Electrons in Atoms Bohr Model of the Atom cjohannesson@satx.rr.com Used by permission of Christy Johannesson nucleus (+) electron (-) Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

How color tells us about atoms Atomic Spectrum

Prism White light is made up of all the colors of the visible spectrum. Passing it through a prism separates it. Author: Thomas V. Green Jr.

If the light is not white By heating a gas or with electricity we can get it to give off colors. Passing this light through a prism does something different. Author: Thomas V. Green Jr.

Atomic Spectrum Each element gives off its own characteristic colors. Can be used to identify the atom. How we know what stars are made of. Author: Thomas V. Green Jr.

These are called line spectra unique to each element. These are emission spectra The light is emitted given off. Author: Thomas V. Green Jr.

Line-Emission Spectrum excited state Wavelength (nm) 410 nm 486 nm 656 nm 434 nm ENERGY IN PHOTON OUT Prism Slits ground state Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Bohr Model electrons exist only in orbits with specific amounts of energy called energy levels Therefore… electrons can only gain or lose certain amounts of energy only certain photons are produced Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Bohr Model Energy of photon depends on the difference in energy levels 6 Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom 5 4 3 2 1 nucleus Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Other Elements Each element has a unique bright-line emission spectrum. i.e. “Atomic Fingerprint” Helium Bohr’s calculations only worked for hydrogen!  Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Bohr’s Experiment Kelter, Carr, Scott, Chemistry A Wolrd of Choices 1999, page 76 Animation by Raymond Chang – All rights reserved.

(a) Electronic absorption transition (b) H2 emission spectrum (top), H2 absorption spectrum (bottom) Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

continuous spectrum gas absorption spectrum absorption spectrum hot source Source: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec11.html emission spectrum emission spectrum

Hydrogen Spectral Lines Lyman series (ultraviolet) Balmer series (visible) Paschen series (infrared) Frequency (hertz) 1016 1015 1014 n = 7 6 5 4 3 2 1

HYDROGEN SPECTRAL LINES (ultraviolet) HYDROGEN SPECTRAL LINES (visible) (infrared) Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Hydrogen Spectral Lines Bohr’s model of the atom accounted mathematically for the energy of each of the transitions shown. ionization IR region E6 E5 656 nm 486 nm 434 nm 410 nm E4 E3 A B C D Paschen (IR) E2 The energy of the light that is emitted from an atom is equal to the difference in energy between the excited state and the ground state. As Bohr predicted, the colors of light indicated by his theory matched those in the line spectrum of hydrogen. • In 1885, Johann Balmer showed that the frequencies of the lines observed in the visible region of the spectrum of hydrogen fit a simple equation:  = constant (1/22 – 1/n2) where n = 2, 4, 5, 6, and these lines are known as the Balmer series. • Johannes Rydberg restated and expanded Balmer’s result in the Rydberg equation: = (1/n21 – 1/n22), where n1 and n2 are integers, n2 > n1, and , the Rydberg constant, has a value of 1.09737 x 107m-1. A B C D E Balmer (Visible) Energy UV region E1 A B C D E F Lyman series (UV) Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 97

Electronic Transitions in the Excited Hydrogen Atom