Atomic Spectra and Atomic Spectra and Electron OrbitalsElectron Orbitals

The Classical AtomThe Classical AtomElectrons orbited the nucleus.Electrons orbited the nucleus.

Problem!!Problem!!Accelerating charges emit radiation (energy)Accelerating charges emit radiation (energy)With less energy the electron orbitals should With less energy the electron orbitals should

shrink and the electrons should spiral into the shrink and the electrons should spiral into the nucleusnucleus

The Bohr AtomThe Bohr AtomElectrons orbit in specific energy levels Electrons orbit in specific energy levels

without radiatingwithout radiatingEach energy level has a specific energyEach energy level has a specific energyTo change energy levels, atoms must To change energy levels, atoms must

either absorb or emit energy (photons)either absorb or emit energy (photons)Link

Ground State vs. Excited StatesGround State vs. Excited StatesGround state is the lowest energy level in Ground state is the lowest energy level in

the atom.the atom.All other energy levels are known as All other energy levels are known as

excited states.excited states.

Quantization of EnergyQuantization of EnergyA result of the Bohr atom is that energy A result of the Bohr atom is that energy

cannot come in any amount it wants, it cannot come in any amount it wants, it must be in specific amounts of energy must be in specific amounts of energy called quantacalled quanta

Like steps or rungs of a ladderLike steps or rungs of a ladder

Electron Volts (eV)Electron Volts (eV)The energies involved in atomic spectra The energies involved in atomic spectra

are very small, making it inconvenient to are very small, making it inconvenient to use Joules as our units of energyuse Joules as our units of energy

1 eV = 1.6x101 eV = 1.6x10-19-19 J JeV eV J J multiply by 1.6x10multiply by 1.6x10-19-19

J J eV eV divide by 1.6x10divide by 1.6x10-19-19

Hydrogen Atom: Energy Levels Hydrogen Atom: Energy Levels and Energies of Transitionand Energies of Transition

2

6.13neVE

22

116.13if nn

eVE

n - energy level of atomn - energy level of atomprimary quantum numberprimary quantum numbern = 1, 2, 3, …n = 1, 2, 3, …

E = -2.176 x 10-18 J n2

Photon EnergyPhoton EnergyE = hf = hc/E = hf = hc/

h = 6.626 x 10h = 6.626 x 10-34-34 J*s J*sc = 3x10c = 3x1088 m/s m/s f – frequencyf – frequency – – wavelength wavelength

EEphotonphoton = = EEatomatom

EEatom atom > 0 if atom absorbs photon> 0 if atom absorbs photonEEatom atom < 0 if atom emits photon< 0 if atom emits photon

Emission SpectraEmission SpectraThe set of energies emitted by an atom.The set of energies emitted by an atom.Energy determines frequency which Energy determines frequency which

determine colordetermine color

Electromagnetic SpectrumElectromagnetic Spectrum

Absorption SpectrumAbsorption SpectrumHow we know the gaseous atmosphere How we know the gaseous atmosphere

surrounding the sunsurrounding the sun

Spectral SeriesSpectral SeriesSpectral series are the set of energies Spectral series are the set of energies

produced when electrons from excited produced when electrons from excited states emit photons and transition down to states emit photons and transition down to the same final energy levelthe same final energy level

Three important series for Hydrogen:Three important series for Hydrogen:Lyman Series: nLyman Series: nff = 1 = 1Ballmer Series: nBallmer Series: nff = 2 = 2Paschen Series: nPaschen Series: nff = 3 = 3

Atoms that aren’t HydrogenAtoms that aren’t HydrogenThe formulae in the previous slides only The formulae in the previous slides only

apply to Hydrogen.apply to Hydrogen.For other atoms, the energies cannot be For other atoms, the energies cannot be

so easily calculated, and will often be so easily calculated, and will often be given to you directly. given to you directly.

Energy Level DiagramEnergy Level Diagram

E = EE = Eff - E - Eii

Ionization EnergyIonization Energy In order to remove an electron from an In order to remove an electron from an

atom you have to add enough energy atom you have to add enough energy make the energy zero.make the energy zero. i.e. if you are at ground level and have an i.e. if you are at ground level and have an

energy of -10.38eV that means you need energy of -10.38eV that means you need 10.38eV if energy to ionize10.38eV if energy to ionize

All left over Energy become All left over Energy become Kinetic EnergyKinetic Energy

ExampleExampleAn atom’s only electron is in the fourth An atom’s only electron is in the fourth

energy level (-2.35 eV). energy level (-2.35 eV). How many different photon energies can be How many different photon energies can be

emitted as this photon returns to the ground emitted as this photon returns to the ground state (-15.8 eV)?state (-15.8 eV)?

What is the frequency of the photon that What is the frequency of the photon that would be emitted if the electron returned to would be emitted if the electron returned to the ground state in a single transition?the ground state in a single transition?