Inside the atomInside the atom

A beginners guide to atomic A beginners guide to atomic physicsphysics

Brief HistoryBrief History

JJ ThomsonJJ ThomsonDiscovered the electronDiscovered the electronWhere is it?Where is it?Plum pudding ModelPlum pudding Model

RutherfordRutherford Gold foil experimentGold foil experiment

Positive Positive nuclear corenuclear core

Problems with classical modelProblems with classical model Coulomb force could make Coulomb force could make

electrons orbit nucleus like electrons orbit nucleus like planets, BUT…planets, BUT… Accelerated charge is Accelerated charge is

supposed to emit EM waves…supposed to emit EM waves…why is atom stable?why is atom stable?

Also, why do atoms emit and Also, why do atoms emit and absorb only discrete absorb only discrete frequencies of radiation?frequencies of radiation?

BOHR ATOMBOHR ATOM Circular electron orbitsCircular electron orbits Only “stationary” orbits allowedOnly “stationary” orbits allowed The energy of each orbit is given by: The energy of each orbit is given by:

EEnn= (-13.6 eV) 1/n= (-13.6 eV) 1/n22 (where n is an integer)(where n is an integer) Photons of the correct energy are absorbed, Photons of the correct energy are absorbed,

“exciting” the atom into a higher energy orbit“exciting” the atom into a higher energy orbit Photons are emitted when the electron makes a Photons are emitted when the electron makes a

transition to a lower energy orbittransition to a lower energy orbit The energy of the photons corresponds to The energy of the photons corresponds to

energy energy difference difference of the two orbits of the two orbits E=hfE=hf

Bohr atom (continued)Bohr atom (continued)

Bohr atom...java applet

• What wavelength of radiation would excite the hydrogen atom from the ground state (E1) to the third energy level?

E1 = -13.6 eV/1 = -13.6 eV E2 = -13.6/9 = -1.5 Ev

hc/ = 12.1 eV = 1243 eV-nm / 12.1 eV = 100 nm

What possible wavelengths of light could What possible wavelengths of light could be emitted when a hydrogen atom “de-be emitted when a hydrogen atom “de-excites” from the 4excites” from the 4thth energy level? energy level?

-0.82 eV-0.82 eV-1.51 eV-1.51 eV-3.4 eV-3.4 eV-13.6 eV-13.6 eV

Stationary orbitsStationary orbits Conservation of energy:Conservation of energy:

KE + PE = EtotKE + PE = Etot½ mv½ mv22 – ke2/r = E – ke2/r = Etotaltotal

Circular motion:Circular motion:F=ma F=ma ke ke22/r/r22 = mv = mv22/r /r ke ke22/r = mv/r = mv22

Combined:Combined:EEtotaltotal= mv= mv22 – ½ mv – ½ mv22 = -½ mv = -½ mv22 OROREtotal = ½ keEtotal = ½ ke22/r – ke/r – ke22/r = -½ ke/r = -½ ke22/r/r

Stationary orbits (continued)Stationary orbits (continued)Which orbits are allowed?Which orbits are allowed?One way of getting the answer is to look at One way of getting the answer is to look at

deBroglie wavelength of electrons.deBroglie wavelength of electrons.=h/mv=h/mvTo “fit” in a particular orbit, 2To “fit” in a particular orbit, 2r=nr=n