• 1st 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.
• 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 travels in waves.
• 2.998 x 108 m/s
• Different colors have different energies.
– Blue end is more energetic than the red end of
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.
• When energy is absorbed, electrons move
from a low to a higher energy level.
• They cannot maintain the higher state and
– When this happens they release the energy
• 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…Emit light & fall back to ground
– Can contain only 2 e-
– Can contain 6 e-
– Can contain 10 e-
– Can contain 14 e-
• 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
Co 1s 22s22p63s23p64s23d7
Electrons & 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 1s22s22p4
can be written as [He] 2s22p4
• Br 1s22s22p63s23p64s23d104p5
can be written as [Ar] 4s23d104p5