EM Spectrum


EM Spectrum

A New Atomic ModelProperties of LightThe Photoelectric EffectEmission-Line SpectrumBohr ModelComparing Models of the Atom

The Dilemma of the Atom Electrons outside the nucleus areattracted to the protons in the nucleus Charged particles moving in curvedpaths lose energy What keeps the atom from collapsing?

Wave-Particle DualityJJ Thomson won the Nobel prize for describing theelectron as a particle.His son, George Thomson won the Nobel prize fordescribing the wave-like nature of the electron.Theelectron isa particle!The electronis an energywave!

The Wave-like ElectronThe electron propagatesthrough space as an energywave. To understand theatom, one must understandthe behavior ofelectromagnetic waves.Louis deBroglie

Properties of LightThe wave description of light:Electromagnetic radiation is a form ofenergy that exhibits wavelike behavior as ittravels through space.Together, all the forms of electromagneticradiation form the electromagneticspectrum.

Light travels as a wave?Dutch Physicist ChristianHuygens suggested thatlight consisted of waves,light traveling away fromits source like waves ofwater traveling awayfrom a rock that hasbeen dropped in water

WavelengthWavelength =the distance between two wavesrepresented by the greek letter -Lamda λMeasured in Nanometers (10 9 nm = 1m)

Wavelength:measured fromcrest to crest, ortrough to trough

Frequency (v) is defined as thenumber of waves that pass agiven point in a specific time,usually one second.Represented by the greek letternu (ν)Measured in cycles/s or Hertz(Hz)or s -1

Frequeny: Waves/secondLonger waves = low frequencyShort waves = high frequencylong wavelength λ LowfrequencyAmplitudeshort wavelength λ AmplitudeHighfrequency

LongWavelength=Low Frequency=Low ENERGYShortWavelength=HighFrequency=High ENERGY

Electromagnetic Spectrum:An arrangement of all electromagneticwaves by decreasing wavelength, andtherefore increasing frequency.

Light is energyIn early 1900's, German scientistMax Planck, confirmed that lightwas indeed a kind of energy, withdifferent colors representingmore energy than others(bluelight carries more energy thanred light)

The Formula:c = λνc – speed of light (m/s)λ – wavelength (m)ν – frequency (1/s or s -1 ) (Hz)

Example problem:Calculate the wavelength of a light with afrequency of 5.10 x 10 14 Hz. Using theEM spectrum, identify what color the lightis.c = λν so, λ = c/νλ = 3.0 x 10 8 m/s5.10 x 10 14 Hz (1/s)λ = 5.9 x 10 -7 mλ = 590nm = yellow

Practice:What is the wavelength of radiation with afrequency of 1.50 x 10 13 Hz? Does thisradiation have a longer or shorterwavelength than red light?λ = c/vλ = 3.0 x 10 8 m/s1.50 x 10 13 Hzλ = 2.0 x 10 -5 m (2.0 x 10 -3 cm)λ = 2.0 x 10 4 nm longer wavelength than red

One more:What is the frequency of radiation with awavelength of 5.00 x 10 -8 m? In what regionof the electromagnetic spectrum is thisradiation?v = c/λv = 3.0 x 10 8 m/s5.00 x 10 -8 mv = 6.0 x 10 15 Hz Ultraviolet region

Light: Dual wave-particle natureIt is now believed that light hastwo natures: a dual nature.* Light is a stream of particles(photons) traveling in waves.* All waves have a frequency, awavelength, and a wave velocity.

The Photoelectric EffectThe photoelectric effect refers tothe emission of electrons from ametal when light shines on themetal.

The Particle Descriptionof LightA quantum of energy is theminimum quantity of energythat can be lost or gained byan atom.

Photoelectric Effect

German physicist Max Planck proposed thefollowing relationship between a quantum ofenergy and the frequency of radiation:E = hvE = energy (in joules)v = frequency (in s −1 , or Hz)h constant now known as Plancksconstant; h = 6.626 × 10 -34 J• s.

Practice:What is the energy of a photon ofmicrowave radiation with afrequency of 3.20 x 10 11 Hz?E = hvE = (6.626 x 10 -34 J•s) (3.20 x 10 11 Hz)E = 2.12 x 10 -22 J

A photon is a particle of electromagneticradiation having zero mass and carrying aquantum of energy.The energy of a particular photon dependson the frequency of the radiation.E photon = hv

Quantization of Energy

Energy of a Photon

The lowest energy state of an atom is itsground state.A state in which an atom has a higherpotential energy than it has in its groundstate is an excited state.

The Hydrogen-Atom Line-Emission SpectrumWhen investigators passed electric currentthrough a vacuum tube containing hydrogengas, they observed the emission of acharacteristic pinkish glow.When a narrow beam of the emitted light wasshined through a prism, it was separated intofour specific colors of the visible spectrum.The four bands of light were part of what isknown as hydrogens line-emissionspectrum.

Absorption and EmissionSpectra

• These are calledline spectra• unique to eachelement.• These are emissionspectra• The light is emitted(given off).Author: Thomas V. Green Jr.

Other Elements Each element has a unique bright-lineemission spectrum.i.e. Atomic FingerprintHelium Bohrs calculations only worked for hydrogen!LCourtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Emission Spectrumof Hydrogen1 nm = 1 x 10 -9 m = a billionth of a meter410 nm 434 nm 486 nm 656 nm

Continuous and Line SpectraVisiblespectrumλ (nm) light400 450 500 550 600 650 700 750 nmNaHCaHg4000 Ao 5000 6000 7000

Flame Emission SpectraPhotographs of flame tests of burning wooden splints soaked in different salts.methane gas wooden splint sodium ioncalcium ioncopper ionstrontium ion

Copyright © 2006 Pearson Benjamin Cummings. All rights reserved.

Bohr Model of the HydrogenAtomNiels Bohr proposed a hydrogen-atommodel that linked the atoms electron tophoton emission.According to the model, the electron cancircle the nucleus only in allowed paths, ororbits.

Bohr Model of the Atom

When an electron falls to a lower energylevel, a photon is emitted, and the processis called emission.Energy must be added to an atom in orderto move an electron from a lower energylevel to a higher energy level. This processis called absorption.

Electron transitionsinvolve jumps ofdefinite amounts ofenergy.This produces bandsof light with definitewavelengths.

Comparing Models of theAtom

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