model of the atom
– Rutherford “plum pudding”
• Next model
– Bohr, 2 years later
– Introduced the idea of energy levels
• The difference between 2 energy levels is a
quantum of energy.
Present ModelPresent Model
• Orbitals are like clouds.
– Show areas where electrons are likely to be
• Like the propellers on an airplane or helicopter.
Light is a Wave…Light is a Wave…
• Light travels in waves.
• 2.998 x 108
• Different colors have different energies.
– Blue end is more energetic than the red end of
Light is a particle?Light is a particle?
• Einstein said light is a particle.
– Observed the photoelectric effect.
• When light strikes metal, electrons are released.
– This only happens at certain frequencies.
Light EmissionLight Emission
• When energy is absorbed, electrons can
move from a lower energy level to a higher
• Electrons cannot maintain the higher state
and fall back to their lower level. When this
happens they release the energy they
– Produces light.
• Ground State
– State of lowest possible energy
– Where e- normally are
• Excited State
– Where e- moves when energy is absorbed
– Can’t stay…
– Emits light when falls back to ground state
Energy LevelsEnergy Levels
– Can contain only 2 e-
– Can contain 6 e-
– Can contain 10 e-
– Can contain 14 e-
Aufbau PrincipleAufbau Principle
• The aufbau (German for “building up”)
principle tells us the order to follow when
writing energy levels and the e- they
• The next slide is extremely important.
Write electron orbital configurations for the elements
Stop and go to doc-cam
Electrons & the Periodic TableElectrons & the Periodic Table
• The periodic table is arranged by increasing
atomic number (# of p+).
• Each element is 1 proton and electron higher than
the one before it.
• It is an acceptable shortcut to write the electron
configuration for larger elements by noting the
symbol of the noble gas on the previous line
before it followed by the remaining orbitals.
• Oxygen 1s2
can be written as [He] 2s2
• Br 1s2
can be written as [Ar] 4s2