Transcript Atomic Spectra - Northeast High School

Atomic Spectra and
Electron Orbitals
The Classical Atom

Electrons orbited the nucleus.

Problem!!
Accelerating charges emit radiation (energy)
 With less energy the electron orbitals should
shrink and the electrons should spiral into the
nucleus

The Bohr Atom
Electrons orbit in specific energy levels
without radiating
 Each energy level has a specific energy
 To change energy levels, atoms must
either absorb or emit energy (photons)
 Link

Ground State vs. Excited States
Ground state is the lowest energy level in
the atom.
 All other energy levels are known as
excited states.

Quantization of Energy
A result of the Bohr atom is that energy
cannot come in any amount it wants, it
must be in specific amounts of energy
called quanta
 Like steps or rungs of a ladder

Electron Volts (eV)
The energies involved in atomic spectra
are very small, making it inconvenient to
use Joules as our units of energy
 1 eV = 1.6x10-19 J

eV  J
 J  eV

multiply by 1.6x10-19
divide by 1.6x10-19
Hydrogen Atom: Energy Levels
and Energies of Transition
13.6eV
E
2
n
E = -2.176 x 10-18 J
n2
 1

1
E  13.6eV  2  2 
n

n
i 
 f

n - energy level of atom
primary quantum number
 n = 1, 2, 3, …

Photon Energy

E = hf = hc/l
h = 6.626 x 10-34 J*s
 c = 3x108 m/s
 f – frequency
 l – wavelength


Ephoton = Eatom
Eatom > 0 if atom absorbs photon
 Eatom < 0 if atom emits photon

Emission Spectra
The set of energies emitted by an atom.
 Energy determines frequency which
determine color

Electromagnetic Spectrum
Absorption Spectrum

How we know the gaseous atmosphere
surrounding the sun
Spectral Series
Spectral series are the set of energies
produced when electrons from excited
states emit photons and transition down to
the same final energy level
 Three important series for Hydrogen:

Lyman Series: nf = 1
 Ballmer Series: nf = 2
 Paschen Series: nf = 3

Atoms that aren’t Hydrogen
The formulae in the previous slides only
apply to Hydrogen.
 For other atoms, the energies cannot be
so easily calculated, and will often be
given to you directly.

Energy Level Diagram

E = Ef - Ei
Ionization Energy

In order to remove an electron from an
atom you have to add enough energy
make the energy zero.

i.e. if you are at ground level and have an
energy of -10.38eV that means you need
10.38eV if energy to ionize
All left over Energy become
Kinetic Energy
Example

An atom’s only electron is in the fourth
energy level (-2.35 eV).
How many different photon energies can be
emitted as this photon returns to the ground
state (-15.8 eV)?
 What is the frequency of the photon that
would be emitted if the electron returned to
the ground state in a single transition?
