoms as a whole are electrically neutral, some other part of the atom must be positive. Thomson conceived the atom as a sphere of positive charge in which negatively charged electrons were embedded; his model is called plum pudding model 2 -
Rutherford s Atom: 1911
Rutherford s Atom: 1911 1. A piece of gold foil is bombarded with α particles 2. Most α particles went through the gold foil, some were deflected and a few bounced backwards. 3. The deflected particles led to the hypothesis of a positive nucleus 2 - at the center, and electrons occupied the volume outside
Rutherford s Atom: 1911 Rutherford envisioned the atom as having a positive charge (the nucleus) around which the electrons orbited Section 9.1
Continuous Spectrum of Visible Light Light of all colors is observed Section 9.3
Line Emission Spectrum for Hydrogen When light from a gas-discharge tube is analyzed only spectral lines of certain frequencies are found Section 9.3
Line Absorption Spectrum for Hydrogen Results in dark lines (same as the bright lines of the line emission spectrum) of missing colors Section 9.3
Spectra & the Bohr Model Spectroscopists did not initially understand why only discrete, characteristic wavelengths of light were Emitted in a line emission spectrum, and Absorbed in a line absorption spectrum In 1913 an explanation of the observed spectral line phenomena was advanced by the Danish physicist Niels Bohr. Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii Section 9.3
434 nm 656 nm 410 nm 486 nm 434 nm -e 5 4 3 2 1 -e 656 nm -e +P 486 nm -e -e 410 nm
Atomic Absorption Atomic line absorption occurs when an electron absorbs a photon and makes a transition from a lower energy to a higher energy E2 e - hv Absorption Spectrum E1 e - 656.3nm
Atomic Emission Atomic line emission occurs when an electron makes a transition from a higher energy to a lower energy by emitting a photon E2 e - hv Emission Spectrum E1 e - 656.3nm
Bohr and the Hydrogen Atom Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii Bohr s possible electron orbits were given whole-number designations, n = 1, 2, 3, n is called the principal quantum number The energy of each orbit is: En 2.178 10 2 n 18 ( J) Section 9.3
En 2.178 10 2 n 18 ( J) Energy Levels of Hydrogen Atom Principal quantum number, n Energy, E n (J) 1 2 3 4 5 6
Transition Energy for Hydrogen Transition Energy Transition E n (J) 6 2 5 2 4 2 3 2 6 5 4 3 2 E 6 E 5 E 4 E 3 E 2 Hydrogen emission lines
Energy Levels of Hydrogen Atom E 1 = -2.178 x 10-18 /1 2 = -2.178 x 10-18 J E 2 = -2.178 x 10-18 /2 2 = -5.445 10-19 J E 3 = -2.178 x 10-18 /3 2 = -2.420 10-19 J E 4 = -2.178 x 10-18 /4 2 = -1.361 10-19 J E 5 = -2.178 x 10-18 /5 2 = -8.712 10-20 J E 6 = -2.178 x 10-18 /6 2 = -6.050 10-20 J
Photon Energy According to Max Planck, the energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength hc E photon hv Where E photon = energy of the photon (in Joules), h = Planck s constant (6.626 x 10-34 J.s), ν = frequency (in Hz)
Determining Photon Energy Example Find the energy in joules of the photons of blue light of frequency 7.50 10 14 Hz. Solution: E = hf = (6.63 10 34 J s)(7.50 10 14 /s) = 49.73 10 20 J ** Note the blue light has more energy than red light Section 9.2
Emission Wavelengths for Hydrogen For the transition: 6 2 hc Energy of the Emitted photon Ei E f -20-19 hc -6.050 10 - (-5.445 10 ) hc -19 4.840 10 9 34 8 10 nm (6.626 10 J. s)(2.998 10 m / s) m -19 4.840 10 J 16 1.98 10 ( nm) 410.4-19 4.840 10
Electron Cloud Model of an Atom The electron cloud is actually a visual representation of the probability distribution of finding the electron. Section 9.7
The Modern Periodic Table
Elements and the Atomic Number Sodium: (as shown in the periodic table) Atomic symbol 11 Na (Atomic number: total number of protons) 22.99 (Average Atomic Mass)
2.2 Elements and Atomic Number Mass Number(A) = 23 (Total number of protons and neutrons) Atomic number(z) = 11 (Total number of protons) Na Protons: 11 Neutrons: 23-11=12 Electrons: 11
Isotopes Isotopes are atoms with identical atomic numbers but different mass numbers.
The Atomic Mass Atomic Mass is a weighted average of all naturally occurring isotopes of the atom in atomic mass units (amu). For Carbon atom, there are 3 naturally occurring isotopes: 12, 13 &14
What is the Atomic Mass of Mg?
Calculate the Atomic Mass of Ni +?
Homework Exercises: 2 3 4 10